Impact of MRI-based planning in transperineal interstitial brachytherapy for locally advanced cervix tumors: Clinical, dosimetric and toxicity outcomes.

Purpose: To compare dosimetric characteristics, clinical outcomes, and toxicity in patients with locally advanced cervical cancer(LACC) treated with transperineal interstitial brachytherapy(P-ISBT), using CT-based planning or MRI-based planning with a monoinstitutional MRI-compatible applicator, Template Benidorm(TB).

Materials and methods: We retrospectively analyzed 106 women treated with P-ISBT for LACC from 2006 to 2022. P-ISBT was CT-based, with Martinez Universal Perineal Interstitial Template(MUPIT) between 2006 and 2012, or MRI-based planning (TB) since 2013. Volumetric, clinical, and dosimetric parameters were compared between applicators.

Results: Forty-six (43.4%) patients were treated with MUPIT, and 60 (56.6%) with TB. Median follow-up was 42 months (4-188). No significant difference was observed in 5-year local control (LC), regional control (RC), distant failure (DF), overall survival (OS), and disease-specific survival (DSS) between applicators. The TB cohort had fewer needles (p = 0.001), halved median CTV volume (p < 0.001), increased median D_90%CTV dose (p < 0.005), and lower D_2cc values in rectum/bladder (p < 0.001). G3 chronic toxicity was 15% in the MUPIT group and 6% in TB (p = 0.15), with no G4 toxicity reported. Rectal hemorrhage was lower in the TB group (p = 0.002). Larger CTV volumes correlated with higher rectal D2cc and rectal hemorrhage (p = 0.001; p = 0.029, respectively), and enterovaginal fistula (p = 0.03; p < 0.001, respectively). Median CTV volume was 152.82 cc (35.3-256.78) in patients with chronic rectal toxicity and 102.9 cc (21.4-329.41) in those without (p = 0.001).

Conclusion: MRI's superior CTV-volume definition results in smaller treatment volumes, lower D_2cc for rectum and bladder, and a trend towards higher CTV D_90%. Rectal hemorrhage was significantly lower in the MRI-based group. A significant correlation was observed between larger CTV-volumes, higher rectal D_2cc and rectal hemorrhage/enterovaginal fistula.