Is there an advantage in designing adapted, patient-specific PTV margins in intensity modulated proton beam therapy for prostate cancer?

Purpose: To investigate robust margin strategies in intensity modulated proton therapy to account for interfractional organ motion in prostate cancer. Methods and Materials: For 9 patients, one planning computed tomography (CT) scan and daily and weekly cone beam CTs (CBCTs) were acquired and coregistered. The following planning target volume (PTV) approaches were investigated: a clinical target volume (CTV) delineated on the planning CT (CTV_ct) plus 10-mm margin (PTV_10mm); a reduced PTV (PTV_Red): CTV_ct plus 5 mm in the left-right (LR) and anterior-posterior (AP) directions and 8 mm in the inferior-superior (IS) directions; and a PTV _Hull method: the sum of CTV_ct and CTVs from 5 CBCTs from the first week plus 3 mm in the LR and IS directions and 5 mm in the AP direction. For each approach, separate plans were calculated using a spot-scanning technique with 2 lateral fields. Results: Each approach achieved excellent target coverage. Differences were observed in volume receiving 98% of the prescribed dose (V_98%) where PTV_Hull and PTV _Red results were superior to the PTV_10mm concept. The PTV_Hull approach was more robust to organ motion. The V_98% for CTVs was 99.7%, whereas for PTV_Red and PTV_10mm plans, V_98% was 98% and 96.1%, respectively. Doses to organs at risk were higher for PTV_Hull and PTV_10mm plans than for PTV _Red, but only differences between PTV_10mm and PTV _Red were significant. Conclusions: In terms of organ sparing, the PTV_10mm method was inferior but not significantly different from the PTV_Red and PTV_Hull approaches. PTV_Hull was most insensitive to target motion.