SMARCB1(INI1)-deficient undifferentiated carcinoma with rhabdoid features of anorectal origin

Background: SMARCB1 (INI1) mutations are often associated with primary soft tissue tumors, though are starting to be recognized as driving mutations in carcinomas of the gastrointestinal tract, often associated with undifferentiated and/or rhabdoid features and extremely poor prognosis. We detail the first case of a SMARCB1-deficient undifferentiated carcinoma with rhabdoid features arising in the anorectal canal and document response to palliative chemoradiation therapy. Case report: A 34-yr old pregnant female presented with a large anorectal canal mass and concurrent pelvic and inguinal lymph node involvement. Biopsy revealed tumor cells with high-grade rhabdoid morphology, immunophenotype supporting a carcinoma of gastrointestinal origin, and loss of SMARCB1 (INI-1) protein expression. 5-Fluorouracil (5FU) and cisplatin chemotherapy with concurrent radiation therapy was initiated with evident radiologic primary tumor regression and palliation of pain. Despite initial therapeutic response, disease rapidly progressed resulting in death 127 days after diagnosis. Conclusion: We report the first case of a SMARCB1 (INI-1)-deficient undifferentiated carcinoma with rhabdoid features of anorectal area. Given the currently limited therapeutic guidance on these types of tumors this case highlights the potential benefit of 5-FU and platinum based chemoradiation therapy, similar to the treatment of anal squamous carcinomas, which appeared to provide effective loco-regional control and palliative benefits.