SMARCB1 (INI1)-Deficient Undifferentiated Rectal Carcinoma Mimicking Sarcoma: A Case Report and Literature Review.

Undifferentiated carcinomas with rhabdoid morphology and loss of SMARCB1 (INI1) are rare, highly aggressive neoplasms. In the colorectum, they account for < 1% of cases and carry a poor prognosis. We report a 41-year-old man with congenital hemophilia A who presented with pelvic fullness and altered bowel habits. Imaging revealed a large rectosigmoid mass without distant metastasis. Core needle biopsy suggested a high-grade sarcomatoid malignancy. Following multidisciplinary planning and perioperative factor VIII replacement, owing to progressive symptoms and concern for impending obstruction, the patient underwent low anterior resection. Histopathology showed rhabdoid morphology with complete loss of nuclear INI1 expression, confirming SMARCB1-deficient undifferentiated colorectal carcinoma. The tumor was staged as pT4aN0, with vascular invasion. The patient received adjuvant chemoradiation and remained alive and disease-free at the time of publication. This case highlights an important diagnostic pitfall, as SMARCB1-deficient colorectal carcinoma may closely mimic sarcoma on limited biopsy specimens. Recognition of rhabdoid morphology and incorporation of INI1 immunohistochemistry into the diagnostic workup are essential for establishing the correct diagnosis and guiding multidisciplinary treatment.