Five patients with a palpable mass at presentation underwent magnetic resonance (MR) imaging. The final diagnosis was myositis ossificans (MO). MR imaging features, particularly after injection of gadopentetate dimeglumine, mimicked those of an inflammatory mass or neoplasm. The lesions were excised in three patients, and the images were correlated with histologic findings. Three different appearances were noted on MR images, corresponding to the stages of maturation of MO. Two cases involved early-stage lesions, and T1-weighted MR images showed a mass with homogeneous intermediate signal intensity. Both lesions showed rim enhancement after contrast agent injection and high signal intensity on T2-weighted images. Pathologic specimens demonstrated stroma with masses of spindle cells in which osteoid production was interspersed. The enhanced rim of the lesion mimicked the expected MR appearance of an abscess or necrotic tumor. Areas of enhancement in adjacent muscle were also seen on postcontrast T1-weighted images. Intermediate-stage MO was present in one case; there was evidence of a thin rim of calcification on plain radiographs and fatty changes in the lesion on T1-weighted images, corresponding with histologic findings. One case of a mature lesion showed a considerable degree of peripheral calcification both on MR images and at histology. MR imaging is nonspecific in the diagnosis of early-stage MO.

Five cases of granular cell myoblastoma have been studied for detection of the neuroectodermal protein S-100. Immunoperoxidase staining on paraffin sections, using an antibody raised against calf brain S-100 protein, was utilized to demonstrate positive cytoplasmic and nuclear reactivity in all cases. Negative staining in adjacent muscle and connective tissue elements was contrasted to in situ control staining of Schwann cells in peripheral nerves and staining of Langerhans cells and melanocytes in overlying stratified epithelia. These observations are interpreted as support for possible Schwann cell origin of granular cell myoblastomas.

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We report two transitional cell carcinomas of the urinary bladder containing numerous osteoclast-type giant cells that stained for vimentin and acid phosphatase (with and without tartrate) and were negative for cytokeratin and lysozyme. One tumour, in a 65-year-old man, was composed of papillary transitional cell carcinoma, invasive poorly differentiated carcinoma with a prominent spindle cell component and numerous osteoclast-type giant cells; repeat curettage 2 months later showed no residual tumour. The second tumour occurred in a 75-year-old woman who underwent a radical cystectomy for a deeply invasive transitional cell carcinoma with a spindle and anaplastic giant cell component and areas containing numerous osteoclast-type giant cells. Osteoclast-type giant cells, which appear to be reactive, should be distinguished from the neoplastic giant cells of giant cell carcinoma.

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