Comparison of dosimetry from bowel bag and individual bowel loop contouring for patients receiving pelvic Stereotactic Ablative Radiotherapy (SABR) radiotherapy


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Rhiannon Howells1,Eleanor Clarke1,Matthew Beasley2,Louise Murray3
1University of Leeds,2Leeds Cancer Centre,3Leeds Cancer Centre, University of Leeds

Abstract

Background

Current practice for patients receiving Stereotactic Ablative Radiotherapy (SABR) for pelvic oligometastatic recurrence is to delineate individual bowel loops (BL) on the planning CT; a highly time-consuming process. Alternatively, a bowel bag (BB) structure outlines the peritoneal cavity, allowing estimation of bowel dose during radiotherapy.

This study investigated whether contouring only restricted BL in close tumour proximity, with the BB contoured to cover the remaining volume, would provide a more efficient method of contouring without sacrificing valuable information.

Method

Planning CTs with contoured BL for twenty patients who received SABR for pelvic oligometastatic nodal recurrence were included. The BB was delineated based on the Radiotherapy Oncology Group atlas. The planning target volume (PTV) was expanded 3 cm laterally and 2 cm superio-inferiorly (PTV3+2), to include BL closest to the tumour. Maximum doses to BL (0.5 cm3 (Dmax0.5cm3) and 5 cm3 (Dmax5cm3)) inside and beyond the PTV3+2 were compared. Differences between doses to the BB and BL beyond the PTV3+2 were evaluated.

Results

Dmax0.5cm3 and Dmax5cm3 for BL within the PTV3+2 were consistently and significantly higher than doses to BL beyond the PTV3+2 (both p<0.001).

The median difference in Dmax0.5cm3 of BL outside the PTV3+2 (BL_outside_PTV3+2) and BB outside the PTV3+2 (BB_outside_PTV3+2) was 0.4 Gy (range: -0.2 to +7.0). In 17/20 patients, dose to BB_outside_PTV3+2 was higher.

Similarly, median difference in Dmax5cm3 of BL_outside_PTV3+2 and BB_outside_PTV3+2 structure was 1.0 Gy (range: -0.1 to +7.1), with dose to BB_outside_PTV3+2 being higher in all patients.

Conclusion

Contouring individual BL in close proximity to the target provides the relevant structures to limit high dose to individual BL during planning based on Dmax constraints. Contouring the BB only beyond the PTV3+2 does not result in any meaningful under-estimation of dose to BL within this region. This process offers substantial time-saving implications for clinicians.