Adenocarcinoma Developing Overlying a Colonic Lipoma: A Case Report

Colonic lipomas are benign mesenchymal tumors composed of mature adipocytes and are most commonly located in the submucosal layer. They are usually asymptomatic and are often managed conservatively. Although adenomas arising overlying colonic lipomas have been reported, the coexistence of adenocarcinoma directly overlying a lipoma is extremely rare. The pathogenic relationship between submucosal tumors and epithelial malignancies remains unclear.

A 76-year-old man was diagnosed with a lipoma in the descending colon (diameter, approximately 30 mm) and was managed with surveillance. After 26 months of follow-up, colonoscopy revealed a newly developed small type 2 ulcerative lesion located directly over the lipoma. A biopsy confirmed adenocarcinoma. Endoscopic ultrasonography demonstrated a homogeneous hyperechoic lesion in the 3rd layer consistent with a lipoma, along with focal thickening of the 2nd layer, without evidence of deep invasion. CT and MRI showed imaging features characteristic of a lipoma, with a peripheral component suspicious for malignancy. The patient was diagnosed with descending colon cancer (cT2N0M0, Stage I) and underwent laparoscopic left hemicolectomy with lymph node dissection. A histopathological examination revealed a lipoma extending from the submucosa into the muscularis propria, while the carcinoma was confined to the mucosa and muscularis propria overlying the lipoma, without invasion into the lipomatous tissue. No histological findings indicative of chronic inflammation were observed.

We report a rare case of adenocarcinoma developing directly over a colonic lipoma during surveillance. Although chronic inflammation has been proposed as a possible mechanism linking submucosal tumors to epithelial malignancies, no such features were identified in this case, and a causal relationship could not be established. While colonic lipomas are generally managed conservatively, careful endoscopic surveillance may be considered in selected patients with relatively large lipomas requiring long-term follow-up.