Aim was to compare the efficacy of interventional radiotherapy (IRT) boost vs. external beam radiotherapy (EBRT) boost after chemoradiation (CCRT) in patients with anal cancer (AC). The P.I.C.O. framework was: in patients with AC [P], is IRT boost [I] superior to EBRT boost [C] in terms of local control (LC), cancer specific survival (CSS), overall survival (OS), distant meta-static free Survival (DMFS), colostomy free survival (CFS) and toxicity [O]? 651 patients were analyzed. The median 5-year locoregional control rates was 87.8% in the IRT boost group versus 72.8% in the EBRT boost group. The 5-year cancer-specific survival rate was 91% in the IRT boost group versus 78% in the EBRT boost group. 5-years overall survival was 74.6% in IRT boost versus 67.7% in the EBRT boost. 5-years disease metastasis-free survival rate was 92.9% in IRT boost group vs. 85.6% for the EBRT boost group. Cancer-free survival rate was 76.8% in the IRT group vs. 63.1% in the EBRT boost group. Acute toxicity above grade 2 was less common in the IRT boost group while chronic toxicity was similar between both groups. IRT boost after CCRT could lead to better outcomes than EBRT boost in treating AC.