A feasibility study of sexual organ-dose sparing volumetric modulated arc therapy in female low rectal cancer patients treated with three-dimensional conformal radiation therapy.

BACKGROUND: Sexual dysfunction is a significant toxicity of pelvic radiation therapy (RT) for female rectal cancer patients and may be more common with intensity-modulated techniques compared to three-dimensional conformal radiation therapy (3D-CRT). However, limited research has evaluated dose sparing of female sexual organs at risk (OARs), particularly erectile tissues that are critical for sexual function. PURPOSE: This feasibility study aimed to demonstrate the practicality of contouring and sparing female sexual OARs in rectal cancer RT by generating sexual organ-sparing volumetric modulated arc therapy (SO-VMAT) plans from a small 3D-CRT cohort and comparing dose distributions to guide future planning. METHODS: Nine female patients with low rectal adenocarcinoma treated with concurrent chemoradiation using 3D-CRT were retrospectively analyzed. Sexual OARs (bulboclitoris, external genitalia, and vagina) were contoured and incorporated into replanned SO-VMAT plans. Institutional dose-volume constraints were applied for standard OARs and planning target volumes (PTVs). SO-VMAT and 3D-CRT plans were normalized to equal PTV coverage, and dose-volume metrics were compared using Wilcoxon signed-rank tests. RESULTS: Contouring and sparing of female sexual OARs was feasible in all cases. Compared to 3D-CRT, SO-VMAT reduced radiation doses to sexual OARs. For the bulboclitoris, SO-VMAT lowered V3000cGy and V4000cGy (both p = 0.02). For the external genitalia, SO-VMAT reduced maximum and mean dose (both p = 0.02) as well as V1000cGy (p = 0.004) and V2000cGy (p = 0.01). Vaginal sparing was observed at V4000cGy (p = 0.03). SO-VMAT provided significant bladder sparing, whereas bowel and femoral head doses remained comparable. Target coverage remained equivalent between techniques. CONCLUSION: This study demonstrates that female sexual OARs can be systematically contoured and effectively spared during rectal cancer RT using SO-VMAT. Compared with 3D-CRT, SO-VMAT achieved meaningful dose reductions while maintaining target coverage. These preliminary findings support multi-institutional studies with patient-reported outcomes to validate clinical relevance and guide integration of female sexual OARs into treatment planning.