Painless perianal mucinous adenocarcinoma: A case report and literature review.

Painless perianal mucinous adenocarcinoma (PMA) is characterized as a rare malignant tumor of the anal canal and rectum; its incidence rate is extremely low, and it has atypical clinical manifestations. In the present case report, a 79-year old male patient is described who presented with a painless perianal mass that had been present for 15 days. Perianal B-ultrasound examinations revealed a well-circumscribed hypoechoic mass with a maximum diameter of 3.31 cm. A pathological examination of the resected tumor specimen revealed that the tumor exhibited an expansive nodular growth pattern, accompanied by irregular infiltration of glandular structures and extensive secretion of extracellular mucus. The columnar and goblet cells presented with low-grade intraepithelial neoplasia. Immunohistochemistry examinations showed positive staining for cytokeratin (CK) 7, CK20, caudal type homeobox 2 and mucin 2. The molecular test results suggested a KRAS p. G12d activating mutation. The pathological diagnosis was PMA. The patient underwent radical local wide resection 1 month later, and no recurrence or metastasis was observed during a follow-up period of 12 months. The present case report provides a real-world reference for the clinical diagnosis, selection of treatment plans and evaluation of the prognosis of painless PMA.