[50] Epithelioid Rhabdomyosarcoma: A Morphologically Distinct Variant.

Vickie Y Jo, Christopher DM Fletcher. Brigham and Women's Hospital, Boston, MA

Background: Rhabdomyosarcoma (RMS) mainly affects children and young adults and is currently classified into embryonal, alveolar, and pleomorphic subtypes. RMS is rare in older adults and in these patients the pleomorphic variant is most common. All RMS subtypes show immunohistochemical evidence of rhabdomyoblastic differentiation. We have identified a seemingly distinct RMS variant that demonstrates epithelioid features, reminiscent of poorly differentiated carcinoma or melanoma, and occurring mostly in older adults.
Design: Fifteen cases of epithelioid RMS identified between 2001 and 2009 were retrieved from consultation files. H&E and immunohistochemical stains were examined. Clinical and follow-up information was obtained from referring pathologists.
Results: Affected patients were 10 men and 4 women (1 unknown sex), with median age of 70 years (range 14-78). Ten tumors were intramuscular and 2 were subcutaneous. Anatomic sites were upper extremity (4), lower extremity (2), head and neck (3), thoracoabdominal wall (2), back (1). Materials for 3 patients were nodal metastases (neck, mediastinum). Lesions ranged in size from 3 to 8.5 cm, having nodular fleshy cut surfaces, necrosis, and infiltrative edges. All tumors demonstrated high grade epithelioid features, with abundant amphophilic-to-eosinophilic cytoplasm, large vesicular nuclei and frequently prominent nucleoli. Only mild or very focal pleomorphism was seen. Three tumors had focal rhabdoid inclusions. Tumor cells showed sheet-like growth with prominent necrosis, and 1 case showed prominent nesting. All tumors exhibited infiltrative growth into adjacent skeletal muscle and fat. Mitotic counts ranged from 4 to 85 per 10 HPF (median 23), with frequent atypical forms. All tumors showed diffuse desmin immunoreactivity; myf4 staining was diffuse to multifocal in 13 cases and more focal in 2 cases. Immunohistochemical stains for keratins and S100 were negative in all cases. Most patients underwent surgical resection with chemotherapy and/or radiation. Follow-up information is available for 10 cases so far. One patient had multiple local recurrences, 2 developed satellite nodules near the primary tumor, 6 developed local lymph node metastases, and 5 patients developed distant metastases (lung, bone, liver). Six patients have died of disease (5 within 1 year, 1 within 5 years).
Conclusions: Epithelioid RMS appears to be a morphologically distinct variant of RMS, affecting primarily older patients. These tumors show close mimicry of carcinoma or melanoma, but all exhibit skeletal muscle differentiation by immunohistochemistry. The clinical course as determined thus far is aggressive.
Category: Bone & Soft Tissue

Monday, February 28, 2011 8:45 AM

Platform Session: Section F, Monday Morning

 

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