Range optimization for target and organs at risk in dynamic adaptive passive scattering proton beam therapy - A proof of concept.

PURPOSE: The purpose of this study was to design and develop a new range optimization for target and organs at risk (OARs) in dynamic adaptive proton beam therapy (PBT).

METHODS: The new range optimization for target and OARs (RO-TO) was optimized to maintain target dose coverage but not to increase the dose exposure of OARs, while the other procedure, range optimization for target (RO-T), only focused on target dose coverage. A retrospective analysis of a patient who received PBT for abdominal lymph node metastases was performed to show the effectiveness of our new approach. The original plan (OP), which had a total dose of 60 Gy (relative biological effectiveness; RBE), was generated using six treatment fields. Bone-based registration (BR) and tumor-based registration (TR) were performed on each pretreatment daily CT image dataset acquired once every four fractions, to align the isocenter.

RESULTS: Both range adaptive approaches achieved better coverage (D95% ) and homogeneity (D5% -D95% ) than BR and TR only. However, RO-T showed the greatest increases in D2cc and Dmean values of the small intestine and stomach and exceeded the limitations of dose exposure for those OARs. RO-TO showed comparable or superior dose sparing compared with the OP for all OARs.

CONCLUSIONS: Our results suggest that BR and TR alone may reduce target dose coverage, and that RO-T may increase the dose exposure to the OARs. RO-TO may achieve the planned dose delivery to the target and OARs more efficiently than the OP. The technique requires testing on a large clinical dataset.