PurposeTo compare toxicity of radiotherapy (RT) with concomitant chemotherapy (CHT) in patients (pts) with anal cancer treated with simultaneous integrated boost (SIB) versus sequential boost (SeqB).MethodsSixty-six patients were treated from 2007 to 2021. Thirty patients underwent to SeqB concurrent to CHT and 37 to SIB-group. Toxicity assessment has been considered in acute and in late toxicities for gastrointestinal (GI), genitourinary (GU), cutaneous (CU) districts, according to Common Terminology Criteria for Adverse Events (CTCAE), Version 5.0. The Wexner scale among summary scoring systems has been used as a tool to measure fecal incontinence. The chi-square test for ordinal variables were used to evaluate the association between patient and treatment characteristics and acute or late severe toxicity. Univariable logistic regression models were fit to evaluate predictive factors associated with fecal incontinence.ResultsMedian follow-up was 61.5 months (IQR, 27.1–121.7 months) for all patients. Severe acute toxicity (≥ G2) was observed in 49 patients (74.2%). Late toxicity (≥ G2) occurred in 13 cases (19.6%). In assessment of cutaneous toxicity, there was also a significant reduction in ≥ G1 in SIB group with 29 patients (80.5%) vs SeqB group with 29 patients (96.6%) (p-value = 0.046). Of both groups 11 patients (16.6%) developed fecal incontinence, 8 (22%) in the SIB group and 3 (10%) in the SeqB.ConclusionSIB for anal cancer treatment results in reduced acute and late cutaneous toxicity compared to SeqB. According to our results the rate of other acute and late toxicities are low and comparable between the two groups.