Inflammatory leiomyosarcoma (LMS) is a newly included rare tumor entity in the group of smooth muscle tumors in the recent WHO classification. Recent studies have shown skeletal muscle expression within this tumor and its proximity with histiocyte-rich rhabdomyoblastic tumor (HRRT).

A 17-year-old male presented with a soft tissue lump over the back of his neck of one-year duration. Radiologically, a lesion measuring 5.9 cm in the largest dimension was seen, extending from the skull base up to the C2 vertebral level, abutting the occipital bone. The initial biopsy was reported as a fibrohistiocytic tumor at the referring laboratory. A microscopic review of the sections from the initial biopsy and subsequent resection revealed a well-circumscribed, cellular tumor composed of plump spindle and polygonal-shaped tumor cells with relatively bland nuclei, moderate to abundant eosinophilic cytoplasm and numerous interspersed histiocytes, including foam cells and lymphocytes. Immunohistochemically, the tumor cells were positive for desmin, MYOD1 and SMA, focally positive for myogenin, while negative for h-caldesmon, SOX10 and S100P. A diagnosis of inflammatory leiomyosarcoma/HRRT was offered. Subsequently, the tumor was tested for MYOD1 (L122R) mutation and was found to be negative. The patient underwent adjuvant radiation therapy and is free-of-disease at 12 months post-treatment.

This case constitutes an extremely rare case of an inflammatory LMS/HRRT, identified in the neck region. This tumor should be differentiated from its close mimics, such as a spindle cell/sclerosing rhabdomyosarcoma, as the latter is treated more aggressively, including with chemotherapy, given its relatively poor prognosis.