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Independent Dose Calculation for GammaPod Treatment

W Lu1*, M Chen2 , N Salehi3 , Y Zhang4 , D Parsons5 , S Jiang6 , X Gu7 , (1) UT Southwestern Medical Center, Dallas, TX, (2) UT Southwestern Medical Center, Dallas, TX, (3) Xcison Inc, ,VA, (4) UT Southwestern Medical Ctr at Dallas, Dallas, TX, (5) UT Southwestern Medical Center, Dallas, TX, (6) UT Southwestern Medical Center, Dallas, TX, (7) UT Southwestern Medical Center, Dallas, TX


(Sunday, 7/14/2019) 2:00 PM - 3:00 PM

Room: Stars at Night Ballroom 4

Purpose: GammaPod is the first stereotactic body radiation therapy system optimized for breast cancer treatment. Its treatment planning system (TPS) uses dose kernels pre-calculated in homogenous density and fixed breast cups. However, the commissioning, QA, and patient treatment may involve various geometry and tissue types, such as PMMA, water, air cavity, bolus; thus, a general-purpose Monte Carlo (MC)-based independent dose calculator is needed for routine clinical use of GammaPod.

Methods: Due to symmetry of GammaPod’s crossfire radiation and as an independent calculator, we used the fluence map instead of the phase space to model the initial photons with a uniform ellipse convolved by a Gaussian-shaped penumbra kernel for each of the two cones on GammaPod. The ellipse size and penumbra kernel were fitted using the scanned dose profiles measured by the in-house built scanner and water cup phantom. The commissioned dose engine was then verified by point dose measurements for 56 different plans in 26 water cups. The calculation engine is implemented as a background service and automatically generates a second dose calculation report after each GammaPod plan export.

Results: The second dose calculation took less than 1 minute with 1-billion particles when running on a Titan-X GPU workstation. The commissioned effective fluence has 19.5mm and 30mm ellipse sizes with the same penumbra (sigma=1.5mm) and 24mm and 37.5mm dosimetric cone sizes (FWHM) for the 15mm and 25mm cones, respectively. The second dose had <0.3mm and <2% difference from measured profiles and point dose for commissioning and plan verifications, respectively, and had a 3D gamma pass rate >90%(2%/1mm) against the TPS dose for breast.

Conclusion: We developed and validated a general-purpose MC dose engine for GammaPod. With proper commissioning and data-flow management, it has been integrated into the clinical workflow as a patient-specific QA tool for GammaPod.


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