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Dosimetric Impact of Beam Size On the Plan Robustness for IMPT in Locally Advanced Squamous Cell Carcinoma of the Head and Neck

H Baba1,2*, K Hotta1 , H Tachibana1 , R Takahashi1 , K Motegi1 , T Akimoto1 , (1)National Cancer Center Hospital East, Kashiwa, (2) University of Tsukuba, Tsukuba,


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: To evaluate dosimetric impact of beam size for intensity modulated proton therapy (IMPT) with/without robust optimization in locally advanced squamous cell carcinoma of the head and neck (SCCHN).

Methods: Four SCCHN patients with neck lymph node metastasis received primary PTV-based intensity modulated photon therapy (IMRT) were selected. For each case, IMPT plans were generated using six different beam size data and two different optimization methods of PTV based non-robust optimization (NRO) and CTV based robust optimization (RO). IMPT planning goal were set for dose coverage of CTV while the organs-at-risks (OAR) were spared as much as possible. A multi-case dose-volume-histograms (DVH) assuming ±5mm setup errors and ±3% range errors were used for plan comparisons of IMRT and IMPT with the various beam sizes using DVH indices including CTV coverage, homogeneity and OAR sparing.

Results: In nominal-case (0mm/0%) DVH evaluations, IMPT shows similar target coverage and better OAR sparing than IMRT regardless of beam size. OAR sparing of IMPT was worse depending on the larger beam size, and NRO-IMPT reduced the OAR doses compared to RO-IMPT. In multi-case DVH evaluations, dose fluctuations of CTV and OAR were larger NRO-IMPT compared to IMRT and RO-IMPT. In NRO-IMPT, CTV (D99%) was decreased by 17 to 23% at maximum. On the other hand, in RO-IMPT, the OAR doses of the parotid gland, oral cavity and larynx were higher than IMRT depending on the larger beam size.

Conclusion: For IMPT plan, PTV margin may not guarantee the target coverage during actual treatment. RO-IMPT was robust against daily treatment fluctuation regardless of beam size, and OAR doses may be reduced compared to IMRT using small beam size.

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


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