Impact of MRI-based planning in transperineal interstitial brachytherapy for vaginal-involving tumors

Purpose: The purpose of the study was to analyze patients with vaginal-involving recurrences of gynecological tumors and primary vaginal tumors, treated with transperineal interstitial brachytherapy (P-ISBT). Dosimetric, clinical, and toxicity analysis of these patients was conducted, incorporating MRI in volume definition and dose-volume dosimetry.

Material and methods: Forty-two patients were retrospectively analyzed. They were treated with radical or adjuvant intent (in hysterectomized patients with high-risk of relapse). P-ISBT was performed with MUPIT (CT-based planning) in 18 patients (42.9%) and with Template Benidorm (TB) (MRI-based planning) in 24 patients (57.1%).

Results: Median age was 59 years (range, 39-78 years). Median clinical target volume (CTV) was 81.4 cc (range, 33.8-286.2 cc) in MUPIT, and 47.5 cc (range, 10-156.4 cc) in TB-treated patients (p = 0.01). Median EQD₂ for EBRT and brachytherapy D₉₀ CTV was 69.2 Gy (range, 27.9-88.8 Gy) in MUPIT, and 77.2 Gy (range, 31-84.3 Gy) in TB. Median rectal D_2cc was 69.2 Gy (range, 23.5-82.6 Gy) in MUPIT, and 66.3 Gy (range, 16.4-75 Gy) in TB. Median bladder D_2cc was 71.5 Gy (range, 23.6-90.8 Gy) in MUPIT, and 66.9 Gy (range, 18.2-78.3 Gy) in TB. Median follow-up was 36.5 months (range, 4-188 months). Local control was 95% at 3 and 5 years. Overall survival was 77% at 3 years, and 66% at 5 years. Disease-specific survival was 81% at 3 years, and 75% at 5 years. Chronic grade 1-2 proctitis presented in 10 patients (24%; 8 treated with MUPIT and 2 treated with TB; p = 0.01). Grade 3 toxicity was documented in 4 patients (9.6%).

Conclusions: MRI-based planning is superior to CT-based planning in P-ISBT. It allows for better definition of CTV, resulting in smaller and more selective treatment volumes. Our results show a tendency towards higher D₉₀ CTV dose and lower rectal/bladder D_2cc dose, leading to less events of late rectal toxicity.