Calcified Colonic Mass

A 90-year-old woman underwent evaluation for rectal bleeding. A rectal mass was detected 10 cm from the anal verge. A rectal endoscopic biopsy showed a well-differentiated adenocarcinoma. Within some of the biopsy fragments, well-formed trabecular bone was seen. The bone was microscopically normal, without atypia or architectural derangement, suggestive of sarcoma (Figure, A).Preoperative abdominal computed tomography (Figure, B) showed a large rectal mass with diffuse rectal wall thickening, infiltration through the bowel wall, and stippling suggestive of microcalcifications. There was an enlarged perirectal lymph node. No liver lesions were seen. The patient received a course of radiotherapy to the pelvis.A lower anterior resection procedure was performed. The resection specimen contained an 8.0 × 6.5 × 3.0-cm, yellow-tan, lobulated lesion involving the full thickness of the rectal wall and extending into the subserosal fat. One regional lymph node contained metastatic tumor.Microscopic examination showed a well-differentiated colonic adenocarcinoma with intense stromal desmoplasia. Well-formed trabecular bone was present (Figure, C and D).What is your diagnosis?AbstractOsseous metaplasia is an uncommon finding in benign and malignant gastrointestinal tumors. It also has been described in neoplasms outside the gastrointestinal tract. The tumors are usually mucinous adenocarcinomas. Pathogenesis is thought to involve bone morphogenic proteins. Osseous metaplasia has no prognostic significance, but an awareness of the phenomenon is necessary to prevent the overdiagnosis of bone invasion by carcinoma or misdiagnosis of metaplastic carcinoma.Heterotopic bone formation, also known as osseous metaplasia, in colonic carcinomas is a rare phenomenon, despite the high incidence of colonic adenocarcinoma. Osseous metaplasia has been described in other adenocarcinomas, including those of the lung, breast, thyroid, parotid, and pancreas.1 In 1923, Hasegawa2 first described 2 cases of rectal adenocarcinoma with bone formation. In 1939, Dukes3 was the first investigator to describe ossification in rectal carcinoma in the English literature.Ossification can occur throughout the gastrointestinal tract and has been described in benign colonic polyps, gastric carcinoid, adenocarcinoma, and mucocele of the appendix.4–6 Ossification is more likely to be found in gastrointestinal tumors of the lower gastrointestinal tract, and most affected tumors have been diagnosed as well-differentiated or moderately differentiated lesions.7 This phenomenon is rare. The overall incidence is approximately 0.4%, as suggested by Dukes.3 The course and prognosis of adenocarcinoma with osseous metaplasia does not differ from that of colonic adenocarcinoma without osseous metaplasia, although it appears to be more common in slow-growing neoplasms in younger individuals.8Ossification refers to the formation of mature (benign) bone elements in the stroma of the neoplasms and is a complex process requiring the presence of osteoblasts.78 Grossly, the tumors can ulcerate, fungate, or infiltrate, and usually have a mucinous cut surface with areas of calcification and metaplastic bone formation.5 By microscopy, they are usually mucinous adenocarcinomas, with the heterotopic bone consisting of osteoblasts surrounding irregularly deposited osteoid.The widely accepted hypothesis for the cell of origin for osseous metaplasia in colonic adenocarcinoma was proposed by Rhone and Horowitz.9 They postulated that ossification might result from metaplasia of pluripotent mesenchymal cells into osteoblasts. The bone morphogenetic proteins (BMPs) are a family of bioactive proteins, of which 20 different types have been cloned.7 Imai et al7 studied the immunohistochemical expression of BMPs in colonic adenocarcinoma and found that BMP-5 and BMP-6 were prominent in the cytoplasm of tumor cells, but were weakly expressed in the osteoblast-like cells adjacent to the nearby bone. BMP-2 and BMP-4 were strongly expressed in the surrounding mesenchymal cells and weakly expressed in the tumor cells and osteoblast-like cells. This pattern suggests that the tumor cells mainly produce BMP-5 and BMP-6, which may induce proliferation of surrounding mesenchymal cells into preosteoblasts and osteoblasts expressing BMP-2 and BMP-4.7 BMP-2 and BMP-4 are potent inducers of osteoblastic differentiation when compared to other BMPs.10The presence of osseous tissue in colonic biopsy material might be confused with colonic adenocarcinoma invading into bone or with a carcinosarcoma (metaplastic carcinoma). In the present case, the resection specimen definitely showed islands of metaplastic bone superficial to the muscularis propria, thus excluding sacral invasion.In summary, osseous metaplasia may occur in both benign lesions and adenocarcinomas of the gastrointestinal tract. Although the pathogenesis of this rare phenomenon remains incompletely understood, current studies suggest a relationship with overexpression of BMPs by tumor cells, inducing the differentiation of surrounding pluripotent mesenchymal cells into osteoblasts. Osseous metaplasia is of no prognostic significance, but an awareness of the phenomenon is important in order to prevent the overdiagnosis of bone invasion by carcinoma or misdiagnosis of metaplastic carcinoma.

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