Histologic study showed mature fat cells with areas of ossification (Fig 4). There was no evidence of mitotic activity.
Lipomas, benign tumors composed of mature adipose tissue, are classified as superficial or deep, based on their location. Superficial lipomas are seen most commonly in the neck, shoulder, and abdominal region. Deep lipomas are rare and can be subclassified as intramuscular, intermuscular, parosteal, or intraosseous. There are several variants of lipomas, including angiolipoma, myolipoma, spindle cell lipoma, osteolipoma, and chondroid lipoma. 9
Ossification within a benign lipoma, a very rare variant of lipoma, was reported first in the late 1950s. 7 Commonly, ossifying lipomas are situated adjacent to the periosteum and are referred to as parosteal lipomas. 1 Fleming et al 1 recommended using this term rather than periosteal lipoma because the periosteum does not contain fatty tissue. Their report, the largest series in the literature, discussed 32 parosteal lipomas that were collected from the literature. The average age of the patients in the series was the fifth decade, and both genders were represented equally. The most common presenting sign was a palpable mass with occasional complaint of local discomfort. Parosteal lipomas occasionally may become so large that they interfere with joint function or press against vital structures. The tumor abuts the periosteal surface of the bone and, as it grows, has the potential to erode the underlying bone. One half (16 of 32) of the reported cases by Fleming et al 1 presented with bony erosion and/or angular deformity of the underlying bone.
In the current case, the lesion was detached bluntly from the underlying bone with no bony attachments. The final diagnosis was obvious only after the entire specimen was resected and examined under the microscope to ensure that the entire specimen appeared benign. Microscopically, the tumor was composed of fatty tissue with occasional areas of matrix ossification. Neither the fatty nor the bony component revealed any evidence of atypia that might have resembled a malignant process.
Plain radiographs revealed a partially ossified mass projecting over the superomedial scapula, but it is difficult to assess the relationship of the mass to the underlying scapula on plain films.
Computed tomography scanning is of substantial value in assessing the nature of such a lesion and in estimating the lesion’s size and the erosive changes that have taken place in the scapula. One clue from the CT scan is the low attenuation of the mass, a finding typical of fatty tissue and similar to subcutaneous fat. The mass, located along the dorsal surface of the scapula, caused bony erosion and displacement of the adjacent deltoid muscle. The images also highlight the bone production within and surrounding the mass.
Although fatty lesions can be determined with reasonable accuracy using MRI, MRI was not obtained for the assessment of this benign lesion. On fat-saturated T1-weighted images, for example, most of the lesion would have been saturated out (dropped signal), giving a result similar to that of subcutaneous fat.
Diagnosis of this lesion based on the history and radiographic findings was challenging. One of the differential diagnoses for the current case was calcified synovial cell sarcoma, which primarily presents in the periarticular regions of the extremities. The reported median age for patients with this type of tumor is 35 years (range, 16–70 years). 3 Synovial sarcoma is seen most commonly (in order of frequency) in the lower extremities, the upper extremities, and the head and neck region. Less than ½ of the patients present with pain and discomfort. In 15% to 20% of the cases, there is calcification and, less frequently, ossification. 4 The diagnosis becomes clear after biopsy.
Myositis ossificans is a benign ossifying process that is found most commonly in the musculature of the extremities. The lesion may be traumatic or nontraumatic. 10 Radiographically, it presents as a well-circumscribed mass that is calcified in the periphery and noncalcified in the central portion. Some areas of the mass may be highly cellular, and it often is confused with extraskeletal osteosarcoma. 10
The term tumoral calcinosis was introduced first in the 1940s. 2 Most commonly seen in patients in the first two decades of life, 6 it usually occurs in the hip and shoulder and is characterized by a periarticular deposit of calcium. Affected patients have no abnormalities in calcium metabolism and commonly present with an asymptomatic, deep, large, and firm mass. Two thirds of the cases involve African-American patients. 6 Occasionally, patients have ulceration develop over the mass, with drainage of yellow, chalky fluid. Radiographically, the lesion presents as a conglomerate of multiple, round opacities separated by radiolucent lines. 8 There are no associated bony abnormalities. This process is benign, and it should be observed clinically unless the mass interferes with joint function.
Osteochondroma, a benign bone tumor, is non-neoplastic in nature. The etiology appears to be secondary to aberrant epiphyseal formation with displacement of physeal cartilage from the perichondral fibrous ring. 5 The displaced cartilage continues to grow at a right angle to the long axis of the bone. Solitary osteochondroma is the most common benign bone tumor, accounting for 35% of the cases. One of the diagnostic findings is the bony connection of osteochondroma to the underlying bone.
An ossifying soft tissue mass in an adult should prompt the clinician to include several benign and malignant lesions in the differential diagnosis. One such benign lesion is ossifying lipoma. This tumor may resemble a malignant lesion because of the erosive changes in the scapula. The best treatment for patients with this tumor is marginal resection without adjuvant therapy.
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2. Inclan A: Tumoral calcinosis. JAMA 121:490–495, 1943.
3. Lewis JJ, Antonescu CR, Leung DH, et al: Synovial sarcoma: A multivariate analysis of prognostic factors in 112 patients with primary localized tumors of the extremity. J Clin Oncol 18:2087–2094, 2000.
4. Maxwell JR, Yao L, Eckardt JJ, Doberneck SA: Case report 878: Densely calcifying synovial sarcoma of the hip metastatic to the lungs. Skeletal Radiol 23:673–675, 1994.
5. Milgram JW: The origins of osteochondromas and enchondromas: A histopathologic study. Clin Orthop 174:264–284, 1983.
6. Pakasa NM, Kalengayi RM: Tumoral calcinosis: A clinicopathological study of 111 cases with emphasis on the earliest changes. Histopathology 31:18–24, 1997.
7. Plaut GS, Salm R, Truscott DE: Three cases of ossifying lipoma. J Pathol Bacteriol (Lond) 78:292–295, 1959.
8. Steinbach LS, Johnston JO, Tepper EF, Honda GD, Martel W: Tumoral calcinosis: Radiologic-pathologic correlation. Skeletal Radiol 24:573–578, 1995.
9. Weiss SW, Goldblum JR: Benign Lipomatous Tumors. In Weiss SW, Goldblum JR (eds). Enzinger and Weiss’s Soft Tissue Tumors. Ed 4. St Louis, Mosby Inc 571–639, 2001.
© 2003 Lippincott Williams & Wilkins, Inc.
10. Weiss SW, Goldblum JR: Osseous Soft Tissue Tumors. In Weiss SW, Goldblum JR (eds). Enzinger and Weiss’s Soft Tissue Tumors. Ed 4. St Louis, Mosby Inc 1389–1417, 2001.