Sclerosing rhabdomyosarcoma presenting in the masseter muscle: a case report

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1615846455818924 webciteRhabdomyosarcoma (RMS) is relatively uncommon in all soft tissue tumors, and also rare in adults older than 45？years [1]. However, it is the most common soft tissue sarcoma in children. In the current WHO soft tissue tumor classification, RMS is divided into three groups: embryonal (ERMS), alveolar (ARMS) and pleomorphic (PRMS) [1]. The sclerosing rhabdomyosarcoma (SRMS) is an exceedingly rare variant first described by Mentzel and Katenkamp [2], which is still controversial. So far, there are less than 40 reported cases (the present case is not included) in English literatures [3-17]. Because of its rarity, it is still unclear whether SRMS belongs to an unusual subtype of ARMS or ERMS, or even a new variant of RMS [6,8,13,17].Microscopically, SRMS has a characteristic constellation of features. The neoplastic cells can be arranged into lobules, small nests, microalveoli and even single-file arrays in an abundant hyalinized, eosinophilic to basophilic matrix that closely resembles primitive osteoid or chondroid material [18]. So, it is easily misdiagnosed, if one unfamiliar with the histological spectrum of this entity. Herein, we report a case of sclerosing rhabdomyosarcoma arising in masseter muscle in a 40-year-old male. In addition to the above histological features, the cribriform pattern resembling adenoid cystic carcinoma could also be seen in focal areas. This may represent a new histological feature, which can cause a diagnostic confusion.A 40-year-old male presented with a painless swelling in the right parotid gland region. The patient reported the swelling was approximately 1？cm in size 3？months ago, and increased rapidly in size recently. Physical examination revealed a 5-cm subcutaneous mass which felt firm and adhered to the adjacent tissue. Ultrasonography revealed a low echo mass measuring 57.1×26.3？mm in the subcutaneous ti