• Yan Xia, Ye Li, Peng Gong, Huifeng Jiang, and Xianbin Zhang.
• Department of Pathology.
• Medicine (Baltimore). 2021 Jun 18; 100 (24): e26105.

RationaleSkeletal muscle tumors are traditionally classified as rhabdomyomas or rhabdomyosarcomas. However, some soft tissue tumors cannot easily be identified as benign or malignant. We report a case of a histiocyte-rich rhabdomyoblastic tumor, with pathologic characteristics distinct from either rhabdomyoma or rhabdomyosarcoma. In contrast to rhabdomyosarcomas, the tumor cells exhibited low mitotic activity, lacking obvious morphologic atypia. Clinically, the tumor followed a very indolent course. Overall, the tumor did not fit classification criteria for either benign or malignant.Patient ConcernsA 58-year-old Chinese man was admitted to Qilu Hospital on September 8, 2018, with a >20 year history of a mass in the middle of the left thigh. A few months prior to admission, he had experienced the pain from the mass extending to the distal left lower extremity. He had no prior history of significant disease or relevant family history.DiagnosesMicroscopically, numerous histiocytes and foamy cells covered the actual tumor cells that were positive for desmin, MyoD1, and myogenin, suggesting striated skeletal muscle cell differentiation. However, cross-striations were not detected in the tumor cells. The tumor was characterized by a non-infiltrative growth pattern and a low level of Ki67. A diagnosis of histiocyte-rich rhabdomyoblastic tumor was suggested.InterventionsThe thigh mass was surgically resected September 12, 2018.OutcomesThe patient recovered well postoperatively, and was free of tumor recurrence or metastasis, followed to September 12, 2020 (23 months).LessonsHistiocyte-rich rhabdomyoblastic tumor cells have minor atypia, indicating possible malignant potential. However, the tumor behavior was quit indolent. Due to the conflicting clinical and pathologic aspects of the tumor, to label it as rhabdomyosarcoma seemed inaccurate, potentially prompting over treatment. Interestingly, mutations were detected in NF1, AXIN2, CHEK2, DNMT3A, KMT2D, and RB1 through next-generation sequencing. These mutations suggest disruptions in Ras signaling, the Wnt pathway, methyltransferases, and the cell cyclepotentially influencing the development of this histiocyte-rich rhabdomyoblastic tumor. This unusual tumor should be incorporated into the WHO Classification of Soft Tissue Tumors owing to its unique characteristics.Copyright © 2021 the Author(s). Published by Wolters Kluwer Health, Inc.

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