Case 7 -
Richard M. DeMay
University of Chicago
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- Fine needle aspiration biopsy
- 45 year old male
- 1 cm tonsillar/parapharyngeal mass
Rhabdomyoma is a benign tumor of mesenchymal origin composed of striated muscle. It is among the
rarest of soft-tissue tumors, representing only about 2% of all tumors showing skeletal muscle
differentiation, and is much rarer than rhabdomyosarcoma. Only about 100 cases have been reported .
There are three types: fetal, adult, and genital (vaginal). The tumor is distinct from cardiac
rhabdomyoma, which is considered a hamartoma often associated with tuberous sclerosis. The head &
neck area is the most common site, particularly for the adult type; rhabdomyoma does not occur in bulky
muscles. Adult rhabdomyomas of the head & neck are thought to arise from skeletal muscle of the
third and fourth branchial arches.
The adult rhabdomyoma is the most common of these rare tumors. It presents as a slowly growing,
painless mass, usually solitary. Patients have a mean age of 50 years, but there is a wide age range
(2–80 years) . Rhabdomyoma is two to four times more common in males than females. It typically is
found in the head & neck area, particularly in the vicinity of the mouth, but has been reported in
various other sites, including the mediastinum, gastrointestinal tract, and prostate . It usually
forms a solitary mass, but occasionally is multifocal. The tumor will recur if incompletely excised.
The FNA biopsy is highly characteristic, reminiscent of regenerating muscle. Myoblasts are very large
round to polygonal to elongated cells. The nuclei are single or multiple, central or peripheral,
uniform, round, and vesicular with conspicuous nucleoli
. Intranuclear cytoplasmic
invaginations may be present .
Mitotic figures are not seen .
The cytoplasm is abundant, dense, granular, and acidophilic (Papanicolaou stain) or basophilic
. Cross-striations, vacuoles, granules, and crystalloids characterize the
cytoplasm. Cross-striations may be visible, in both the Papanicolaou and Diff-Quik stains, in scattered
cells and in cell blocks
. Cytoplasmic vacuoles contain either glycogen or lipid
. Spider cells are myoblasts with peripheral cytoplasmic vacuoles between
which thin legs of cytoplasm extend from the dense central body to the cell membrane, giving a spidery
appearance (best appreciated in cell blocks or tissue)
. Rod-shpaed or needle- shaped
crystalloids of Z band material may occur in rare cells mimicking alveolar soft part sarcoma
Occasional cells are huge. There is usually a component of small spindle cells.
Electron microscopy shows thick and thin myofilaments and masses of Z-band material. Desmin and
myoglobin are usually strongly positive. Muscle specific actin is positive in some cells, but smooth
muscle actin and S-100 are negative
The differential diagnosis includes granular cell tumor, hibernoma, and malignant tumors with skeletal
muscle differentiation. Granular cell tumor lacks both cross striations and crystals, but S-100 is
usually positive. Hibernoma cells have distinct intracytoplasmic lipid droplets. Also, oncocytic tumors
and acinar tumors containing zymogen granules may be considered. Aspiration of non-neoplastic skeletal
muscle could mimic rhabdomyoma
. Alveolar soft part sarcoma lacks cross-striations.
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