Browsing by Author "Wang, Zhiheng"

Single isocenter multiple target (SIMT) stereotactic radiosurgery (SRS) provides an effective treatment of brain metastases. However, the delivered dose to the target and to the surrounding tissue affect SRS outcomes. The accuracy of dose calculation is the key challenge in SIMT SRS application. Two dose calculation algorithms, analytical anisotropic algorithm (AAA) and Acuros XB, have been implemented in a commercial treatment planning system (TPS), Varian Eclipse. The purpose of this project is to compare SIMT SRS dose distribution obtained by Acuros XB dose calculation algorithm and commonly used AAA and to investigate the effects of size, distance to isocenter and heterogeneity.

40 clinical cases with 189 targets were used to evaluate the dose distribution differences. All plans were generated using the Eclipse treatment planning system (TPS) v13.6 and calculated using Analytical Anisotropic Algorithm (AAA). Each patient plan consisted of 2 to 14 targets and treated using volumetric modulated arc therapy (VMAT). These plans were recalculated for the purpose of this project using Acuros XB using the same geometry, voxel resolution and monitor units. Parameters used for plan comparison included planning tumor volume (PTV) coverage to 99%, 95%, and 1%, PTV minimum dose, PTV mean dose, PTV maximum dose, and whole brain V3Gy, V6Gy and V12Gy. The dosimetric accuracy was evaluated based on gamma pass rate with threshold criteria 3%/1mm on SRS MapCHECK.

The two algorithm comparison results showed large dose discrepancies in the PTV minimum dose (-7.9% to 5.3%), occurring also in small field size range and in the small range of distance from PTV to skull. The average difference for D1%, D95% and D99% were 0.8%, 0,2% and 0.03%, respectively. However, the difference of D99% revealed up to -6.1%. Relative to Acuros XB, V3Gy, V6Gy and V12Gy decrease by 1.3%, 0.2% and 2.3% with AAA. Gamma analysis demonstrated higher gamma pass rate for AAA compared to Acuros XB (99.9%,97.9%).

Sizable dose difference were found in AAA and Acuros XB, particularly in the PTV minimum dose and PTV coverage to 99%. Heterogeneity and tumor size introduced uncertainty for dose calculation. However, the dose difference showed no dependence on the distance from isocenter to lesions. AAA showed better agreements between calculated and delivered planar dose distributions than Acuros XB.

Since 2010, Duke University Medical Center has used the single-isocenter technique to treat patients with multiple brain metastases. The purpose of this project is to compare treatment planning techniques for these patients who received treatment. First, we want to determine if volumetric modulated arc therapy (VMAT) or dynamic conformal arc therapy (DCAT) is the better method for treatment for incoming patients. Next, we want to know if using U-frame or frameless masks provide better plan quality. Lastly, we want to test the use of a stationary couch to simulate imaging while treating with the moving gantry. DCAT plans were created for each of the 40 single-isocenter patients who received VMAT at Duke University Medical Center from 2016 to 2018. These patients were randomly selected based only on the number of metastases, from 2 to 14. We created the DCAT plans using 5 couch positions, 2 collimator angles, and 100° arcs on BrainLab Elements. We modeled U-frame and frameless masks using 100° and 180° arcs, respectively. To simulate imaging, we kept the couch at 0° while using only 180° arcs. The clinical VMAT plans delivered to the 40 patients had an average conformity index of 1.47 and average gradient index of 8.57. Average whole-brain V3 Gy and V5 Gy were 14.07% and 5.80%, respectively. In comparison, using DCAT the conformity index was 1.75 and the gradient index was 6.87. Whole-brain V3 Gy and V5 Gy were 11.25% and 5.59%, respectively. The frameless mask plans had conformity and gradient indexes of 1.68 and 6.39 and V3 Gy and V5 Gy of 11.39% and 5.09%, respectively. Using VMAT for the imaging cases, we found conformity and gradient indexes of 1.59 and 11.99 and V3 Gy and V5 Gy of 18.04% and 8.41%. Using DCAT for the imaging cases had conformity and gradient indexes of 2.02 and 9.86 and V3 Gy and V5 Gy of 14.21% and 7.25%, respectively. Overall, VMAT plans had higher conformity index with lower gradient index at the cost of healthy brain protection compared to DCAT. Frameless masks also increased the conformity index and decreased the gradient index with no significant impact on low doses to the brain. The use of imaging while treating should be considered with the benefit when imaging on a case-by-case basis.

Purpose: Stereotactic Radiosurgery (SRS) indicates a single fraction, large prescription dose, and high dose gradient radiation treatment for intracranial lesions. The stereotactic immobilization and localization techniques assure the submillimeter accuracy which is required for precise target dose delivery and normal tissue sparing during the treatment. The invasive stereotactic frame-based localization system was the gold standard in SRS for many years based on its high reliability; however, the invasive nature has significant disadvantages in causing patient discomfort and increasing clinical burdens. Therefore, based on the development of on-line imaging techniques and the improvement of computer technology, the noninvasive frameless localization system combined with image-guided patient setup correction is currently widely applied in SRS treatments. This work was designed to compare the setup accuracy between the frame-based and frameless localization system. In addition, the localization and dosimetric accuracy of the treatment after cone beam computed tomography (CBCT) image-guided localization were also evaluated in this work.

Material and Method: At Duke University Radiation Oncology Department, SRS treatments are performed with Novalis TX system. Commonly, the invasive BrainLab Headring system is used for patients with trigeminal neuralgia; on the other hand, the non-invasive BrainLab U-frame system with thermoplastic mask immobilization is applied for the patient with other brain lesions. Both 2D kV images and CBCT images are acquired from the on-board imager (OBI) system for patient localization. The couch shift data from 288 patients with 394 brain lesions treated by single fraction SRS were collected to calculate the setup discrepancies between both frame-based and frameless stereotactic localizations. The systematic setup errors were represented by the mean discrepancies in each direction and the corresponding standard deviations were taken as the random errors in patient setup. In addition, the discrepancies in RMS were also calculated in three transitional directions as the magnitude of the setup errors. 19 patients with 20 brain lesions immobilized with the noninvasive BrainLab U-frame system and thermoplastic mask were selected to evaluate the localization and dosimetric accuracy of CBCT image-guided SRS by the BrainLab iPlan Phantom Mapping module. The contours of the PTV and critical organ were transferred directly from the planning CT and MR images to the CBCT images after image registrations. The distances between the CBCT isocenter coordinates and the PTV center coordinates after couch shifts applied were considered as the residual errors of image-guided setup correction. Moreover, the delivered dose distributions could be calculated and analyzed by copying the original treatment plans to the CBCT images and assigning the treatment isocenters on the CBCT images according to the couch shifts acquired after planning CT and CBCT image registrations. The CT electron density calibration curve used for original plans was also applied for the CBCT-based planning.

Results: The patient treatment records showed that the radial systematic errors (mean) and the random errors (STD) measured by the OBI system were 1.29±1.36 mm and 1.52±2.28 mm (mean±STD) for the BrainLab Headring system and the U-frame system, respectively. In addition, the radial discrepancies in RMS which represented the magnitude of the shifts were 2.07 mm and 2.73 mm for the BrainLab Headring system and the U-frame system, respectively. The residual errors of the image-guided setup correction evaluated from the distances between CBCT isocenter and the PTV center was 0.49±1.06 mm (mean±STD), and the radial discrepancy in RMS was 1.15 mm. With the dosimetric accuracy of treatment delivery, the average minimum dose, mean dose, and maximum dose in PTV of the original plans and the CBCT-based plans were 95.45%±3.80% and 92.88%±3.25%, 110.59%±1.81% and 110.11%±2.40%, 116.55%±3.11% and 115.93%±2.78%, respectively. In the original plans, the average 100% prescription dose coverage of GTV and PTV were 99.99% and 99.81%, respectively. On the other hand, in the CBCT plans, the average 100% prescription dose coverage of GTV and PTV were 99.90% and 98.20%, respectively. The average conformity index of the original plans and the CBCT plans were 1.846 and 1.863, respectively. Only the difference in average PTV coverage between the two plans was considered statistical significant among all dosimetric parameters.

Conclusion: In this study, the invasive BrainLab Headring system demonstrated better localization accuracy than the non-invasive U-frame system. The image-guided system provided larger improvement to the U-frame system than the Headring system in minimizing systematic errors, random errors, and the discrepancies in RMS. In addition, the CBCT-based plans also showed a comparable dosimetric treatment quality relative to the original plans. Only the average PTV coverage in CBCT-based plans was considered poorer than the original plans and the differences was found statistical significant. This deviation may result from the residual discrepancies after image-guided patient localization. However, the treatment goal (100% of GTV covered by 100% prescription dose) can still be achieved by the margin expansions from GTV to PTV. Furthermore, in our work, the rotational image-guided corrections were not considered because of the phantom mapping module limitations. These rotational discrepancies may also cause the degradations in PTV coverage during treatment delivery especially for lesions with irregular shape. Therefore, the future work can be focused on designing a comprehensive evaluation method which can also take the rotation discrepancy corrections into consideration.