Neoplasm of anal canal are <5% of the malignancy of gastrointestinal tract. Majority of the cancers are squamous cell and adenocarcinoma. Primary small cell carcinoma of the anal canal is a very rare diagnosis with only handful of cases reported in literature. 66-year-old female with multiple co-morbidities presented with rectal pain. She had initially noticed hemorrhoids six months prior to presentation. CT abdomen and pelvis showed bilateral inguinal lymphadenopathy and diverticulosis. Patient then had colonoscopy which showed mass in the anal canal, initial biopsy showed squamous cell carcinoma with basaloid features. Lymph node biopsy was also positive for metastatic cancer likely anal in origin. PET scan showed metastatic lesion in the lumbar vertebra. Review of slides was done before starting chemotherapy as component of small cell carcinoma was also seen. Due to diagnostic dilemma, patient was initially started on Carboplatin and Paclitaxel first cycle as it is effective against both squamous and small cell carcinoma. Repeat evaluation of the slides confirmed small cell cancer as the diagnosis and chemotherapy was changed to carboplatin and etoposide. Patient received 3 cycles of the carboplatin and etoposide after which atezolizumab was added. She received total of 6 cycles of the chemotherapy. Repeat PET scan showed partial favorable response. Due to rarity of the disease, there was significant delay in diagnosis of the cancer. Diagnosis of small cell carcinoma in the anal canal relies on neuroendocrine differentiation. There is no formal evidence-based recommendation regarding treatment. They are considered equivalent to pulmonary small cell carcinoma and treated in similar manner.
Niroula S, Karki UJ, Ghimire B, Thapa S. Small cell carcinoma of the anal canal: a rare entity. Poster presented at: The American College of Physicians Michigan Chapter Residents Day / Medical Students Day; 2022 May 13; Dearborn, MI.
American College of Physicians Michigan Chapter Residents Day / Medical Students Day, May 13, 2022, Dearborn, MI.