This video provides a detailed histopathological review of adult rhabdomyoma, a rare, benign neoplasm of skeletal muscle lineage, using a case presented in a 50-year-old man's floor of the mouth. The discussion emphasizes the tumor's characteristic multi-nodular, sharply circumscribed architecture. Microscopically, the cells are epithelioid, exhibiting deeply eosinophilic, granular cytoplasm, and abundant glycogen deposits. These deposits lead to prominent cytoplasmic vacuolation, creating the pathognomonic "spider cells." The video also highlights the presence of essential features like cross striations and jumbled rod-like crystals. Differential diagnosis focuses on distinguishing rhabdomyoma from granular cell tumor (based on cell border clarity and circumscription) and rhabdomyosarcoma (based on atypia and mitotic activity). The tumor is confirmed to be Desmin positive and is clinically noted to be unrelated to Tuberous Sclerosis.
Adult Rhabdomyoma: Histopathology and Differential Diagnosis
0:04 Case Introduction: The example involves an adult rhabdomyoma from a 50-year-old man, growing slowly under the tongue in the floor of the mouth.
0:13 Architectural Features: The tumor is benign and often multi-nodular, displaying very sharp circumscription. A small subset of cases can be multifocal.
0:35 Cellular Morphology: Composed of sheets of richly, deeply eosinophilic (reddish), polygonal/epithelioid cells with round nuclei and abundant granular cytoplasm.
1:09 Key Feature: "Spider Cells": The cells often exhibit prominent cytoplasmic vacuolation, which is due to abundant glycogen. When the vacuoles rupture during processing, the remaining eosinophilic cytoplasm surrounds the nucleus with spidery processes connecting to the cell membrane. These are the hallmark of adult rhabdomyoma.
2:06 Defining Characteristics: Cross striations are usually present in these skeletal muscle neoplasms, although they may require searching. Additionally, the tumors may contain funny elongated rod-like crystals, likened to "jack straws" (pickup sticks).
3:53 Clinical Context: These tumors typically occur in the pharynx, larynx, head, and neck of adults over 40, and are more common in men (approx. 3:1 ratio). They are totally benign but can recur locally. Unlike cardiac rhabdomyoma, they are not associated with Tuberous Sclerosis.
Rhabdomyoma cells have sharply circumscribed cell borders and the lesion is sharply circumscribed.
GCT cells usually have poorly formed cell borders (syncytial arrangement) and the lesion is typically poorly circumscribed (infiltrative). GCT also stains diffusely S100 positive, while Rhabdomyoma stains Desmin positive.
7:04 DDx – Rhabdomyosarcoma (RMS): RMS is ruled out by the lack of significant pleomorphism and minimal/absent mitotic activity in rhabdomyoma. RMS contains other cells (spindle, round blue) in addition to rhabdomyoblasts.
8:22 Other DDx: Other differential entities include Alveolar Soft Part Sarcoma (TFE3 positive), Paraganglioma (nested pattern, neuroendocrine markers positive), and Hibernoma (sharply circumscribed lipid vacuoles).
9:44 Takeaway: Adult rhabdomyoma is a distinct lesion recognizable by its abundant eosinophilic cytoplasm, sharp cell borders, and spider cells. The full digital slide image is available on Kikoxp.com for further exploration.