Browsing by Author "Wu, Qiuwen"

Purpose: The usage of conformal electron beam therapy techniques in treating superficial tumors on uneven surfaces has often lead to undesired outcomes such as non-uniform dose inside the target and a wide penumbra at boundary of the target. The dynamic electron arc radiotherapy (DEAR) technique has been demonstrated to improve dose distribution and minimize penumbra. The aim of this study is to investigate the feasibility and the accuracy of DEAR technique in irradiating surgical scars.

Method: 3D scar coordinates, a series of connected points along a line extracted from CT images were used. A treatment plan was designed to irradiate the scar with a uniform dose. An algorithm was developed to produce a DEAR plan consisting of control points (CP) corresponding to various positions along machine mechanical axes as a function of MU. Varian’s Spreadsheet based Automatic Generator (SAGE) software was used to verify and simulate the treatment and also to generate the plan in XML format. XML file was loaded on a TrueBeam Linac in research mode for delivery. The technique was demonstrated in i) a straight line scar on the surface of a solid water phantom and ii) curved scar on the surface of a cylindrical phantom. Energy used was 6MeV and a 6x6 cm2 applicator fitted with a 3x3 cm2 cutout. Dose at the surface and dmax were measured with Gafchromic film. Dose profiles calculated from the Eclipse eMC and Virtual Linac Monte Carlo tool were compared to film dose measurement.

Results: The dose profile analysis show that the TrueBeam Linac can deliver the designed plans for both straight line and arc scars to a high degree of accuracy. The root mean square error (RMSE) value for the line scar is approximately 0.00350 Gantry angle and it is 0.0349 for the arc scar. This is due to the fact that in the straight line delivery the gantry angle is static so it has a

higher degree of agreement than in the arc delivery. RMSE values for the straight line scar has an overall high degree of agreement compared to the arc scar because the arc scar delivery has more mechanical axes motion during delivery.

Conclusion: The DEAR technique can be used to treat various line targets (scars) i.e. straight or curved lines, to a high degree of accuracy. This treatment modality can help reinvigorate electron therapy and make it a clinically viable treatment option.

Ionizing radiation is a powerful tool in the fight against cancer. Its potentially lethal effect on cells can halt or even irradicate tumor growth by destroying malignant tumor cells. This is a positive effect on unwanted tumors, but a negative one for surrounding healthy tissue. The goal of radiation therapy is to irradiate a tumor with a dosage as close to the desired amount as possible while sparing the healthy surrounding tissue. Intensity Modulated Radiation Therapy (IMRT) gives the ability to shape a dose distribution through modulating the intensity of the radiation field at different points. This creates a 2D intensity pattern that is linked to a 3D dose distribution through the transport of radiation through the patient. Determining this intensity pattern is a highly coupled numerical optimization problem that relies on a set of objective inputs. These inputs are determined by a human planner and iteratively updated to reach an optimal plan to be delivered to the patient. These constraints depend on the treatment site and may vary based on the patient anatomy. Determining these constraints is a time-consuming problem for cases involving the Pancreas or in the Head and Neck region. For the Pancreas, several gastrointestinal (GI) structures, namely the Stomach, Bowel, and C-Loop, are usually nestled closely to the tumor. This introduces a tradeoff between providing a necessary dose to the target or completely preserving those important organs. The Head and Neck region also poses problems in sparing organs proximal to the tumor such as the parotid glands and oral cavity. Head and neck tumors can also be very large and asymmetric with large overlaps with surrounding organs at risk. The goal of this thesis was to develop and to investigate a framework for automated treatment planning. This involves being able to calculate the dose and optimal fluence and developing a machine learning model to create relevant optimization structures and set constraints. The steps for the thesis are as follows. (i) First, a dose calculation algorithm was developed that is computationally cheap. Machine learning algorithms will rely on numerous calculations of the dose and thus it must be fast and lightweight. There are many commercial algorithms available, but for these purposes it is best to develop a custom engine specifically for the task at hand to minimize cost. This was accomplished by using an analytical definition of a finite-sized pencil beam model parameterized for both depth and off-axis distance and fitted to the beams used in delivering treatment. The addition of variable kernel width was added in to reduce computational cost in both the speed and storage of the calculation. (ii) Second, an optimization engine was developed to quickly find an optimal fluence map given a constraint set. The optimization problem relies on knowing the absorbed dose from a finite sized beamlet to a specific point for all points and beamlets. This is quite expensive, and work must be done to reduce this cost. Analysis was performed to ascertain the effect the cost reduction techniques introduced into the dose calculation would have on the optimization problem. The optimization algorithm was then evaluated to determine the optimal kernel truncation length. (iii) The problem of handling overlapping structures with contrasting constraints has been formulated in a way that an auto-planning system can handle. Pancreas SBRT plans with a simultaneous integrated boost (SIB) are good example of this situation. Previous auto-planning frameworks were modified to specifically deal with the dose gradient around these proximal regions. A reinforcement learning agent was then trained to plan for these scenarios. (iv) Finally, the coupling of plan states and potential actions has been elucidated for determining the control points for structure’s volume effect constraints. Principal Component Analysis (PCA) along with geometric properties such as inflection points and points of maximum curvature were used to correlate the states of a dose-volume histogram to control actions. This was studied and implemented into the beginnings of an automated treatment planning system and demonstrated with Head and Neck cases. The system’s state and action transition probabilities were also investigated to ascertain the stability of the learning process and to ensure the state definition was complete and satisfied the properties of a Markov decision process. The automated system was tested to ascertain the ability of the computer agent to learn how to plan with multiple goals and was shown to be capable of learning techniques providing a foundation for computer automated and aided planning.

BackgroundStereotactic body radiation therapy (SBRT) is a noninvasive alternative treatment for patients who cannot accept surgery. It delivers higher ablative total radiobiological doses to the tumor in fewer fractions. The dose can be as high as 20Gy per fraction (compared to 1.8 to 2 Gy in conventional radiotherapy treatment) and the number of fractions are usually less than 5 (compared to 30 in conventional treatment). Thus, it requires high conformal dose distribution and minimal exposure of surrounding healthy tissues for the patients. The major challenges of SBRT for lung cancer include respiratory motion, positioning uncertainty and intra-fraction stability. Breath hold is usually not suitable for lung cancer patients because they cannot hold the breath for a long time (typically 20 seconds or more) for treatment. Current treatment strategies for Lung SBRT include free breathing (FB), gating at the end of exhale (GE), gating at the end of inhale (GI), and real-time tracking (RT), which are illustrated in the figure below (Fig.a). Most lung SBRT patients are treated in free breathing. It delivers dose to all the tumor motion trajectory, thus requires field size larger enough to cover all the possible tumor locations, creating extra dose to normal tissue. This results in lesion size limitation, so FB is most widely used in small tumors or tumors with less motion, such as those located in the upper lobes of the Lung. The advantages of FB are its low technical requirements and high treatment efficiency (100% duty cycle). For gating technology, the radiation is limited to several specific respiratory phases at the end of exhale or inhale in which the target motions are reduced. It monitors motion and reduce treatment volume, which offers the dosimetric advantage of lower doses to organs at risk. The duty cycle of gating depends on the threshold and is usually less than 30%. Gated treatment can be executed in two strategies: GE and GI. These two strategies each has their own pros and cons. For GE, the tumor position is typically more reproducible than GI. However, GI has larger lung volume due to inspiration and this can lead to lower lung toxicity. In real-time tracking, the beam moves with the tumor movement and delivers the dose to the exact tumor position. It is the most effective strategy because it has theoretically zero motion margin. Another advantage is high treatment efficiency with the beam always on during treatment. The drawback of RT is that it is highly technical demanding and there may be some time delay in tumor detection, decision making and taking actions. Therefore, it is more complex than other strategies and could result in additional potential errors. In addition, the dose delivery accuracy of real-time tracking is limited by the 4DCT resolution and this will be introduced in the following section.

Purpose To reduce the motion margin in dose delivery, we developed a novel bipolar (BP) strategy that requires the patient to hold the breath at the end of the inhale and exhale under audio/video coaching. By only delivering the dose to the tumor during breath-holding, the dose is given to the exact tumor position. This reduces normal tissue toxicities to a great extent. Moreover, the treatment duty cycle is higher than conventional gating technology. In this study, we want to evaluate the feasibility of BP breathing pattern and also compare BP with other strategies geometrically and dosimetricly. Methods BP Feasibility Analysis: Feasibility analysis is conducted to see if the bi-polar breathing pattern is sustainable and comfortable for patients to breathe in a long time. 8 volunteers are included in this study to breathe following the FB/RT, GE, GI and BP breathing patterns under audio coaching. The respiratory signals acquisition time for each pattern is more than three minutes. A custom MatLab program is developed for data analysis. The period repeatability, breath-hold repeatability and treatment efficiency are calculated and compared for each strategy. Geometric Evaluation: 10 previously treated lung SBRT patients with 4DCT were selected retrospectively, each having tumor motion ≥ 1cm. The tumor size at end-exhale (EE) ranges from 0.1 to 22.7 and 80% cases larger than 1cc. 60% tumors located at the lower lobe of the lung. Lung volume and tumor position were used to determine the end-exhale (EE) and end-inhale (EI) phases. The GTV was contoured at each 4DCT phase to determine the ITV for each strategy. PTV is formed by 1mm expansion from ITV. The lung volume, ITV and PTV in BP were compared with FB, GE, GI and RT. All the values are normalized to GI to include all the patients for comparison. Dosimetric Evaluation: IMRT and VMAT plans were generated for each patient with a prescription dose of 60 Gy in 5 fractions. All the plans are completed in the Varian Eclipse System (Version 15.5). The energy we used is 6MV and the calculation algorithm is AAA. 100% dose is normalized to 95% volume. All the plans should meet the RTOG 0813 protocol. IMRT uses 7-9 beams and VMAT uses 1-2 arcs. OARs includes lungs, spinal cord, esophagus, trachea, heart etc., were contoured. For BP and RT, a custom MatLab program was used to summate the plans and calculate the DVHs. Parameters include V5Gy, V13Gy, V20Gy and MLD (mean lung dose) were compared for each strategy in both VMAT and IMRT plans. Tumor Motion Modeling: The purpose of this section is to prove the observed volume in 4DCT is larger than real tumor volume due to tumor motion and the limited number of phases in 4DCT. Both phantom and patient study are included in tumor motion modeling to verify our assumption. QUASAR phantom with a white ball inside (radius: 1.5cm) was used for phantom data acquisition. 5 breathing patterns using same motion amplitude was acquired. The BPM (breath per minute) are 15, 20, 25, 30,33, respectively. 10 previously treated lung SBRT patients with 4DCT were selected for patient study. These patient data are the same as patient data in geometric evaluation section. We developed a MatLab program to calculate the theoretical volume (simulated volume) in each 4DCT phase. By comparing the simulated volume with the observed volume, we want to verify that the observed tumor volume is larger than the real tumor volume in each 4DCT phase and it is a function of real tumor volume and tumor motion.

ResultsFeasibility Analysis: BP breathing pattern is found to be comfortable and sustainable over 3 minutes. This may be longer if we test for longer time. The period repeatability and breath-hold repeatability are at 1.00±0.03 and 1.00±0.04. It is higher than GE(with breath-hold) and GI (with breath-hold), indicating a better repeatability for BP. Treatment efficiency of BP can be more than 65% under audio coaching. It may be improved with the video coaching, longer period of breath holding and patient training. Geometric Evaluation: Using GI as reference, ITV in FB is the largest among all 5 strategies and it is significantly larger than BP. That’s because FB delivers dose to the whole tumor motion trajectory thus creating a large tumor motion margin. The ITVs in RT and BP are similar and smaller. They are approximately one third of FB. The ITV in FB is about twice of the ITVs in GE and GI. Generally, PTV shows a similar trend with ITV. FB is significantly larger than other strategies and it is approximately 2.5 times of RT and BP. The PTV of GE and GI are similar and they are about 56% of the FB. BP is a little bit smaller than RT because the fast-moving tumor in limited phases. Comparing with ITV and PTV, they basically follow the same trend. However, the difference between BP and FB are narrowed due to 1mm expansion from ITV to PTV. In PTV, BP is 58% less than FB, and in ITV, BP is 67% less than FB. Dosimetric Evaluation: In IMRT, all the dose are normalized to GI. FB is the highest for V5Gy, V13Gy, V20Gy and MLD in all strategies and BP is significantly smaller than FB. The reason for that is the largest PTV for FB and smallest PTV for BP. V5Gy, V13Gy, V20Gy and MLD in RT and BP are similar and smaller than GE, GI. They are approximately 20%-30% lower than FB. Although GE and GI have similar PTVs, the dose in GI for V5Gy, V13Gy, V20Gy and MLD are much smaller than GE due to lager lung volume in GI. In VMAT, the evaluation parameters are the same as in IMRT. They basically show similar trend with IMRT. The dose for all parameters in FB is the largest and BP is also significantly smaller than FB. RT and BP is similar and smaller and they are approximately 10%-20% lower than FB for all the parameters. These values are smaller than IMRT. Overall, the improvement from FB to BP is slightly larger in IMRT than VMAT.

Tumor Motion Modeling:In phantom data analysis, for all the cases, the simulated volume achieves minimum at EE and EI phases due to minimum motion, and it increases with larger motion. Observed volumes agree with simulated volume at most phases. In patient data analysis, the agreement is not as good as phantom. The simulated volume achieves its minimum at EE and EI phases and the tumor volume is larger in other phases. Although the observed volumes do not perfectly agree with the simulated volume for most patient cases, they basically follow the same trend. The possible reasons can be tumor location (connecting to diaphragm or vessels) and patients’ irregular respiratory repeatability. More patient data with clear and isolated margin should be included in the future. Conclusion The respiratory experiment demonstrates that the bi-polar breathing pattern is feasible for lung SBRT. It can sustain a long treatment time with a high duty cycle. Moreover, compared to the breathing pattern of GE and GI, BP is more regular, comfortable and thus more sustainable than other breathing patterns. The tumor volume in each 4DCT phase can be inaccurate due to tumor motion and limited resolution in 4DCT. It is actually a function of real tumor volume and tumor motion. Thus, the treatment volume in RT overestimates the actual tumor volume since it used the observed volume as GTV. For the tumors with larger motion (≥1cm) in this research, BP has a significantly smaller ITV in geometric evaluation; therefore, a smaller treatment volume (PTV) compared to FB, GE and GI. The dosimetric evaluation of IMRT and VMAT shows lower V5Gy, V13Gy, V20Gy, and MLD in BP, especially when comparing to FB. This will lead to lower lung dose in treatment. RT has similar geometric and dosimetric results with BP. However, RT is more complicated than BP in implementation and dose delivering.

Background: The verification of VMAT delivery accuracy is widely performed with measurement-based QA methods and gamma index test evaluation. Having the gantry speed as an element of modulation requires that VMAT QA methods resolve the gantry angle accuracy during delivery. EPIDs have increasingly been used for VMAT QA and its minimal size limitation make it advantageous for the measurement of large fields. In this work, we implemented a gantry-resolved EPID-based QA method for patient-specific QA and evaluated its performance for large VMAT fields based on gamma index analysis and process-based tolerance and action limits.

Materials and Methods: Our method created gantry-resolved pseudo-3D dose distributions from XIM files acquired in TrueBeam using dosimetry mode acquisition. The method was used for the evaluation of 35 large VMAT fields measured with two different EPID models and two energies, accounting a total of 140 fields divided into 4 different processes. Predicted portal dose distributions were calculated based on MU information contained in the image headers. An independent calibration procedure that only requires MATLAB for the full implementation of the method was developed. Gamma index analyses were performed with a two-step calculation algorithm that increases accuracy in steep dose gradient regions. Acquisition artifacts causing MU information variability and banding patterns were addressed. Gamma pass rates for pseudo-3D and composite 2Ddose distributions were used to calculate process-based tolerance and action limits following the TG-218 methodology.

Results: The methods to increase gamma index calculation accuracy and reduce artifacts greatly improved the performance and reduced the variation of our results. An independent calibration procedure was successfully implemented. All calculated tolerance limits were stricter than the action limits, and no gamma pass rate from the pseudo-3D distributions with global normalization fell outside of the recommended universal action limit of 90%.

Conclusion: We have demonstrated that our software is suitable for use in patient-specific QA of large VMAT fields. Our results met the recommendation of TG-218. The differences in performance among the processes illustrated they are affected by different sources of variation, indicating that improvements are possible to obtain stricter process-specific tolerance and action limits.

Keywords: VMAT QA, EPID, gantry-resolved, gamma analysis, TG-218, tolerance limits

Electron beam therapy represents an underutilized area in radiation therapy. While electron radiation therapy has existed for many decades and electron beams with multiple energies are available on linear accelerators – the most common device to deliver radiation therapy – efforts to advance the field have been slow. In contrast, photon beam therapy has seen rapid advancements in the past decade, and has become the main modality for radiation therapy treatment.

This doctoral research project comprises the development of a novel treatment modality, dynamic electron arc radiotherapy (DEAR) that seeks to address challenges to clinical implementation of electron beam therapy by providing a technique that may be able to treat specific patient subsets with better outcomes than current techniques. This research not only focused on the development of DEAR, but also aimed to improve upon and introduce new tools and techniques that could translate to current clinical electron beam therapy practice.

The concept of DEAR is presented. DEAR represents a new conformal electron therapy technique with synchronized couch motion. DEAR utilizes the combination of gantry rotation, couch motion, and dose rate modulation to achieve desirable dose distributions in patient. The electron applicator is kept to minimize scatter and maintain narrow penumbra. The couch motion is synchronized with the gantry rotation to avoid collision between patient and the electron cone.

First, the feasibility of DEAR delivery was investigated and the potential of DEAR was demonstrated to improve dose distributions on simple cylindrical phantoms. DEAR was delivered on Varian’s TrueBeam linac in Research Mode. In conjunction with the recorded trajectory log files, mechanical motion accuracies and dose rate modulation precision were analyzed. Experimental and calculated dose distributions were investigated for a few selected energies (6 MeV and 9 MeV) and cut-out sizes (1x10 cm2 and 3x10 cm2 for a 15x15 cm2 applicator). Our findings show that DEAR delivery is feasible and has the potential to deliver radiation dose with high precision (RMSE of <0.1 MU, <0.1° gantry, and <0.1 cm couch positions) and good dose rate precision (1.6 MU/min). Dose homogeneity within ±2 % in large and curved targets can be achieved while comparable penumbra to a standard electron beam on a flat surface can be maintained. Further, DEAR does not require fabrication of patient-specific shields, which has hindered the widespread use of electron arc therapy. These benefits make DEAR a promising technique for conformal radiotherapy of superficial tumors.

Next, an accurate dose calculation framework for DEAR was developed since current commercial dose calculation systems cannot handle the dynamic nature of the DEAR. Comprehensive validations of vendor provided electron beam phase space files for Varian TrueBeam linacs against measurement data were assessed. In this framework, the Monte Carlo generated phase space files were provided by the vendor and used as input to the downstream plan-specific simulations including jaws, electron applicators, and water phantom computed in the EGSnrc environment. The phase space files were generated based on open field commissioning data. A subset of electron energies of 6, 9, 12, 16, and 20 MeV and open and collimated field sizes 3×3, 4×4, 5×5, 6×6, 10×10, 15×15, 20×20, and 25×25 cm2 were evaluated. Measurements acquired with a CC13 cylindrical ionization chamber and electron diode detector and simulations from this framework were compared for a water phantom geometry. The evaluation metrics include percent depth dose, orthogonal and diagonal profiles at depths R100, R50, Rp, and Rp+ for standard and extended source-to-surface distances (SSD), as well as cone and cut-out output factors. Agreement for the percent depth dose and orthogonal profiles between measurement and Monte Carlo were generally within 2% or 1 mm. The largest discrepancies were observed for depths within 5 mm from the phantom surface. Differences in field size, penumbra, and flatness for the orthogonal profiles at depths R100, R50, Rp, and Rp+ were within 1 mm, 1 mm, and 2%, respectively. Simulated and measured orthogonal profiles at SSDs of 100 and 120 cm showed the same level of agreement. Cone and cut-out output factors agreed well with maximum differences within 2.5% for 6 MeV and 1% for all other energies. Cone output factors at extended SSDs of 105, 110, 115, and 120 cm exhibited similar levels of agreement. The presented Monte Carlo simulation framework for electron beam dose calculations for Varian TrueBeam linacs for electron beam energies of 6 to 20 MeV for open and collimated field sizes from 3×3 to 25×25 cm2 were studied and results were compared to the measurement data with excellent agreement.

DEAR uses the superposition of many small fields for its delivery, as such accurate planning requires the knowledge of accurate small field dosimetry. Prior research has shown that previous versions of the clinically used eMC dose calculation algorithm (Varian Medical Systems, Inc., Palo Alto, CA) cannot accurately calculate small static electron fields, leading to discrepancies in the dose distributions and output. Further, the clinical treatment planning system, Eclipse, currently does not support the planning of dynamic electron radiation therapy. Therefore, the aforementioned validation was extended to small fields and compared to dose calculations from the treatment planning system.

Subsequently, small field optimization was explored. Monte Carlo simulations were performed using validated Varian TrueBeam phase space files for electron beam energies of 6, 9, 12, and 16 MeV and square (1x1, 2x2, 3x3, 4x4, and 5x5 cm2) and circular (1, 2, 3, 4, and 5 cm diameter) fields. Resulting dose distributions (kernels) were used for subsequent calculations. The following analyses were performed: (1) Comparison of composite square fields and reference 10x10 cm2 dose distributions and (2) Scanning beam deliveries for square and circular fields realized as the convolution of kernels and scanning pattern. Preliminary beam weight and pattern optimization were also performed. Two linear scans of 10 cm with/without overlap were modeled. Comparison metrics included depth and orthogonal profiles at dmax. (1) Composite fields regained reference depth dose profiles for most energies and fields within 5%. Smaller kernels and higher energies increased dose in the build-up and Bremsstrahlung region (30%, 16 MeV and 1x1 cm2), while reference dmax was maintained for all energies and composite fields. Smaller kernels (<2x2 cm2) maintained penumbra and field size within 0.2 cm, and flatness within 2 and 4% in the cross-plane and in-plane direction, respectively. Deterioration of penumbra for larger kernels (5x5 cm2) was observed. Balancing desirable dosimetry and efficiencies suggests that smaller kernels should be used at the target edges and larger kernels in the center of the target. (2) Beam weight optimization improves cross-plane penumbra (0.2 cm) and increases the field size (0.4 cm) on average. In-plane penumbra and field size remain unchanged. Overlap depends on kernel size and optimal overlap results in flatness ±2%. Dynamic electron beam therapy in virtual scanning mode is feasible by employing small fields to achieve desired dose distributions and acceptable efficiencies.

Further, tools to generally improve upon limitations in Monte Carlo simulations for electron beams were investigated. The phase space file contains a finite number of particle histories and can have very large file size, yet still contains inherent statistical noises. A characterization of the phase space file was investigated to overcome its inherent limitations. To characterize the phase space file, distributions for energy, position, and direction of all particles types were analyzed as piece-wise parameterized functions of radius. Subsequently, a pseudo phase space file was generated based on this characterization. Validation was assessed by directly comparing the original and pseudo phase space file, and by comparing the resulting dose distributions from Monte Carlo simulations using both phase space files. Monte Carlo simulations were run for energies 6, 9, 12, and 16 MeV and all standard field sizes 6x6, 10x10, 15x15, 20x20, and 25x25 cm2. Percent depth dose and orthogonal profiles at depths R100, R50, and Rp were evaluated. Histograms of the original and pseudo phase space file agree very well with correlation coefficients greater than 0.98 for all particle attributes. Dosimetric comparison between original and pseudo dose distributions yielded agreement within 2%/1mm for PDDs and profiles at all depths for all field sizes 6x6, 10x10, 15x15, 20x20, and 25x25 cm2 and energies 6, 9, 12, and 16 MeV. Phase space files were found to be successfully characterized by piece-wise distributions for energy, position, and direction as parameterized functions of radius and polar angle. This facilitates generation of sufficient particles at any statistical precisions.

Additionally, new hardware for improved DEAR capability was investigated. Few leaf electron collimators (FLEC) or electron MLCs (eMLC) are highly desirable for dynamic electron beam therapies as they produce multiple apertures within a single delivery to achieve conformal dose distributions. However, their clinical implementation has been challenging. Alternatively, multiple small apertures in a single cut-out with variable jaw sizes could be utilized in a single dynamic delivery. A Monte Carlo simulation study was performed to investigate the dosimetric characteristics of such an arrangement. Investigated quantities included: Energy (6 and 16 MeV), jaw size (1x1 to 22x22 cm2; centered to aperture), applicator/cut-out (15x15 cm2), aperture (1x1, 2x2, 3x3, and 4x4 cm2), and aperture placement (on/off central axis). Three configurations were assessed: (a) single aperture on-axis, (b) single aperture off-axis, and (c) multiple apertures. Reference was configuration (a) with the standard jaw size. Aperture placement and jaw size were optimized to maintain reference dosimetry and minimize leakage through unused apertures to <5%. Comparison metrics included depth dose and orthogonal profiles. Configuration (a) and (b): Jaw openings were reduced to 10x10 cm2 without affecting dosimetry (gamma 2%/1mm) regardless of on- or off-axis placement. For smaller jaw sizes, reduced surface (<2%, 5% for 1x1 cm2 aperture) and increased Bremsstrahlung (<2%, 10% for 1x1 cm2 aperture) dose was observed. Configuration (c): Optimal aperture placement was in the corners (order: 1x1, 4x4, 2x2, 3x3 cm2 for quadrants I, II, III, and IV) and jaw size were 2x2, 2x2, 3x3, and 7x7 cm2 and 7x7, 7x7, 10x10, and 10x10 cm2 for apertures: 1x1, 2x2, 3x3, 4x4 cm2 and energies 6 and 16 MeV, respectively. Asymmetric leakage was found from upper and lower jaws. Leakage was generally within 5% with a maximum of 10% observed for the 1x1 cm2 aperture irradiation. Multiple apertures in a single cut-out with variable jaw size can be used in a single dynamic delivery, thus providing a practical alternative to FLEC or eMLC.

Based on all the results from this project, DEAR has been found to be a feasible technique and demonstrates the potential to improve electron therapy.

Purpose

Methods and materials

Results

Conclusions

Purpose: Total Body Irradiation (TBI) is typically performed at extended source-to-skin distance (SSD), and the treatment planning is done by simple point dose calculation based on measurement data. The goal of this study is to validate the use of a computerized treatment planning system (TPS, Eclipse) in TBI planning. Typical TPS is commissioned with beam data collected at standard SSD of 100 cm. Extrapolation is required when planning treatments at extended SSDs beyond 400 cm. This requires comprehensive validation.

Methods: Rectangular water phantoms of different sizes were created in Eclipse. Various dosimetric factors including absolute output at extended SAD, phantom scatter factors, off-axis ratios, TMRs, spoiler factors, and compensator transmission factors were calculated in Eclipse using Analytical Anisotropic Algorithm (AAA, V10) and compared with the TBI commissioning measurements at the same geometry. Homogeneous patient phantoms approximating patient body sizes were created with MATLAB and imported into Eclipse. Dose distributions were calculated in Eclipse, and representative point doses were compared with measurements obtained during treatment and those from standard manual calculation.

Results: Dosimetric factors calculated in Eclipse showed good agreement with TBI commissioning measurements. The differences were all within 2% in the range of clinical situations, with most of them within 1%. When comparing in vivo dose measurements and Eclipse calculations, umbilicus (central axis) dose showed better agreement than the lung. Along the midline, doses at head and umbilicus agreed better with prescribed dose than doses at neck, lung, and lower legs.

Conclusion: This work demonstrates that Eclipse can be used for TBI planning at extended SSD with beam data from standard SSD. The Eclipse provides not only dose at certain points, but also full 3D dose distributions that allow more detailed evaluation of the patient treatment plan, hence reducing the need for time-consuming in vivo dose measurements.

Purpose: Xerostomia and dysgeusia are two of the most common and severe

complications for patients undergoing head and neck chemoradiotherapy. HPV positive

patients with oropharyngeal squamous cell carcinoma (OPSCC) have higher survival

rates than HPV negative patients. This research aimed to determine the impact of oral

cavity dosimetry to patient reported toxicity outcomes for 6 and 12 months.

Methods: Multivariate ordinal logistic regress was used to analyze 244 patients who

received deescalated chemoradiotherapy for OPSCC. For each patient both the oral

cavity as a whole and separate substructures of the oral cavity were analyzed to

determine the importance of said structures on xerostomia and dysgeusia.

Results: The results of the analysis showed the oral cavity was a significant predictor of

xerostomia at 6 months post treatment (p=0.0384), however not at 12 months.

Conversely, the oral cavity was not a significant predictor of dysgeusia at 6 months but

was at 12 months (p=0.0092). Additionally, baseline presence of xerostomia was a

predictor at all time points and age was a predictor of dysgeusia at 12 months (p=0.005).

Conclusion: The floor of mouth and oral tongue are the only substructures of the oral

cavity associated with xerostomia and dysgeusia at 6 and 12 months. Our data also

agrees with past studies suggesting the contralateral parotid gland and contralateral

submandibular gland should be spared to limit toxicities at 6 and 12 months.

Purpose: Total body irradiation (TBI) is to deplete patient’s bone marrow and suppress the immune system by delivering uniform dose to patient’s whole body with a relatively low dose rate. The widely used total body irradiation (TBI) protocol in many institutions is to extend the source to surface distance (SSD) to over 400 cm in a large treatment room. The TBI techniques currently used at Duke University Medical Center is anteroposterior (AP)/posteroanterior (PA) technique and bilateral technique. Though bilateral technique TBI is executed with simpler treatment planning and setups in a more comfortable position, it could not provide adequate shielding to lungs and kidneys using blocks like AP/PA TBI technique. However, the whole process of block fabricating and verification is labor-consuming and time-costing. This project aims to develop a better AP/PA TBI treatment method in recumbent position, which provides better sparing for lungs and kidneys in any treatment room.

Methods: In this study, we considered different treatment techniques (three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated radiation therapy (IMRT)), different treatment position (on the floor or on the couch), and different setups (gantry rotation and platform movement). TBI treatment plans were simulated in Eclipse treatment planning system by using both water equivalent phantom and patient CT image. Prescription for the treatment plans was 200 cGy per fraction with 4 fractions. The dose homogeneity should within ±10% of the prescription dose. Dose constraints for kidney and lung are 25% of the prescription dose. In 3DCRT TBI, we applied multi-leaf collimators (MLCs) for OARs shielding and used boost field to provide adequate dose to lungs and kidneys. For IMRT TBI, an iterative optimization algorithm was generated for increasing dose uniformity. By using IMRT, dynamic multi-leaf collimators (DMLCs) provided shielding for kidneys and lungs, which were considered in fluence map optimization. Volume dose and dose profiles were used to analyze the dose uniformity. Measurements with solid water phantom in treatment room were performed to verify the simulation results. IMRT QA with portal imager was performed for phantom.

Results: 3DCRT could not ensure the dose homogeneity and dose deliver accuracy at the same time. To ensure the dose homogeneity in 3DCRT TBI, patient/platform position should be changed between field or applying customized wedge to compensate the inverse square law. For IMRT, the optimization algorithm has excellent performance for both phantom and patients. The dose homogeneity in the mid-plane of both phantom and patients were less than ±5% of the prescription dose after a few iterations. Lungs and kidneys could receive around 25% prescription dose. The simulation and measurement results agree with each other. No additional physical compensators or partial transmission blocks were needed. Portal dose and predict dose perfectly agreed with each other. CR film worked well in positioning. Surface dose enhancement under blocked field was observed.

Conclusion: IMRT technique performed much better than 3DCRT in TBI treatment. In this study, we develop an AP/PA recumbent position IMRT TBI technique that could be used in any linac room. This technique can ensure high dose homogeneity, provide better sparing to lungs and kidneys, and reduce the complexity of TBI treatment planning without the need of labor-intensive compensators and partial transmission blocks.

Purpose: Total body irradiation (TBI) is commonly performed using opposing photon beams of maximum field size at extended SSD. Partial transmission blocks (PTB) are utilized to shield critical organs such as the lungs and kidneys. Both phantom measurement and convolution algorithms confirmed that PDD under PTB deviates significantly from those regions without the blocks. The relationship is complex and depends on many factors. In this study, we investigated the dosimetry under the PTB using the Monte Carlo tool.Methods: The photon phase space (PSP) for Truebeam linac from MyVarian was used as input in the EGSnrc package. The PSP was analyzed and separated into primary (originating from the target) and scatter ( extra-focal source originating from flattening filter) components. It was hypothesized that they behave differently in the presence of PTB which is responsible for the uncommon dosimetry. In this study, a virtual filter was developed to simulate the PTB of any transmission factors in EGSnrc. Further, a concept of scatter photon enhancement ratio (SPER=〖scatter〗_PTB/(〖primary〗_PTB+〖scatter〗_PTB )/〖scatter〗_open/(〖primary〗_open+〖scatter〗_open ) ) was proposed to quantify how the scatter photons’ dose contribution changes with SSD, block size, block-to-patient distance, and transmission factor. Results: Scatter accounts for 12% for 6X, but only 5% for 6XFFF. MC result of the virtual PTB filter agrees well with the measurement for PDD (<1.5%). For a clinical PTB of size 6x12 cm2 at the surface, the SPER at 5cm depth increases from 2.01 to 3.27 when SSD 100 to 400cm; decreases from 3.38 to 1.09 when block-surface-distance 15010cm; and decreases from 8.38 to 2.60 when PTB transmission factor 0 to 30%. Conclusion: The dosimetry under PTB for TBI can be explained by the different behavior of the primary and scatter photon components. MC allows the separation and independent investigation of each component. For in vivo dose measurement under PTB, the measurement needs to be interpreted carefully using the correct dosimetry.

Abstract

Background: Monte Carlo and square root are two commonly used calculation methods to predict electron cutout factors in clinic. Monte Carlo is accurate but time consuming, while square root is faster but less accurate for cutouts with highly irregular shapes.

Purpose: Simplify and develop an efficient residual neural network model to predict electron cutout factors accurately and instantly.

Methods: 281 clinical cutouts were screened, and 12 groups were designed combining four different electron energies (6, 9, 12 and 15 MeV) and three different cones (10, 14 and 20 cm). The cutout factors of 35 previously used electron cutouts were calculated by Monte Carlo simulation and also measured with a solid water phantom and an ion chamber for validation of Monte Carlo accuracy. To solve the issue of sparse training data, 600 cutout samples were created for each group based on modifications of previously screened clinical cutouts. Cutout factors of these 600 samples were calculated with Monte Carlo simulation. 400 samples were randomly selected as training data, 50 as validating and the remaining 150 as testing. 1D Distance histograms were calculated as model input instead of 2D images to accelerate the training process. 1D Residual neural network with four residual blocks and three linear blocks was used. Performance of the trained model was evaluated with testing data, and the accuracy of the model was compared with square root method for eight selected cutouts with highly irregular shapes.

Results: The Monte Carlo calculated cutout factor agreed with the measurement within 0.7±0.5% on average. During the training process, the model started to converge within 20 epochs with 30 seconds. For model prediction, mean errors and maximum discrepancies for each energy and cone combinations were all within 1%, and the average overall error was 0.2±0.16%. Compared to square root method, the trained model performed better for cutouts with highly irregular shapes.

Conclusion: An efficient residual neural network model was simplified and developed, which is capable of estimating electron cutout factors accurately and instantly.

Total skin irradiation (TSI) with electron beam has been very effective for patient with Mycosis fungoides. We recently developed and implemented a technique of laying down position for patients who are too frail for the standard standing position. In this study, we validated these measurements with Monte Carlo (MC) simulation which can provide more information on dose distributions and guidance on further optimization of the technique. The laydown technique consists of 6 equi-spaced beam directions relative to the patient cranial-caudal axis, similar to the standup technique. For the AP/PA directions (vertex fields), patient is placed directly under the gantry at 195cm source-to-skin distance (SSD) and 3 overlapping fields with gantry angles 60˚ apart are used. For the four oblique directions, patient is repositioned on the floor parallel to the gantry rotation axis at SSD of 305 cm with gantry at 300˚. A customized 0.25 mm Cu filter was placed in the linac interface mount to further broaden the beam. Each treatment fraction consists of 10 fields and 3 of them are unique. The Monte Carlo simulation was performed within the EGSnrc environment, using the phase space file provided by the linac vendor. The following quantities were studied and compared with the measurements: for each field/direction at the treatment SSDs, the percent depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on the flat water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. Cu filter increases the beam FWHM by 44% but also reduces the output by 60%. The central regions within ±10% of the prescription dose were 170×70 cm2 for vertex fields and 140×80 cm2 for oblique fields. Profiles and output factors for both vertex fields and oblique fields agreed within 3% between MC and measurements. Vertex fields has dmax at (0.55: MC; 0.67: measurement)cm and R80 at (1.15; 1.40)cm, oblique field has dmax at (1.05; 0.86)cm and R80 at (1.55; 1.40)cm. When all fields are combined on the cylindrical phantom, the dmax shifted toward surface region. The composite dose distribution has the surface dose at (99.0; 95.2) %, dmax at (0.15; 0.15)cm, and R80 at (0.55; 0.75)cm. The maximum X-ray contamination at the central axis was (2.2; 2.1)%, and reduced to 0.2% at 40 cm off the central axis. Cylindrical phantom of 20 cm and 40 cm diameters for patient size simulation shows the surface dose of 93% and 103%, compared to 30 cm diameter. The Monte Carlo results in general agree well with the measurement, which provides secondary support in our commissioning procedure. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, and the ability to investigate and optimize techniques such as different filter design, SSD and field size variations.

Purpose/Objective(s): The standard total skin irradiation (TSI) procedure for patients with Mycosis fungoides at our clinic is the Stanford technique where dual electron beams are directed toward patient standing at an extended source to skin distance (SSD) of 300 cm. Patients rotate along the cranial-caudal axis in 6 directions to get full coverage to skin surface. The purposes of this study are to validate the commissioning dosimetric data using Monte Carlo (MC) systems, and to investigate the effect of scattering filter on the standard stand-up technique with a single beam.

Materials/Methods: The first MC system is the EGSnrc environment with BEAMnrc and DOSXYZnrc packages, which has been the standard MC simulation system used in the radiation therapy field. In this study, extended SSD with electron beam was tested, which is not a common use of EGSnrc. The second system is the VirtuaLinac, a recently developed cloud-based application from Varian for research purpose based on GEANT4 platform. For both MC systems, the same phase space files which have been previously validated were used. At each direction, a dual-field electron beam with jaws opening of 36 × 36 cm2 and gantry angle at ±19° degrees from horizontal direction was used. The following quantities were studied and compared with the measurements during commissioning: for each field/direction at the treatment SSD, the percentage depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on a flat solid water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. For the investigation part, the materials (Cu, Fe, Au, Zn, Ag) were chosen because of their stability and availability. The thickness ranges from 0.05 mm to 0.55 mm, depending on characteristics of materials. The extended source to skin distance (SSD) from 250 cm to 350 cm were studied. For each material, we vary the thickness and SSD, to evaluate following quantities: percent depth dose (PDD), profiles and output at dmax, and compared them with the standard dual beams at treatment SSD.

Results: For the dual-field electron beam from one direction, the average(maximum) difference in profiles between EGSnrc/VirtuaLinac and measurement were -5.5% (-8.7%) and 0.9% (2.0%). Both dmax (1.1 cm) and R50 (2.1 cm) in PDD of both MC systems agreed well with the measurements within 1 mm. The X-ray contamination at 15 cm depth was 0.5%/0.6% for EGSnrc/ VirtuaLinac, compared with the measured value of 0.8%. The output was -2.4%/-3.2% difference for EGSnrc/VirtuaLinac when compared with measurement. When radiation from all six directions are combined on a cylindrical phantom, the ratio of output at the surface from 6 directions to a single direction, defined as B-factor, is 3.1 from both MC systems and the measurement. The dmax also shifted toward the surface at 0.15 cm. The X-ray contamination of all fields was 1.2 % and 1.3% for EGSnrc and VirtuaLinac, compared with 2% in the measurements. For the investigation part, no material shows acceptable profile flatness (±10% within the central 160 cm) at 250 cm SSD. At 300 cm SSD, Au (0.1 mm), Ag (0.25 mm), and Cu (0.45 mm) are acceptable. Zn (0.45 mm) requires 325 cm SSD to meet the requirement. For these 4 configurations, the dmax is 0.87-0.99 cm, similar to dual beam (0.97 cm); R50 is 1.85-1.91 cm, compared with dual beam of 2.06 cm; the output ranges from 0.025-0.029, lower than the dual beam (0.080). The composite fields for 4 configurations, the dmax is 0.1 cm, compared with dual beam (0.16 cm). The surface dose is 97%, similar to dual beam (96%). B-factor is 3.3-3.4, compared with dual beam (3.1). The maximum x-ray contamination is 3%, slightly higher than dual beam (2%).

Conclusions: The results from both Monte Carlo systems in general agree well with the measurement for the validation part. Furthermore, MC results suggest the stand-up TSI technique can be implemented using a single beam if the customized filter is used. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, thus become the foundation for investigations for future technique optimizations.