Adult-Type Rhabdomyoma of the Omohyoid Muscle

Abstract

Rhabdomyomas are benign tumors composed of mesenchymal tissue and having a histologic appearance similar to skeletal muscle. Extracardiac rhabdomyomas are rare, and the majority of the adult subtype occur in the head and neck (H&N) region. Diagnosis can be challenging due to fine-needle aspiration (FNA) and core needle biopsy being suspicious for sampling error from surrounding muscle or concerning for rhabdomyosarcoma. We present a case of a slowly enlarging left neck mass in the strap musculature of a 45-year-old Hispanic male. Multiple FNA and core biopsies failed to establish a diagnosis, and excisional biopsy was pursued revealing a hypertrophied left inferior belly of the omohyoid muscle. Histological analysis was diagnostic of an adult-type extracardiac rhabdomyoma, with complete surgical excision being the gold standard treatment for these tumors. The patient had an uneventful recovery. Skeletal muscle tumors of the H&N are uncommon, and benign extracardiac rhabdomyoma must be considered in the differential diagnosis to prevent unnecessarily aggressive intervention and appropriate patient counseling before and after surgery.

Figure 1

MRI T1 FatSat axial showing lesion (star) within left omohyoid muscle.

Figure 2

Gross coarsely lobulated cut surface of excised rhabdomyoma. Note: mass appears discolored due to formalin fixation.

Figure 3

10x magnification of H&E stained histology specimen of rhabdomyoma shows tightly packed, variously sized, round or polygonal cells separated from one another by thin fibrous septa and small vascular channels (yellow arrows). The cells have eosinophilic, striated cytoplasm with occasional intracytoplasmic vesicles (black arrows), vacuoles (red arrows), peripherally placed vesicular nuclei, and one or two prominent nucleoli. Mitotic figures and evidence of necrosis were absent.

Figure 4

40x magnification of H&E stained histology specimen of rhabdomyoma shows tightly packed polygonal cells separated by thin fibrous septa, vascular channels, striated cytoplasm, and spider cells (arrow), characterized by a central acidophilic mass connected to a condensed rim of cytoplasm at the periphery by thin strands of cytoplasm. Mitotic figures and evidence of necrosis are absent.