Sclerosing rhabdomyosarcoma presenting in the masseter muscle: a case report

Abstract

Sclerosing rhabdomyosarcoma (SRMS) is exceedingly rare, and may cause a great diagnostic confusion. Histologically, it is characterized by abundant extracellular hyalinized matrix mimicking primitive chondroid or osteoid tissue. So, it may be easily misdiagnosed as chondrosarcoma, osteosarcoma, angiosarcoma and so on. Herein, we report a case of SRMS occurring in the masseter muscle in a 40-year-old male. The tumor showed a diverse histological pattern. The tumor cells were arranged into nests, cords, pseudovascular, adenoid, microalveoli and even single-file arrays. Immunostaining showed that the tumor was positive for the Vimentin, Desmin and MyoD1, and was negative for CK, P63, NSE, CD45, CD30, S-100, CD99, Myoglobin, CD68, CD34, CD31, and α-SMA. Based on the morphological finding and immunostaining, it was diagnosed as a SRMS. In addition, focally, our case also displayed a cribriform pattern resembling adenoid cystic carcinoma. This may represent a new histological feature which can broaden the histological spectrum of this tumor and also may lead to diagnostic confusion.

Virtual slides: The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1615846455818924.

Figure 1

Magnetic resonance imaging revealing the tumor. A and B, Magnetic resonance imaging revealed an oval mass about 5.2× 3.1 cm in the right lower masseter. The tumor was relatively well circumscribed.

Figure 2

Morphological changes of the tumor. A, The tumor infiltrated the normal skeletal muscle tissues (red arrow). B, Focally, the tumor cells showed a pattern closely resembling adenoid cystic carcinoma. C, The cells were arranged into cords or single-file arrays paralleled with each other. D, The cells surrounded the thick-walled blood vessel, and showed a characteristic constellation of features. E, Primitive chondroid matrix and cells with intracytoplasmic vacuole could be seen in focal areas. F, The tumor cells showed adenoid or microalveoli pattern in abundant basophilic matrix, and an atypical mitosis could be seen (green arrow). G, Occasionally, hyalinized, eosinophilic keloid-like collagen (black arrow) could be seen. H, Pseudovascular lumens were filled with numerous red cells. I, In extensive myxoid stroma, scarcely scattered cells could be seen.

Figure 3

Immunohistochemical staining for CK, α–SMA, Myoglobin, MyoD1, Desmin and Ki-67 in the tumor. A, The tumor cells were negative for CK. B, α–SMA was expressed in the wall of blood vessel, but not expressed in tumor cells. C, Myoglobin immunostaining highlighted the entrapped striated muscle cells, but was not positive in tumor cells. D, The tumor cells showed a diffuse positive immunostaining for MyoD1. E, The majority of the cells were positive for Desmin. F, Ki-67 index was approximate 60%.