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Proliferative myositis with bone/osteoid formation

There are several reports in the literature pointing out to the risk of misinterpreting pseudomalignant osseous tumours as osteogenic sarcoma because of the high cellularity and mitotic activity of tumour cells [1-5]. However, in most of such cases the proper differentiation between sarcoma and pseudomali-gnant proliferation is possible. Despite a charac-teristic clinical picture of pseudomalignant changes, there are several morphological features that enable differentiation between sarcoma and the pseudo-malignant tumour.
Clinically, pseudomalignant proliferations are characterized by a very rapid growth, counted in days or weeks, in contrast to a much longer growth typical of sarcomas [5-8].
Morphologically, the lack of high nuclear and/or nucleolar atypia and atypical mitoses, as well as the demonstration of zonal maturing of the bone phenomenon, evident in cases of myositis ossificans [6, 7] and checkerboard pattern typical of proliferative myositis [6, 8, 9] are the most useful features in the diagnosis of pseudomalignant changes.
The checkerboard pattern of the growth is
a striking feature of the histological texture of the presented case. It is characterized by alternating areas of proliferating fibroblasts or myofibroblasts and remnants of infiltrated muscle tissue (Fig. 1-4). However, the skeletal muscle fibres are relatively unaffected except for the presence of secondary atrophy, with neither sarcolemmal proliferation nor any evidence of skeletal muscle regeneration [6, 8, 9] (Fig. 1 and 3). The other conspicuous histological sign of proliferative myositis is the presence of large basophilic ganglion-like cells (Fig. 4 arrow) that usually have single eccentrically situated nuclei. The unusual feature of the described tumour is the presence of foci of osteoid formation (Fig. 3 asterisk). However, a few cases of proliferative myositis with bone and/or osteoid formation were reported [1, 6]. Such cases should be differentiated mainly from myositis ossificans. The key features of differential diagnosis between these two entities, as well as extraskeletal osteogenic sarcoma are depicted in the table I.

References
1. Dahl I, Angervall L. Pseudosarcomatous lesions of the soft tissue reported as sarcoma during a 6-year period (1958-1963). Acta Pathol Microbiol Scand Sect A 1977; 85: 917-930.
2. Horie Y, Morimura T. Fibro-osseous pseudotumor of the digits arising in the subungal...


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