Radiation generates an abscopal response and complete resolution of metastatic squamous cell carcinoma of the anal canal: a case report.

The abscopal effect is a rare phenomenon in the setting of radiation therapy (RT) for metastatic cancer where tumor regression occurs distant from the site of treatment. A proposed mechanism of the abscopal effect is the activation of a systemic antitumor immune response by localized RT. We report the first case, to our knowledge, of the abscopal effect in squamous carcinoma of the anal canal with metastases to pelvic lymph nodes, liver and bone. After palliative RT to the pelvis with sensitizing chemotherapy, complete response was observed not only in the primary tumor but also in bone and multiple liver metastases at 4 months after treatment, an effect that remained durable at 4-year follow-up. Understanding of the abscopal effect and the immune mechanisms associated with anal cancer may lead to new avenues of research to improve outcome for patients with this rare disease.