A 72-year-old male presented with a history of weakness and constipation, for which he underwent colonoscopy, revealing a submucosal mass in the right colon, compressing on the lumen extrinsically. The mucosa overlying the mass was intact and colonoscopy showed an adjacent 12 mm polyp, which turned out to be a tubular adenoma. He had history of removal of previous tubular adenomas on prior colonoscopies. Computed tomography (CT) imaging showed a 7.0 cm lobulated mass arising from mid-ascending colon with exophytic growth and foci of calcifications. CT findings were consistent with a mesenchymal tumor and carcinoma was less likely. No fatty component was reported (Figure 1). Subsequently, the patient underwent laparoscopic right hemicolectomy and tumor was resected.
Gross examination of the right hemicolectomy specimen demonstrated a firm and calcified intramural mass in the posterior wall of ascending colon, extending into the mesocolic fat, without involvement of the mucosa. Microscopically, distinct histological patterns were identified, with the majority of the tumor being composed of a central ossified core surrounded by a minor component of a high-grade mesenchymal nonlipogenic tumor and adipose tissue. The central bone forming component showed areas of osteoid matrix with highly atypical cells resembling a high-grade osteosarcoma as well as areas of more mature appearing woven bone trabeculae with intervening mildly atypical spindle cells similar to a low-grade osteosarcoma (LGOS). A thin rim of a high-grade sarcoma composed of atypical spindled and pleomorphic cells was identified in the periphery of the bony mass. In addition, small foci of mature-appearing adipose tissue containing rare atypical hyperchromatic stromal cells were detected adjacent to the high-grade sarcoma and at the retroperitoneal resection margin, raising consideration of a WDL (Figures 2 and 3). The findings suggested that WDL may have arisen in the retroperitoneum and secondarily involved the colon with a DDL component with heterologous osseous differentiation.
On immunohistochemical (IHC) studies, SATB2 was diffusely positive in the atypical cells between bony trabeculae and in high-grade sarcomatous areas without evident bone formation. High and low molecular weight keratin (pancytokeratin), CD117, DOG-1, desmin, SMA, and S-100 stains were negative. Fluorescence in situ hybridization (FISH) revealed high-level amplification of MDM2-gene, supporting the diagnosis of retroperitoneal WDL with DDL showing heterologous osteosarcomatous differentiation and forming a mass involving the ascending colon.