Quantification of vaginal motion associated with daily endorectal balloon placement during whole pelvis radiotherapy for gynecologic cancers

Abstract Background and purpose To quantify intra-treatment vaginal motion in women treated with daily endorectal balloon (ERB) placement and external beam radiotherapy for gynecologic cancers. Materials and methods Eighteen post-hysterectomy women with gynecologic cancers underwent computed tomography (CT) simulation scans and daily treatment with ERB. Fiducial markers were placed at the vaginal apex prior to simulation and patients were counseled on a pre-treatment bladder filling protocol. Weekly to biweekly verification CT scans were used to calculate the intra-treatment change in bladder volumes, rectal volumes, and fiducial coordinates along all axes. The planning target volume (PTV) margins required to encompass 95% of intra-treatment fiducial movement were calculated using the van Herk margin recipe. Results The median bladder volume was 223 (range, 29–879) cc for verification CT scans. Mean intra-treatment fiducial displacements were 1.7 (range, 0–9.1) mm, 2.9 (range, 0–15.5) mm, and 2.5 (range, 0–11.8) mm along the left–right (L/R), superior–inferior (S/I), and anterior–posterior (A/P) axes, respectively. The van Herk PTV margins were 3 mm (L/R), 10 mm (S/I) and 7 mm (A/P). Conclusion When compared to existing studies, the use of daily ERB with post-hysterectomy radiotherapy reduces vaginal motion along the A/P axis. The impact of variable bladder filling on vaginal motion is most evident along the S/I axis.