Rhabdomyosarcoma

Rhabdomyosarcoma is the most common primary orbital malignancy of childhood. The average age of onset is 5–7 years. The classic clinical picture is that of a child with sudden onset and rapid progression of unilateral proptosis. However, patients in their early teens may experience a less dramatic course, with gradually progressive proptosis developing over weeks to more than a month. There is often a marked adnexal response with edema and discoloration of the eyelids. Ptosis and strabismus may also be present. A mass may be palpable, particularly in the superonasal quadrant of the eyelid, and it may cause proptosis if sufficiently large (Fig 5-13A, B). However, the tumor may be retrobulbar, may involve any quadrant of the orbit, and may in rare cases arise from the conjunctiva. If the patient has an unrelated history of trauma to the orbital area, this can lead to a delay in diagnosis and treatment.

If a rhabdomyosarcoma is suspected, the workup should proceed urgently. CT and MRI can be used to define the location and extent of the tumor (Fig 5-13C). A biopsy should be performed, usually through an anterior orbitotomy approach (Fig 5-13D). If the lesion is focal and has a pseudocapsule, it may be possible to completely remove the tumor. If this is not practical, there is some indication that the smaller the volume of residual tumor, the more effective is the combination of adjuvant radiation and chemotherapy in achieving a cure. In diffusely infiltrating rhabdomyosarcoma, a large biopsy specimen should be obtained to provide adequate tissue for frozen sections, permanent light-microscopy sections, electron microscopy, and immunohistochemistry. Cross-striations are often not visible on light microscopy and may be more readily apparent on electron microscopy.

Figure 5-13 Orbital rhabdomyosarcoma. Lesion presenting as a right upper eyelid mass causing (A) inferolateral globe displacement and (B) proptosis. C, T2-weighted MRI shows a mass in the right superonasal quadrant. D, Anterior orbitotomy through the upper eyelid provided tissue for pathologic analysis.

(Courtesy of Bobby S. Korn, MD, PhD.)

The physician should palpate the cervical and preauricular lymph nodes of a patient with orbital rhabdomyosarcoma to evaluate for regional metastases. Chest radiography, bone marrow aspiration and biopsy, and lumbar puncture should be performed to search for more distant metastases. Sampling of the bone marrow and cerebrospinal fluid is best performed, if possible, with the patient under anesthesia at the time of the initial orbital biopsy.

Rhabdomyosarcomas arise from undifferentiated pluripotent mesenchymal elements in the orbital soft tissues, not from the extraocular muscles. They may be grouped into the following 4 categories:

• Embryonal. This is by far the most common type, accounting for more than 80% of cases. The embryonal form is typically found in the superonasal quadrant of the orbit. The tumor is composed of loose fascicles of undifferentiated spindle cells, only a minority of which show cross-striations in immature rhabdomyosarcomas on trichrome staining. Embryonal rhabdomyosarcomas are associated with a good 5-year survival rate (94%).

• Alveolar. This form typically occurs in the inferior orbit and accounts for 9% of orbital rhabdomyosarcomas. The tumor displays regular compartments composed of fibrovascular strands in which rounded rhabdomyoblasts either line up along the connective tissue strands or float freely in the alveolar spaces. This is the most malignant form of rhabdomyosarcoma; the 5-year survival rate for the alveolar subtype is 65%.

• Pleomorphic. Pleomorphic rhabdomyosarcoma is the least common and best-differentiated form overall, most frequently affecting young adults. In this type, many of the cells are straplike or rounded, and cross-striations are easily visualized with trichrome stain. The pleomorphic variety has the best prognosis (5-year survival rate of 97%).

• Botryoid. This rare variant of embryonal rhabdomyosarcoma appears grapelike. It is not found in the orbit as a primary tumor; rather, the botryoid variant occurs only through secondary extension from the paranasal sinuses or the conjunctiva. Rhabdomyosarcoma that occurs in the head and neck region outside the orbit, including the sinus cavities, has a lower 5-year survival rate (50%–71%) than rhabdomyosarcoma in the orbit, likely because tumors in the head and neck region produce fewer signs and symptoms.

• Crist W, Gehan EA, Ragab AH, et al. The Third Intergroup Rhabdomyosarcoma Study. J Clin Oncol. 1995;13(3):610–630.