Ocular Pathology

Anatomy and pathology of the human eye. Use it to review eye pathology for Ophthalmology Board Review or OKAP.

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Monday, September 11, 2006

What is a rhabdomyosarcoma?

Rhabdomyosarcoma of the Orbit
Definition: malignant neoplasm with skeletal muscle differentiation
Incidence/Prevalence: the most common primary malignant orbital tumor of childhood

Etiology: The alveolar subtype of rhabdomyosarcoma has a consistent chromosomal translocation, t(2;13)(q35;q14). PAX3 and FKHR genes on respective chromosomes 2 and 13 are juxtaposed as a result of the translocation.
Clinical Findings:
Rhabdomyosarcomas of the orbit usually present with sudden and rapidly progressive proptosis. As shown in the image, there is accompanying swelling of the eyelids, and conjunctival chemosis. The average age of onset is 7-8 years. The tumor is slightly more common in boys. Rhabdomyosarcomas usually involve the superior part of the orbit, so the eye may be displaced downward as in the case shown. The notable absence of erythema, calor and fever separates rhabdomyosarcoma from cellulitis due to an adjacent sinus infection.

Radiologic Studies: The tumor often appears deceptively well circumscribed on scan. The lamina papyracea is often eroded in up to 60% of patients. The lateral orbital wall should be carefully examined for erosion. Rhabdomyosarcoma is not a highly vascular tumor.
Histopathology: Orbital rhabdomyosarcomas are classified slightly differently and have a better prognosis (overall five-year survival of 92%) than do their extraorbital counterparts.
Three histologic types of orbital rhabdomyosarcoma are recognized:

1. embryonal (the most common),
2. alveolar, and
3. differentiated.
Embryonal rhabdomyosarcoma may develop in the conjunctiva and may present as grapelike submucosal clusters (botryoid variant). Histologically, spindle cells are arranged in a loose syncytium with occasional cells bearing cross-striations, which are found in about 69% of embryonal rhabdomyosarcomas.
Alveolar rhabdomyosarcoma has the worst prognosis in untreated or non-orbital rhabdomyosarcomas, but in recent studies the prognosis is no different than for other orbital types. The pattern produced is alveolar, with tumor cells forming and lining trabecular spaces. A reticulin stain shows thin septae that mark this tumor.
Differentiated rhabdomyosarcomas feature numerous cells with striking cross-striations, muscle differentiation. In the image series here, one sees a cellular tumor with a slightly myxoid background (arrow 1), hemorrhage (arrow 2). At higher magnification numerous strap cells with abundant pink cytoplasm are seen in the H&E stained section (arrow 3). A trichrome stain reveals numerous cells with clear cross striations (arrow 4) . Note also that the nuclei are extremely atypical with marked enlargement, variation in size and shape, numerous dark chromocenters and nucleoli and irregular chromatinic rims (arrow 5). Immunohistochemical reactivity for desmin and muscle-specific actin may be identified. Electron microscopy is often helpful, especially in the less well-differentiated cases of embryonal rhabdomyosarcoma, to demonstrate the typical sarcomeric banding pattern.
Treatment: A combination of surgical intervention, chemotherapy and radiotherapy has been very effective.
Prognosis: Orbital rhabdomyosarcomas are classified slightly differently and have a better prognosis (overall five-year survival of 92%) than do their non-orbital tumors.

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