Primary leiomyosarcoma of the descending colon initially non-diagnostic on biopsy: a case report

Primary leiomyosarcoma (LMS) of the colon is an exceptionally rare malignancy, accounting for less than 0.1% of colorectal cancers. Because of its rarity and nonspecific presentation, preoperative diagnosis is challenging, and optimal management strategies remain undefined.

A 47-year-old woman with no prior medical history underwent screening colonoscopy, which revealed a 2.5-cm ulcerative mass in the proximal descending colon. Biopsies performed twice, including a follow-up examination one month later, showed only chronic inflammation. Computed tomography demonstrated eccentric wall thickening with minimal pericolic infiltration, raising suspicion for malignancy. The patient underwent laparoscopic radical en bloc resection of the splenic flexure and descending colon with D2 lymphadenectomy, followed by stapled colo-colic anastomosis. Histopathology revealed a 4.2 × 3.5 cm spindle-cell tumor with high cellularity, moderate pleomorphism, > 100 mitoses/50 HPF, and invasion extending to the subserosal layer. Immunohistochemistry showed positivity for smooth muscle actin and desmin, and negativity for CD34, CD117, DOG-1, and S-100. The Ki-67 index was 70–80%. Resection margins and all 13 lymph nodes were negative. The postoperative course was uneventful, and the patient remains disease-free at 11 months.

Colonic LMS is a rare and aggressive tumor that often evades preoperative diagnosis, especially when biopsies are inconclusive. Surgical resection with negative margins is the mainstay of treatment, and close surveillance is essential due to the high risk of recurrence.