SU-E-J-49: Distal Edge Activity Fall Off Of Proton Therapy Beams.

PURPOSE To characterize and quantify the distal edge activity fall off, created in a phantom by a proton therapy beam METHOD AND MATERIALS: A 30×30×10cm polymethylmethacrylate phantom was irradiated with a proton therapy beam using different ranges and beams. The irradiation volume is approximated by a right circular cylinder of diameter 7.6cm and varying lengths. After irradiation, the phantom was scanned via a Philips Gemini Big Bore™ PET-CT for isotope activation. Varian Eclipse™ treatment planning system as well as ImageJ™ were used to analyze the resulting PET and CT scans. The region of activity within the phantom was longitudinally measured as a function of PET slice number. Dose estimations were made via Monte Carlo (GATE) simulation. RESULTS For both the spread out Bragg peak (SOBP) and the mono-energetic pristine Bragg peak proton beams, the proximal activation rise was steep: average slope -0.735 (average intensity/slice number) ± 0.091 (standard deviation) for the pristine beams and -1.149 ± 0.117 for the SOBP beams. In contrast, the distal fall offs were dissimilar. The distal fall off in activity for the pristine beams was fit well by a linear curve: R2 (Pierson Product) was 0.9968, 0.9955 and 0.9909 for the 13.5, 17.0 and 21.0cm range beams respectively. The good fit allows for a slope comparison between the different ranges. The slope varied as a function of range from 1.021 for the 13.5cm beam to 0.8407 (average intensity/slice number) for the 21.0cm beam. This dependence can be characterized: -0.0234(average intensity/slice number/cm range). For the SOBP beams, the slopes were significantly less and were also less linear: average slope 0.2628 ± 0.0474, average R2 =0.9236. CONCLUSION The distal activation fall off edge for pristine proton beams was linear and steep. The corresponding quantities for SOBP beams were shallower and less linear. Philips has provided support for this work.