Is There an Advantage in Designing Adapted, Patient-Specific PTV Margins in Intensity Modulated Proton Beam Therapy for Prostate Cancer?

Abstract

To investigate robust margin strategies in intensity modulated proton therapy to account for interfractional organ motion in prostate cancer. 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. 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. 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.