[A Resected Case of Leiomyosarcoma of the Transverse Mesocolon with Difficult Preoperative Diagnosis].

The patient was a woman in her 70s who presented to our hospital for further examination and treatment after a palpable mass was noted in the upper right area of the umbilicus. Contrast-enhanced CT revealed a 52 mm heterogeneously enhancing mass located ventral to the horizontal part of the duodenum. MRI showed a mass with heterogeneous signal intensity in the right upper abdomen. Due to the discrepancy in tumor location between CT and MRI, a non-epithelial tumor originating from the bowel or mesentery was suspected;however, the exact site of origin could not be determined. Given the presence of well-developed feeding vessels, a biopsy was avoided, and surgical resection was performed for diagnostic and therapeutic purposes. Intraoperatively, the tumor was found within the transverse mesocolon and showed portal vein invasion. Therefore, resection of the transverse mesocolon tumor, partial colectomy of the transverse colon, and combined resection of the portal vein were performed. Gross examination of the resected specimen revealed a well-demarcated, white-colored tumor measuring 60 mm in maximum diameter within the transverse mesocolon. Immunohistochemical analysis showed positivity for SMA and desmin, and negativity for c-kit, CD34, and S-100. The Ki-67 labeling index was approximately 30%. Based on these findings, the tumor was diagnosed as a primary leiomyosarcoma of the transverse mesocolon. The patient has been followed postoperatively without adjuvant therapy, and no signs of recurrence have been observed during the 10 months since surgery.