Faculty Bibliography

Dosimetric accuracy of a volumetric modulated arc therapy (VMAT) plan is directly related to the beam model, particularly with multileaf collimator characterization. Inappropriate dosimetric leaf gap (DLG) value can lead to a suboptimal beam model, with significant failure in patient-specific quality assurance (PSQA) of VMAT plans. This study addressed the systematic issue of beam modeling and developed a practical method to determine the optimal DLG value for a beam model. Several complex VMAT plans were selected for the quality assurance analysis using the variable DLG values. The results of three-dimensional (3D) Gamma analysis as a function of the DLG at 3%/3 mm, 2%/2 mm, and 1%/1 mm criteria were fitted by a polynomial curve. The DLG value corresponding to the maximum Gamma passing rate for each polynomial fitting function was derived, and the average was calculated to be the optimal DLG value for each model. The 3D Gamma analysis was repeated with the optimal DLG value to verify the dosimetric accuracy of each VMAT case by PSQA. Gamma passing rates are seen to vary considerably with the DLG values and different analysis criteria (3%/3 mm, 2%/2 mm, and 1%/1 mm) for each case. The optimal DLG derived for each model was 1.16 mm and 1.10 mm, much larger than the measured value (about 0.3 mm). The beam models with the optimal DLG was able to produce an average Gamma passing rate of 97.1% (range, 94.6%- 99.1%) at 3%/3 mm and 93.5% (range, 89.0%- 96.5%) at 2%/2 mm for one beam model, and 97.1% (range, 94.8%- 99.1%) at 3%/3 mm, and 93.3% (range, 88.8%- 96.7%) at 2%/2 mm for another. The overall accuracy of dose calculation for VMAT plans should be optimized with a compromise of varied modulation complexities in a beam model. We have developed a practical method to derive the optimal DLG value for each beam model based on the Gamma passing criterion. This technique should be applicable in general for all beam energies and patient cases.

Purpose: We implemented an HDR program to treat small, superficial skin lesions with a Leipzig-style cone (Varian GM11010080). Patients will be simulated with a 3D printed model of the applicator to ease in the placement of the virtual applicator. The patient treatment is planned in Eclipse using a virtual applicator which is built into Varian's solid applicator library. A hand calculation using the published data provides an independent check of the calculated treatment time. Materials and Methods: First, the length of the applicator and Transfer Guide Tube (GM19001010) were verified using a racetrack ruler and measurement wire. A well chamber and electrometer were used to verify the central dwell position in the surface applicator. EBT3 GAFChromic Films and OSLD nanoDots were exposed using all four Leipzig-style cones: 3.0, 3.5, 4.0 and 4.5 cm, at a depth of 0.3 cm to verify dose profile and output, respectively. The solid model applicator and cones were converted directly from the Varian XML library into Stanford PLY format and printed using PLA filament on a MakerBot Replicator 2. Results: The total length of the Leipzig-style applicator and the transfer guide tube was measured as 99.3 cm. The max dwell positon, 98.7 cm, was determined after 4 repeated measurements of outputs at various dwell positions. Both of these measurements agree with Varian's published values in the Instructions for Use manual. The dose profiles from our films demonstrate a 1.68 +/- 0.85 cm spread of the 80% isodose line which is comparable to the published values of approximately 1.6 cm. The OSLD nanodot measurements for absolute dose are within 5% of the calculated values. The 3D-printer model of the applicator was CT simulated and compared to the applicator in Varian's solid applicator library (see image 1). Conclusions: An HDR program treating small skin lesions is implemented by utilizing the Varian Leipzig-style applicator. Using a 3D-printed model of the applicator for CT simulation, the placement of the virtual applicator in treatment planning can be achieved more accurately