High-Tailored Anal canal Radiotherapy (HIT-ART): Long term results of MR image-guided interventional radiotherapy (brachytherapy) boost.

Purpose: The aim of this study is to investigate the outcomes in terms of survival, treatment compliance and toxicity in a cohort of patients treated with personalized chemoradiation (RCT) followed by image guided Interventional Radiotherapy (IG-IRT), stratified by clinical baseline stage and treatment response.

Methods And Materials: We analyzed patients with histologically proven squamous anal carcinoma treated using intensity modulated radiation therapy (IMRT) with curative intent, and IRT boost. External beam radiation was delivered using personalized IMRT technique with a simultaneous-integrated boost (SIB) to deliver 45 to 55 Gy in 25 fractions according to clinical stage. After the end of the RCT patients underwent clinical and imaging re-evaluation, and according to initial stage of disease and tumor response, a radiotherapy boost was administered through IG-IRT, performing MRI with IRT applicator on site (trans-anal position) and defining GTV on MRI imaging. All patients' treatments and outcomes data were collected by an internal database. The endpoints were in terms of overall treatment time (OTT), toxicity, clinical complete response (cCR), locoregional failure (LRF), colostomy free survival, overall survival (OS) and disease-free survival (DFS). Univariate survival analysis with Kaplan- Meier curves was performed to allow between-group comparison.

Results: 73 patients treated between February 2012 and January 2023 were included in the analysis. 72.6 % were female, with a median age of 65.4 years (range 39.1-89). 62 were staged T2-T4 (T2 42.5.5%, T3 19.2%, T4 27.4%) and 50 (68.5%) had positive nodes. Combined RCT was administered mainly using mitomycin C and 5-fluorouracil. The mean dose received by patients was 50 Gy; after a median time of 43 days (range 9-128) a sequential boost dose was delivered in 1-2 fractions (median dose of 4 Gy). OTT median was 85 days (range 44-225). The median follow-up period was 50.6 months. LRF was 17.8%, and at 3 and 5 years LC was 80.1% and 77.2%, CFS was 80.5 and 77.8%, DFS was 79.3% and 75.9% and OS 95.1%, 84.3% and 61.4% at 10 years. Moreover, we verified that OS was higher in patients who had an OTT lower than 93 days. Toxicity was acceptable: 45.2% of patients experienced a G1-G2 abdominal discomfort (only 5.5% had G3).

Conclusions: Our experience confirms that MR Image Guided Interventional Radiotherapy boost is a feasible treatment option with a significant impact on outcomes.