SU‐E‐T‐292: CBCT Imaging and the Assessment of PTV Margin Size for Rectal Cancer Patients Treated Prone on Belly Board

Abstract

Purpose: The goal of this work was to use CBCT images taken at the time of treatment to derive PTV margin sizes that would account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board (CDR Systems Inc.). Methods: Twenty‐four patients (12 male and 12 female) were included in this study. CBCT data was acquired once every 5 fractions for a total of 5 images per patient. A 3D‐3D bony anatomy auto‐match was performed offline and the residual difference used as a surrogate for inter‐fractional positional errors of the CTV. Systematic and random variations in CTV position were evaluated in a manner consistent with that of Stroom et al and used in PTVmargin = 1.96Σ + 0.7σ. The influence of hypothetical intra‐fractional motion was included in the margin evaluation by introducing the following values: 1.0, 2.0 and 3.0mm.Results: PTVmargin required to account for inter‐fraction positional errors was found to be (AP, SI, LR) = (5.2 mm, 3.1 mm, 2.8 mm). If we assume any intra‐fractional motion to be similar to that presented by Xu et al, then the required PTVmargin increases to (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm). A 7.0 mm AP expansion is consistent with that quoted in the “Elective Clinical Target Volumes in Anorectal Cancer: An RTOG Consensus Contouring Atlas“, which recommends a margin between 7.0 and 10.0 mm. However, 7.0 mm is 2.0 mm greater than the 5.0 mm margin specified in the RTOG 0822 trial. Conclusions: A PTVmargin expansion of (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm) will account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board.