A Rare Case of Primary Rectal Squamous Cell Carcinoma and the Use of Cytokeratin Markers.

Lower gastrointestinal cancers are commonly adenocarcinoma of the colon and rectum and squamous cell carcinoma (SCC) of the anus. Rectal squamous cell carcinoma (SCC) is a rare gastrointestinal tract malignancy, as rectal SCC is assumed to be from the migration of anal squamous cells. However, primary rectal SCC is rarer. Here, we present a case of a 63-year-old male who was found to have rectal SCC that was very close to the anus. Through literature review, it was noted that SCC and adenocarcinoma of rectal origin stain positive for cytokeratin CAM 5.2 and not the anal canal lesions. This patient's tumor was positive for CAM 5.2. The patient was treated with 5-fluorouracil and mitomycin C with radiation therapy for five weeks. The post-therapy repeat PET scan showed complete resolution of the tumor and oligometastasis. Unfortunately, the 20-week follow-up PET CT showed para-aortic and retrocrural lymph nodes consistent with malignancy. This case emphasizes the use of immunohistochemical stains for diagnosis and treatment planning in patients with rectal SCC. Once the diagnosis was confirmed, the patient was treated as anal SCC. The importance of differentiating between rectal and anal SCC can be argued, although the treatment is the same; however, the prognosis is worse based on nodal involvement in rectal SCC. Patients with early intervention have a five-year overall disease-free survival of greater than 80%.