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Impact of Beam Size On the Plan Robustness for Robust Intensity Modulated Proton Therapy in Head and Neck Cancer

H baba1,2,3*, K Hotta1,2 , H Tachibana1,2 , K Motegi1,2 , Y Sugama4 , T Akimoto1,2 , (1) The National Cancer Center, Research Center for Innovative Oncology (2)The National Cancer Center, Hospital East (3) University of Tsukuba (4) Aizawa hospital


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To determine optimal beam size for intensity modulated proton therapy (IMPT) with/without robust optimization for head and neck (H&N) cancer.

Methods: A retrospective analysis of H&N cancer patient including regional lymph nodes disease involvement who received PTV-based intensity modulated photon therapy (IMRT). In this study, IMPT plans were generated with the two approaches of PTV based non-robust optimization (NRO) and CTV based robust optimization (RO) for the patient. In the IMPT planning, six beam sizes (air-sigma 2.5-8 mm, 3.5-10 mm, 4-12 mm, 4.5-14 mm, 5-16 mm and 6-18 mm) in line scanning were used. PTV was generated by 5 mm expansion of the CTV. Robust optimization was performed using the parameters of 5 mm setup and 3% range uncertainties. IMPT planning goal were set for dose coverage of CTV while the organs-at-risks (OARs) were spared as much as possible. A multi-case dose-volume-histograms (DVH) assuming ± 5mm setup errors (± 3% range errors for only IMPT) were used for plan comparisons of IMRT and IMPT with the various beam sizes using DVH indices including CTV coverage, homogeneity and OARs sparing.

Results: In the multi-case DVH evaluations, RO-IMPT shows similar target coverage and better OARs dose sparing than IMRT and NRO-IMPT. RO-IMPT and IMRT plans were more robust than NRO-IMPT plan. RO-IMPT did not show dose fluctuations regardless of the beam sizes, however more dose fluctuations were appeared as the beam size of NRO-IMPT decreased.

Conclusion: Even in treatment conditions with some uncertainties, RO-IMPT with any beam size may provide better OARs sparing than IMRT while keeping target coverage.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by research research grants from the Japan Agency for Medical Research and development (AMED) .


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