BACKGROUND: Radiotherapy currently plays a key role in pelvic malignancies’ management. Excellent outcomes have been reported on its association with chemotherapy for the treatment of the anal carcinoma. Despite that, the combined use of chemo- and radiotherapy and the high doses administered seem to be strongly associated with early and late onset side effects. METHODS: We reported a case of a 72 years old woman, affected by anal squamous cell carcinoma. She underwent chemotherapy, and then radiotherapy, with good results. RESULTS: During a regular MR control, the patient developed anaphylactic reaction to Gadolinium, and after that a rectosigmoid ischemia with total necrosis of the posterior rectal wall was diagnosed and surgically treated with Hartmann procedure. CONCLUSION: In our case we faced with the rapid and severe degeneration of pelvic anatomy determined by the sum of vascular alterations following hypovolemic shock and pelvic tissues alteration after radiotherapy. It seems essential not to underestimate the exponential outcome of a similar unusual combination of events.