Anal extraskeletal osteosarcoma in a man: a case report and review of the literature

BACKGROUND: Osteosarcoma is a common malignancy of bone that usually occurs in individuals in the age range of 0–24 years. Extraskeletal osteosarcoma is a rare tumor presentation which originates in non-bony tissues. Extraskeletal osteosarcoma comprises 2–5% of all osteosarcomas and less than 1% of all soft tissue sarcomas. As compared to bone-derived osteosarcoma, extraskeletal osteosarcoma occurs in older age groups. Extraskeletal osteosarcoma has a poorer prognosis than bone osteosarcoma. To the best of our knowledge, this is the first case of extraskeletal osteosarcoma in the anal region. CASE PRESENTATION: A 70-year-old Hazara man presented to a private hospital with the chief complaints of constipation, bloody defecation, and pain during defecation of 1.5 months’ duration. His past history was unremarkable. A digital rectal examination showed a solid growth in the middle part of his anus. A colonoscopic examination was done and showed a solid mass in his anal region. A computed tomography scan revealed an irregular mural thickening in the anal canal with heterogeneous enhancement. The maximum length of the involved segment was measured to be 4.5 cm. No suspicious lesions were noted in other organs. An abdominoperineal resection was performed on our patient. A 22 cm in length resected segment of his colon, consisting of the lower sigmoid, rectum, and anus was sent to us for histopathological examination. Gross examination revealed a polypoid dark-gray tumor measuring 5 × 3 × 2 cm. The cut section revealed gray and white appearance with firm-to-hard consistency and foci of ossification. Microscopic examination revealed normal anorectal mucosa and a spindle cell malignant neoplasm with osteoid formations. No evidence of epithelial carcinoma was noted. Immunohistochemical stains were positive for stabilin-2 and negative for cytokeratin, which confirmed the diagnosis of osteosarcoma. CONCLUSION: Extraskeletal osteosarcoma of the colon is rare and presence of the tumor in the rectum and anal region is extremely rare. Radiology, colonoscopy, and histopathology with immunostaining are required for the diagnosis. The accurate diagnosis of extraskeletal osteosarcoma is important as it has a different regimen of treatment with poorer prognosis compared to primary osteosarcoma of the bone.