Browsing by Subject "VMAT"

Purpose: To develop an automatic collimator setting optimization algorithm to improve dosimetric quality of pancreas Volumetric Modulated Arc Therapy (VMAT) plans for Stereotactic Body Radiation Therapy (SBRT).

Methods: Fifty-five pancreas SBRT cases were retrospectively studied. Different from the conventional practice of initializing collimator settings manually, the proposed algorithm simultaneously optimizes the collimator angles and jaw positions which are customized to the patient geometry. This algorithm includes two key steps: an iterative optimization algorithm via simulated annealing that generates a set of collimator settings candidates, and a scoring system that choose the final collimator settings based on organs-at-risk (OARs) sparing criteria and dose prescription. The scoring system penalizes 3 factors: 1) jaw opening ratio on Y direction to X direction; 2) unmodulated MLC area within the jaw aperture in a dynamic MLC sequence; 3) OAR shielding capability by MLC with MLC aperture control constraints. For validation, the other 16 pancreas SBRT cases were analyzed. Two dual-arc plans were generated for each validation case, an optimized plan (Planopt) and a conventional plan (Planconv). Each plan was generated by a same set of auxiliary planning structures and dose-volume-histogram (DVH) constraints in inverse optimization. Dosimetric results were analyzed and compared. All results were tested by Wilcoxon signed-rank tests.

Results: Both plan groups had no statistical differences in target dose coverage V95% (p=0.84) and Root Conformity Index (p=0.30). Mean doses of OARs were improved or comparable. In comparison with Planconv, Planopt reduced maximum dose (D0.03cc) to stomach (-49.5cGy, p=0.03), duodenum (-63.5cGy, p<0.01), and bowel (-62.5cGy, p=0.01). Planopt also showed lower modulation complexity score (p=0.02), which implies its higher modulation complexity of the dynamic MLC sequence.

Conclusions: The proposed collimator settings optimization algorithm successfully improved dosimetric performance for dual-arc VMAT plans in pancreas SBRT. The proposed algorithm was demonstrated with great clinical feasibility and readiness.

With the advent of new, more efficient, rotational therapy techniques such as volumetric modulated arc therapy (VMAT), radiation therapy treatment precision requires evolving quality assurance. Two dimensional (2D) detector arrays have shown angular dependence that must be compensated for by the creation of angular correction factor tables. Currently available correction factor tables have several underlying assumptions that leave room for improvement: first, these correction factors assume that the response of all ion chambers is identical for each angle; second, that the ion chamber array response from gantry angles 0°-180° are equivalent to the response from 180°-360° and, third, that the response is independent of the direction of rotation.

Measurements were acquired using a 2D ion chamber array (MatriXX®, IBA Dosimetry) for static open fields delivered every 5° around the MatriXX while dose was calculated using Eclipse v8.6 (analytic anisotropic algorithm, Varian Medical Systems). Customized correction factors were created by dividing the calculated dose by the measured dose for each ion chamber. Two measurement positions were used in the creation of the custom correction factors: a coronal position in which the couch was included, and two sagittal orientations in which the couch was not included.

The correction factors were verified using open field arcs and VMAT patient plans, where measured doses were compared to calculated doses using gamma analysis (3%, 3 mm). Narrow fields were also delivered clockwise and counterclockwise in order to investigate the effect of the internal structure of the ion chamber array.

The angular response of the individual ion chambers appears to vary significantly (1 &sigma &le 4.6%). The response from 0°-180° vs. 180°-360° is significantly different (paired t-test yields p<0.0001). Custom correction factors do enhance the agreement between measured and calculated doses for open field arcs and VMAT patient plans compared to the default correction factors. The direction of rotation appears to affect the dose to the penumbra region of narrow fields, which could affect VMAT patient specific quality assurance.

The custom correction factor tables, using measurements for individual ion chambers over a full 0°-360° range, allows for improved accuracy in measurements by the 2D ion chamber array. However, even the corrected measurements still showed discrepancies with the calculated doses for VMAT plans.

Abstract

Purpose: Updated recommendations for pre-treatment QA of patient-specific intensity modulated radiation therapy (IMRT) and Volumetric modulated arc therapy (VMAT) quality assurance (QA) were recently published by the AAPM task group TG-218. While the traditionally most common QA analysis is to use a Gamma index with dose & spatial analysis criteria of 3% & 3mm, respectively, TG-218 recommends a tighter spatial tolerance of 2mm for standard IMRT QA, and that even tighter tolerances should be considered for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). Our purpose is to report our experience with applying the TG-218 recommendations to a large clinical SRS and SBRT program. In addition, a new SRS technique was recently developed at Duke, called Conformal Arc Informed Volumetric Modulated Arc Therapy (CAVMAT), which is designed to be less sensitive to configuration and delivery errors. We measured the agreement of CAVMAT for pre-treatment QA and compared it to the current standard (VMAT) to evaluate whether CAVMAT is more robust to delivery errors than VMAT.

Methods: We re-analyzed the pre-treatment QA with respect to the TG-218 recommendations. For Portal Dosimetry (Varian Medical Systems, Palo Alto, CA), this included IMRT brain (n=25) and SBRT / hypofractionated image guided radiotherapy (HIGRT) cases that utilize flattened photon beams (n=18). For Delta4 (ScandiDos, Madison, WI) this included single target SRS (n=24), multiple target SRS (n=25), SBRT cases using VMAT (n=74), and SBRT cases using IMRT with FFF photons (n=23). For ArcCHECK (Sun Nuclear, Melbourne, FL)), we take 25 single target VMAT SRS cases and 25 multiple target VMAT SRS cases. For SRS MapCHECK(Sun Nuclear, Melboume, FL), we analyze 10 multiple target VMAT SRS cases with 16 targets. A Gamma analysis was performed with 6 spatial/dose criteria combinations: 3%/3mm, 3%/2mm, 3%/1mm, 2%/1mm, 4%/1mm, 5%/1mm. We then calculated the TG-218 action limit and tolerance limit per plan type and compared to the “universal” TG-218 action limit of 90% having a Gamma <1.

To compare CAVMAT and VMAT, log file analysis and pre-treatment QA was performed for 10 patients with 20 plans (10 VMAT, 10 CAVMAT) with 46 targets in total. 10 VMAT plans were re-planned using CAVMAT, and the dosimetric effect due to treatment delivery errors was quantified for V6Gy, V12Gy, and V16Gy of healthy brain along with the maximum, average and minimum doses of each target. Gamma analysis of VMAT and CAVMAT plans was performed using Delta4 and SRS MapCHECK with 3% / 1mm, 2% / 1mm, 1% / 1mm criteria to assess the agreement during patient specific quality assurance.

Result: For Portal Dosimetry QA of IMRT brain and SBRT/HIGRT using a 3%/1mm criteria, the TG-218 action limit was 99.68, and 90.14, respectively; with 3.68% and 3.68% of cases failing the universal 90% criteria. For Delta4 QA of single target SRS, multiple target SRS, and SBRT IMRT with FFF using a 3%/1mm criteria, the TG-218 action limit was 93.64, 97.12, and 92.01, respectively; with 0%, 0%, and 0% of cases failing the universal 90% criteria. For Delta4 QA of SBRT VMAT using a 4%/1mm criteria, the TG-218 action limit was 94.47, with 100% passing. For ArcCHECK QA of single target and multiple target SRS VMAT using a 3%/2mm criteria, the TG-218 action limit was 98.06 and 96.59 respectively, with 100% passing. For SRS MapCHECK QA of multiple target SRS VMAT cases using 3%1mm criteria, the TG-218 action limit was 99.24 with 100% passing.

The average increase in V6Gy, V12Gy, V16Gy due to treatment delivery errors as quantified using the trajectory logfile was 0.94 ± 1.43, 0.90 ± 1.38%, and 1.23 ± 1.54% respectively for VMAT, and 0.035 ± 0.14%, 0.14 ± 0.18%, and 0.28 ± 0.24% for CAVMAT. The average change to target maximum, average, and minimum dose due to delivery errors was 0.53 ± 0.46%, 0.52 ± 0.46%, and 0.53 ± 0.56%, for VMAT, and 0.16  0.18%, 0.11  0.08%, and 0.03  0.24% for CAVMAT. There was no significant difference in magnitude of MLC discrepancies during delivery for VMAT and CAVMAT. For Gamma analysis with strict 1% / 1mm criteria, the average passing rate of VMAT gamma analysis is 94.53 ± 4.42%, while that of CAVMAT is 99.28 ± 1.74%.

Conclusion: For most QA devices, spatial tolerance of pre-treatment QA for SRS/SBRT can be tightened to 1mm while still maintaining an in-control QA process. The gamma criteria to 3%/1mm for all SRS cases and SBRT with IMRT and transitioning to a 4%1mm criteria for SBRT with VMAT have a spatial tolerance that is appropriate for the radiotherapy technique while not resulting in an excessive false positive failure rate. The CAVMAT treatment planning technique resulted in superior gamma analysis passing rate for each gamma analysis criteria.

Since the publication of TG119 in 2009, new techniques have emerged in the field of radiation therapy including VMAT (Volumetric Arc Therapy) and the FFF (Flattening Filter Free) mode in Varian linear accelerators. Our goal in this work is to verify the feasibility of using TG119 to test the commissioning of VMAT and FFF systems and to set a benchmark for other institutions to use.

We created 48 plans of the five sites given in TG119 in addition to a "real" HN case. For each site, we planned IMRT and VMAT using 6MV and 10MV, FF and FFF modes (6*2*2*2 = 48 plans). All our plans were created on the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) and delivered on three beam-matched TrueBeam linear accelerators (Varian) at Duke University Medical Center.

Measurements were taken using ion chamber, film, and a pseudo-3D diode array (Delta4), and compared to the planned doses. Confidence limits were determined using the approach of TG119 (CL = average mean deviation + 1.96 * standard deviation). We used the student's paired t-test to determine any statistically significant differences between IMRT and VMAT, FF and FFF for 6MV and 10MV.

The majority of the ion chamber measurements (94%) agreed with the planned doses within 3%. The majority of errors > 3% involved the HN IMRT plans, either TG119 or "real". For film measurements, we used gamma parameters of 3%/3mm with a 20% threshold. All films met Duke's acceptability criteria of <= 10% of pixels failing gamma. As for Delta4, gamma parameters of 3%/3mm with a 5% threshold were used. All plans met Duke's acceptability criteria of 90% of pixels passing (average 99.7% +/- 0.8%). A second analysis was performed using 2%/2mm gamma parameters, where almost all plans met the 90% passing rate criteria (average 98.9% +/- 2.5%).

Confidence limits were established for ion chamber (3.1%), film (6%), and Delta4 (3.1%) measurements. All the confidence limits were comparable to TG119 institutions. We recommend that non-clinical plans (e.g. 10MV HN plans) not be included in TG119 evaluations. We also recommend that film continue to be used as the gold standard of multi-dimensional measurements, rather than be replaced by diode-based technology.

Purpose: Linac based radiosurgery to multiple metastases is commonly planned with VMAT as it effectively achieves high conformality to complex target arrangements. However, as the number of targets increases, VMAT may struggle to block between targets, which may lead to highly modulated and/or nonconformal MLC trajectories. This phenomenon is particularly apparent in multi-target geometries as targets often share the same MLC leaf pair, creating an MLC opening and yielding insufficient inter-target collimation. Given the complex geometries, multi-target SRS necessitates high degrees of dosimetric accuracy. Dosimetric accuracy may be impacted by beam commissioning as a single dosimetric leaf gap (DLG) must be selected and used for all targets, geometries, and MLCs. Conformal Arc Informed VMAT (CAVMAT) aims to reduce healthy tissue dose by utilizing simplified MLC trajectories and by producing more conformal dose distributions. It is hypothesized that the simplified leaf motion and reduced complexity of CAVMAT may reduce sensitivity to commissioning and treatment delivery uncertainty.

Materials & Methods: CAVMAT is a hybrid treatment planning technique which combines the conformal MLC trajectories of dynamic conformal arcs with the MLC modulation and versatility of inverse optimization. CAVMAT has three main steps. First, targets are assigned to subgroups to maximize MLC blocking between targets. Second, arc weights are optimized to achieve the desired target dose, while minimizing MU variation between arcs. Third, the optimized conformal arc plan serves as the starting point for limited inverse optimization to improve dose conformity to each target. Twenty multifocal VMAT cases were re-planned with CAVMAT with 20Gy applied to each target. The total volume receiving 2.5 Gy, 6 Gy, 12 Gy, and 16 Gy, conformity index, treatment delivery time, and the total MU were used to compare the VMAT and CAVMAT plans. Of the 20 VMAT plans, 10 were selected and replanned with CAVMAT, at DLG values of 0.4 mm, 0.8 mm, and 1.2 mm and the change in V6Gy [cc], V12Gy [cc], V16Gy [cc], and target dose was quantified. The 10 VMAT and CAVMAT plans were delivered to a Delta4 QA phantom and dose agreement was quantified using gamma index with 3%/1mm, 2%/1mm, and 1%/1mm criteria. Trajectory log files were collected and analyzed to quantify MLC positioning errors during delivery. 16 targets were selected, with at least one target from each plan, and were delivered to an SRS Mapcheck QA phantom to evaluate dose difference per DLG.

Results: After replanning the 20 VMAT cases, CAVMAT reduced the average V2.5Gy[cc] by 25.25±19.23%, V6Gy[cc] by 13.68±18.97%, V12Gy[cc] by 11.40±19.44%, and V16Gy[cc] by 6.38±19.11%. CAVMAT improved conformity by 3.81±7.57%, while maintaining comparable target dose. MU for the CAVMAT plans increased by 24.35±24.66%, leading

to an increased treatment time of ~2 minutes. For the DLG analysis, the 10 VMAT plans

demonstrated an average sensitivity to variation of V6Gy [cc], V12Gy [cc], V16Gy [cc] of 35.83 ± 9.48%/mm,

34.12 ± 6.61%/mm, and 39.22 ± 8.41%/mm, respectively, compared to 23.18 ± 4.53 %/mm, 22.45± 4.28 %/mm, and 24.88 ± 4.91 %/mm for CAVMAT. VMAT was found to be roughly twice as sensitive as CAVMAT to changes in target doses for a varying DLG. For the plans delivered to the Delta4 CAVMAT demonstrated an improved dose agreement, with the strictest criteria of 1%/1mm resulting in a passing rate of 94.53 ± 4.42% for VMAT compared to 99.28 ± 1.74% for CAVMAT. Log file analysis demonstrated CAVMAT’s improved resistance to treatment delivery uncertainties, though the difference compared to VMAT is not expected to be substantial.

Conclusions: The CAVMAT technique successfully eliminated insufficient MLC blocking between targets prior to the inverse optimization, leading to less complex treatment plans and improved tissue sparing. CAVMAT is more robust to dosimetric and treatment delivery uncertainties and better maintains target dose and healthy tissue sparing. The reduced complexity, inherent tissue sparing, improved conformity, and reduced sensitivity to uncertainties indicates CAVMAT to be a promising method to treat brain metastases.

Abstract

Purpose

Single-isocenter multi-target (SIMT) Volumetric Modulated Arc Therapy (VMAT) technique can produce highly conformal dose distributions and short treatment delivery times for the treatment of multiple brain metastases. SIMT radiosurgery using VMAT is primarily limited to linear accelerators utilizing 2.5mm leaf width MLCs. We explore feasibility of applying this technique to linear accelerators utilizing MLCs with leaf width of 5mm to broaden the applicability of SIMT radiosurgery using VMAT to include the greater number of linear accelerators with standard 5mm MLCs.

Methods

Twenty patients with 3-10 intracranial brain metastases originally treated with 2.5 mm leaf width MLCs were re-planned using standard 5mm leaf width MLCs and the same treatment geometry (3-5 VMAT arcs). Conformity index, low (V30%), and moderate (V50%) isodose spill were selected for analysis. V12Gy was also analyzed for single fraction cases. We tested the effects of several strategies to mitigate degradations of dose quality values when 5 mm leaf width MLCs were used; these included duplicating each VMAT arc with altered collimator angles by 10°, 15°, and 90°, and adding 1-2 VMAT arcs, with all arcs equally spaced.

Results

Wider MLCs caused small changes in total MUs (5827±2334 vs 5572±2220, p=.006), and Conformity Index (CI) (2.22%±0.05%, p=.045), but produced more substantial increases in brain V30%[%] and V50%[%] (27.75%±0.16% and 20.04%±0.13% respectively, p < .001 for both), and V12Gy[cc] (16.91%±0.12%, p < .001).

Conclusion

SIMT radiosurgery delivered via VMAT using 5mm leaf width MLCs can achieve similar CI compared to that using 2.5mm leaf width MLCs but with moderately increased isodose spill, which can be only partially mitigated by increasing the number of VMAT arcs.

Purpose: (Project 1): Increased interest in the Radiation Oncology Physics community regarding sensitivity of pre-treatment IMRT/VMAT QA to delivery errors has led to the development of DVH based analysis for pre-treatment QA. This paradigm shift necessitates a change in the acceptance criteria and action tolerance for QA. Here we present a knowledge based technique to objectively compare degradations in the DVH to the range from prior clinically accepted plans after adapting to the new patient's anatomy. We apply this knowledge based method to low risk prostate radiotherapy.

(Project 2): To address many of the physics challenges associated with single isocenter radiosurgery for multiple intracranial metastases (SIRMIT). Because the Varian High Definition MLC has variable leaf width with thicker leaves located >4cm from the isocenter (0.5cm vs. 0.25cm), this raises the question of whether the dose falloff and plan quality is inferior for targets located distal from the isocenter (>4cm). We hypothesize that such an effect will be greater for smaller targets, and we test this hypothesis and evaluate various isocenter placement strategies including one that favors placement of smaller targets closer to the isocenter.

Methods and Materials: (Project 1): DVHs for relevant organs at risk from a population of prior patients' plans were adapted using a machine learning algorithm to establish the DVH range specific to the patient's anatomy. The population of prior plans consisted of 198 (for OARs) and 40 (for PTVs) prostate cancer patients that had previously been planned and treated using IMRT. We then applied this technique to evaluate for single arc VMAT plans the clinical effect of six types of delivery errors: systematic offsets in a single centrally located MLC leaf; systematic leaf bank offsets, random normally distributed fluctuations in MLC position, systematic lag in gantry angle of the MLCs behind their intended position(s), fluctuations in dose rate, and delivery of each VMAT arc with a constant rather than a variable dose rate.

(Project 2): 11 SIRMIT plans with the isocenter placed at the PTV centroid were retrospectively analyzed to determine the relationship between relevant dosimetric indices and distance from the isocenter (e.g. relative tumor volume). We investigated three isocenter placement strategies for four prior SIRMIT patients that had larger variations in target volume; these strategies include (1) centroid, (2) Eclipse's built-in method, and (3) an "inverse center of mass" (ICM) method that weights the isocenter placement more heavily towards smaller lesions. Three VMAT SIRMIT plans were prepared with and without avoiding entrance / exit geometry through the eyes. Dose was calculate within the TPS and measured on an anthropomorphic phantom using Optically Stimulated Luminescence (OSL) dosimeters.

Results: (Project 1): QUANTEC suggests V75Gy dose limits of 15% for the rectum and 25% for the bladder, however the knowledge based constraints were more stringent: 8.48±2.65% for the rectum and 4.90±1.98% for the bladder. 19±10mm single leaf and 1.9±0.7mm single bank offsets resulted in rectum DVHs worse than 97.7% (2σ) of clinically accepted plans; all other errors fell within the clinically acceptable (2σ) rectum range. PTV degradations fell outside of the acceptable range for 0.6±0.3mm leaf offsets, 0.11±0.06mm bank offsets, 0.6±1.3mm of random noise, and 1.0±0.7° of gantry-MLC lag.

(Project 2): Values of conformity and gradient fall-off tended to be of higher quality for larger tumors (>1cc) close to isocenter. Moving the isocenter towards smaller tumors (ICM isocenter placement) yielded increased conformity and decreased gradient indices for these metastases. However, we observed a greater decrease in conformity and increase in gradient indices for larger lesions. Considering arc geometry entrance and avoidance angles for the eyes reduces lens dose from around 0.5-2.3Gy to ≤0.1Gy for a 20Gy SIRMIT plan.

Conclusions: (Project 1): Utilizing a group of prior treatment plans, a machine learning algorithm may be used to determine the distribution for each DVH that would have been achieved had the prior plans been prepared using the new patient anatomy, and degradations leading to statistical outliers may be identified. Using a knowledge based approach to QA evaluation enables customized QA criteria per treatment site, institution and or physician; and can often be more sensitive to errors than criteria based on organ complication rates.

(Project 2): CI and GI values were poorer for small distal targets, especially >6cm. Large tumors benefited from centroid placement more than small tumors did from ICM. It is necessary to consider arc geometry avoidance angles to adequately reduce lens dose.