[ What is RHABDOMYSARCOMA ?   ]

RHABDOMYOSARCOMA

Rhabdomyosarcoma is the most common soft tissue tumor in children accounting for approximately 5% of all pediatric cancers. It is a malignancy that arises from the same embryonal mesenchyme that is destined to give rise to striated skeletal muscle. This highly malignant tumor accounts for well over half the soft tissue sarcomas in childhood. Other less frequently encountered soft tissue sarcomas include fibrosarcoma, mesenchymoma, synovial sarcoma, and liposarcoma.

Although showing slight predominance in males, there is apparently no racial variance in incidence of this tumor. Most cases do not appear to exhibit a familial tendency; however, five families have been reported in which there was a second child with a soft tissue sarcoma and a high frequency of multiple primary cancers in young adult relatives (Li-Fraumeni syndrome). There appears to be a higher incidence of rhabdomyosarcoma in individuals whose families are of low socioeconomic status, who are exposed to chemical and chemical pollution, and whose fathers are cigarette smokers.

There are four major anatomic sites of involvement: the head and neck region (35-40%), the genitourinary tract (20%), the extremities (15-20%), and the trunk (10-15%). Rhabdomyosarcoma may occur at any time in the childhood years or even in adulthood; however, the peak incidence is in the 1-5 year age group, with 10% of cases occurring in the first year of life and 70% appearing in the first decade.

Symptoms and signs of rhabdomyosarcoma can usually be attributed to the mass lesion or to obstructive phenomena. For example, nasopharyngeal tumors may put pressure on the eustachian tubes and produce chronic otitis media, and a bladder or prostate tumor can cause ureteral or rectal obstruction. Evaluation of children with soft tissue masses should include a complete exam of the suspected site. The local tumor extent is further defined by radiographic procedures including CT scans or MR imaging. Rhabdomyosarcoma may spread either by local extension or by metastasis by the venous and lymphatic systems. When metastases develop, 74% will become manifest within 6 months of diagnosis, and 83% will appear by one year. The most frequent sites involved by metastases are the regional lymph nodes, lungs, liver, bone marrow, bones, and brain. The most commonly used staging system in the U.S. is that developed by the Intergroup Rhabdomyosarcoma Study Group.

The Intergroup Rhabdomyosarcoma Staging System is based upon disease extent and resectability, defining groups 1-4 by local disease status, the involvement of regional lymph nodes, and the extent of residual tumor after primary surgery. Group 1 includes patients with localized disease which is completely resected. Group 2 is patients with localized disease with microscopic residual or regional disease. Group 3 includes patients with incomplete resection or biopsy with gross residual disease. Finally group 4 is distant metastatic disease present at diagnosis.

Approximately 15% of patients are classified as group 1, 25% as group 2, 40% group 3, and 20% group 4. The characteristic cellular element of rhabdomyosarcoma is the rhabdomyoblast, a primitive skeletal muscle cell with eosinophilic cytoplasm (depending on the degree of differentiation cross-striations, longitudinal myofibril, or some suggestion of their formation.

Tumors can be classified into four histologic types:
1. Embryonal rhabdomyosarcoma - this is the most common type and it predominantly effects children under 15 years of age in the head and neck region and genitourinary tract.
2. The botryoid type - this tumor is a variant of the embryonal type and arises as a polypoid, grape-like lesions in mucosal-lined hollow organs such as the vagina and urinary bladder.
3. The alveolar type - this tumor is a more aggressive tumor occurring in children and young adults which commonly involves the muscles of the extremities or trunk.
4. The pleomorphic type - this type is rhabdomyosarcoma. It is seen mainly in adults and is usually seen rising in muscles of the extremities.

The treatment of rhabdomyosarcoma has evolved into combined modality therapy. Surgical resection of the primary tumor usually offers the best prospect for local tumor control and limited disease presentations. Integrating chemotherapeutic and radiotherapeutic modalities with surgery in the scheme of treatment has decreased the indications for radical surgery, permitting more limited surgical resection or potentially eliminating the need for surgery altogether. Effective chemotherapeutic agents include vincristine, cyclophosphamide, dactinomycin, adriamycin, ifosfamide, and VP-16. Coordinated use of surgery and radiation therapy with systematic, early chemotherapy has substantially increased the proportion of long term, disease-free survivals likely to be cured. Radiation therapy is not necessary in patients with localized, completely resected tumors.

More than 50% of children with rhabdomyosarcoma now enjoy long term survival after combined modality therapy. When considered from the standpoint of stage, about 80% of patients with early, resected lesions (group 1) are cured, as compared to 50% with more advanced, unresectable local regional presentations (groups 2 and 3). Only 20% or fewer patients with metastatic lesions (group 4) survive for 3 or more years. 90% of deaths from rhabdomyosarcoma occur within the two year period following diagnosis, and, for this reason, patients surviving free of disease beyond this point are considered to be long term survivors with a high probability of cure. Survival is as well influenced by the site of primary tumor with orbital and genitourinary sites being more favorable than other head and neck, extremity, pelvic, or trunk primaries.

REFERENCES

Rudolph, A. Pediatrics pp 1121-1123, 1987.

Altman & Schwartz. Malignant Diseases of Infancy, Childhod and Adolescence . pp 424-434, 1983.

Adeyemi-Doro, HO et al. Primary malignant tumors of the hand. The Journal of Hand Surgery, Nov. 10, 1985, pp 815-820.