Program Information
Evaluation of Surface Dose for Various Treatment Sites Using Mevion S250 Passively Scattered Proton Therapy System
H Jin*, S Ahmad , Oklahoma Univ. Health Science Ctr., Oklahoma City, OK
Presentations
SU-I-GPD-T-128 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To evaluate the surface dose for different treatment sites using Mevion S250 passively scattered proton therapy system.
Methods: The surface dose was measured at 1 mm depth by delivering a factional dose using a PTW Advanced Markus chamber in a water tank for 14 beams from four different treatment sites (1 brain: 3 Small beams (target prescription=54.0 Gy(RBE)), 1 CSI: 5 Large beams (30.0 Gy(RBE)), 2 lungs: 2 Large (61.2 Gy(RBE)) and 2 Small (66.0 Gy(RBE)) beams, and 1 prostate: 2 Deep beams (45.0 Gy(RBE)). Verification plans for the treatment plans were made in a virtual water phantom using Varian Eclipse treatment planning system (TPS) and the calculated surface doses were compared with measurements. To minimize the gradient effect of range compensator (RC), the comparison was performed without RC. However, the surface doses were estimated in the actual treatment plans with RC.
Results: Mean surface-to-mid-SOBP dose ratios of all beams for each plan (measured/calculated) were 78.0/74.8% (brain), 93.4/92.4% (CSI whole brain), 88.6/87.8% (CSI spine), 89.4/86.6% (lung-Large), 81.0/79.7% (lung-Small), and 89.8/77.3% (prostate). The surface doses calculated by TPS agreed well with measurements (<~3% difference) except for the Deep prostate beams (~13% difference) with relatively small field (<~5 cm diameter). The surface dose to prescription for each beam in the actual plans ranged from 25.8% (brain vertex) to 117.5% (CSI lower spine). The surface dose was generally proportional to ratio of modulation to range (target length).
Conclusion: The surface dose using the passively scattered proton therapy system tends to be higher than comparable photon therapy due to absence of distinct dose build-up and less number of beams. Eclipse TPS predicted the surface dose well; however the measured surface dose is higher than calculation for Deep beams with small field (note that the brain target is smaller than the prostate target).
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