Malignant Peripheral Nerve Sheath Tumor of the Rectum Characterized by Focal S100 Protein Expression.

This case describes a malignant peripheral nerve sheath tumor (MPNST) arising in the ischiorectal fossa, an anatomical location that is infrequently reported in the scientific literature. The report provides new anatomical and clinical insights into a rare and aggressive neoplasm that is frequently misdiagnosed due to nonspecific symptoms and overlapping histopathological features with other rectal or perianal tumors. A 52-year-old woman with no significant medical or oncologic history presented with mild rectal bleeding and a painful perianal mass, initially presumed to be thrombosed hemorrhoids. Physical examination revealed a tender, erythematous anal region with prolapsed tissue and persistent bleeding. She underwent a Ferguson hemorrhoidectomy and examination under anesthesia as the initial surgical approach. Intraoperatively, a friable exophytic lesion was identified 4 cm from the anal verge, occupying approximately 70% of the rectal lumen. Biopsies initially suggested a well-differentiated squamous carcinoma with sarcomatoid features. Further imaging and histopathological evaluation led to an abdominoperineal resection with hysterectomy due to vaginal invasion. Final pathology revealed a high-grade spindle cell neoplasm with lymphovascular and perineural invasion. Immunohistochemistry showed focal S100 positivity (in an irregular pattern) and negativity for AE1/AE3, HMB45, smooth muscle actin (SMA), and CD117. Although SOX10 immunostaining and molecular testing were not performed due to institutional limitations and lack of access to advanced diagnostic resources, the diagnosis of MPNST was supported by compatible histological features and a broad immunohistochemical panel, including markers such as BCL2, CD34, CDX2, P63, CK7, and CK20, and a pan melanoma panel. This combination of findings effectively ruled out major differential diagnoses such as sarcomatoid carcinoma, melanoma, and gastrointestinal stromal tumors (GIST), supporting the final diagnosis of MPNST. The patient recovered uneventfully from surgery. This case illustrates the diagnostic complexity of MPNST in atypical anorectal locations and emphasizes the need for thorough histopathological and immunohistochemical assessment. Early recognition and complete surgical excision are crucial for improving prognosis in such rare presentations. The limitations related to unavailable molecular testing were acknowledged in the discussion section.