Dr. Jared Gardner presents a detailed case study of a myopericytoma, an uncommon benign spindle cell tumor. This tumor is defined by its characteristic whirling and swirling pattern of spindled myoid cells (pericytes) concentrically wrapping around numerous small blood vessels. The specimen also features areas exhibiting classic hemangio-pericytic (staghorn) branching vessels. A particularly noteworthy aspect of this case is its intravascular growth pattern, where the benign tumor fills the lumen of a thick-walled blood vessel. The video uses immunohistochemistry (IHC) to confirm the diagnosis, showing strong positivity for Smooth Muscle Actin (SMA) in the pericytic cells, weak/patchy positivity for Desmin, and negativity for endothelial markers (ERG, CD31) in the tumor cells. Myopericytoma is discussed in relation to its differential diagnoses (Glomus tumor, Angioleiomyoma) and its recognized molecular and morphological relationship to myofibroma (both share PDGFRB mutations).
A Detailed Look at Intravascular Myopericytoma
0:00 Myopericytoma Definition: Myopericytoma is an uncommon benign spindle cell tumor composed of myoid cells (pericytes—modified smooth muscle cells) which exhibit a distinctive whirling and swirling growth pattern.
0:00:38 Classic Morphology: The spindled tumor cells wrap concentrically around multiple small blood vessels present throughout the lesion, recapitulating the arrangement of normal pericytes.
0:01:54 Staghorn Vessels: In addition to the whirling pattern, myopericytoma can display branching, dilated, or ectatic "staghorn" vessels. This descriptive pattern is known as a hemangio-pericytic pattern, retained from older soft tissue tumor terminology.
0:03:47 Intravascular Growth: This specific case demonstrates an intravascular myopericytoma, where the tumor fills the lumen of a thick, muscular-walled blood vessel. This finding, while unusual, is recognized and does not indicate malignancy for myopericytoma.
0:04:45 Relationship to Myofibroma: Myopericytoma and myofibroma are now considered to exist on a spectrum (per WHO 2020 classification) due to morphological similarities and shared PDGF receptor beta (PDGFRB) gene mutations.
Glomus Tumor: Also pericytic, but composed of round and monotonous cells, not spindled cells.
Angioleiomyoma: Typically occurs in similar sites (distal extremities) but is composed mainly of true smooth muscle cells with prominent eosinophilic (pink) cytoplasm, unlike the bluer spindled cells of myopericytoma.
0:07:55 Immunohistochemistry (IHC): Lesional cells stain strongly positive for Smooth Muscle Actin (SMA), confirming their myoid nature.
0:09:23 Desmin Staining: While myopericytoma is usually desmin negative, this case showed weak, patchy positivity. Strong, diffuse desmin expression would typically favor an Angioleiomyoma.
0:10:34 Vascular Stains (ERG/CD31): Endothelial markers (ERG and CD31) are negative in the tumor cells, but strongly highlight the background vessels, confirming the tumor’s pericytic origin rather than an endothelial (vascular) origin.
0:12:31 Clinical Outcome: Myopericytoma is a totally benign tumor with a low risk of local recurrence, even if margins are positive. (This case occurred in a 45-year-old man on the lower leg.)