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Dosimetric Study for Shallow-Seated Tumor Using Passive/active Scanning Proton Beam

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C Toramatsu

C Toramatsu*, T Matsuura, H Nihongi, S Takao, N Miyamoto, S Shimizu, R Kinoshita, K Umegaki, H Shirato,

SU-E-T-506 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose:
To investigate the possibility of using a single spot scanning proton beam to treat superficial lesions

Methods:
A cylindrical phantom with a simulated superficial target (it seated 0.5-4cm depth from the surface, volume: 270cm³) was created in Eclipse treatment planning system. Three proton plans were generated: (a) a single AP uniform scanning beam with aperture and range compensator; (b) a single AP spot scanning beam with a pre-absorber. The location and thickness of the pre-absorber were calculated using Geant4 to Monte Carlo code to make use of the available spot scanning beams to get a conformal plan. (c) a five-beam spot scanning beam plan using multi-field optimization. The prescription is 54 cobalt grey equivalent (CGE) which covers 95% of the target. The target coverage, lateral penumbra at 2 and 4cm depth in water, the doses to normal tissue (phantom-target) and skin (2mm from the surface) were evaluated and compared for three plans.

Results:
The mean doses to the target are comparable within 2.4% for all three plans. The conformity indices (at 95%) are 1.36, 1.04 and 0.98 for plan (a), (b) and (c) respectively. The lateral penumbra (80% to 20%) for plan (a), (b) are both 0.73 cm, while it is 3.75 cm for plan (c). The skin dose which received more than 40 (CGE) from plan (a) is 10% higher than that of other two plans. Plan (c) has 70% higher mean doses to normal tissue than that of plan (a) and (b).

Conclusions:
Each plan provides good coverage of target. And in this study, it showed that, with a properly designed pre-absorber, it is possible to use a single spot scanning beam to treat superficial lesion. The plan provides good target coverage and maintains normal tissue sparing in the mean time.

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