Dome-type carcinoma of the rectum mimicking a submucosal tumor: a case report and literature review

Abstract

Dome-type carcinoma (DC) has been recognized as a rare variant of adenocarcinoma, which arises in gut-associated lymphoid tissue. It has a specific morphologic feature of a dome-like protrusion associated with lymphoid tissue. We report a case of a DC of the rectum in an asymptomatic 58-year-old male. A 2-cm sized, well-demarcated, round mass masquerading as a submucosal tumor (SMT) was identified in the rectum and was resected by endoscopic submucosal dissection. The tumor was revealed as an adenocarcinoma with submucosal invasion of 3,700 µm, which consisted of dilated cystic glands and the lymphoid stroma with reactive germinal centers. On immunohistochemistry, the tumor cells revealed retained expression for mismatch repair proteins. Laparoscopic surgical resection was subsequently performed. DC is considered a distinctive subtype of colorectal adenocarcinoma with characteristic morphology and low-grade malignant potential. Careful detection of the overlying mucosal lesion is crucial to differentially diagnose DC from SMT.

Keywords: Adenocarcinoma; Carcinoma; Colorectal neoplasms; Lymphoid tissue; Morphology.

Fig. 1

(A) Colonoscopy identifies a mucosa-covered, protruding, round mass with shallow erosion, originating from the rectum. (B) Endoscopic ultrasound shows a well-demarcated, heterogeneous, and hyperechoic submucosal mass, emanating from the mucosal layer of the rectum.

Fig. 2

(A) The microscopic pathological examination reveals a well-delimited expansile tumor, measuring 1.6 × 1.6 × 0.4 cm, invading the submucosa (H&E stain, ×12.5). (B) The neoplastic glands are composed of pseudostratified cuboidal to columnar and mucin-depleted (absence of goblet cells) epithelial cells with eosinophilic cytoplasm and vesicular nuclei. Focal tumor-infiltrating lymphocytes and intraluminal pink material from the neoplastic glands are also noted (H&E stain, ×200). (C, D) The pink material shows intensive periodic acid–Schiff-diastase-positive (C, ×40) and Alcian blue-positive staining (D, ×40). (E, F) The tumor-infiltrating lymphocytes consist of mixed CD3- (E, ×200) and CD20-positive cells (F, ×200).