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Comprehensive Comparison of Compensator Border-Smoothing Margin in Proton Therapy of Wobbling Scattering System

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T Tsai

T Tsai*, T Wu , C Yeh , C Lee , S Lee , Chang Gung Memorial Hospital, Guishan Dist, Taoyuan City


SU-I-GPD-T-166 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: This study is focused on effect of compensator border-smoothing (BS) margins in proton therapy of wobbling scattering beam in Sumitomo equipment at Chang-Gang Memorial Hospital. This parameter can remove sharp gradient and allow for adequate penetration even in the case of misalignment or patient motion. We also investigate the impact of different BS margin between MLC and block system.

Methods: In Eclipse planning system v13.0, we change proton energy 110-230MeV and field size 7-17cm following different wobbling radius to observe the change of penumbra and field size at middle of SOBP for different compensator BS margins 0-2cm. Besides, remove block and set MLC to same snout position, then repeat BS margins study. After planning comparison, film dosimetry is used to evaluate in different BS margins.

Results: When the compensator BS margins increase from 0 to 2cm, the penumbra is also increasing but not changed until 7mm in same beam condition. When BS=0mm, the penumbras are 6.1mm, 7.4mm, 9.7mm, and 12.5mm respectively for 110MeV, 150MeV, 190MeV, and 230MeV with block and MLC. We observe the penumbras increase as energy and wobbling radius increasing, are 7.1mm, 8.6mm, 10.5mm, and 13.4mm respectively for 110MeV, 150MeV, 190MeV, and 230MeV with block of BS=7mm. And the penumbras are 6.8mm, 8.4mm, 10.4mm, and 13.4mm respectively for 110MeV, 150MeV, 190MeV, and 230MeV with MLC of BS=7mm. We found some trend of penumbras depend on energy and field size. This study can afford clinical judgment for compensator BS setting.

Conclusion: When proton energy less than 150 MeV, the compensator border-smoothing effect of using block is more obvious than using MLC. Appropriate BS margins not only provide important clinical reference but also improve planning efficiency.

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