Melanomas: radiobiology and role of radiation therapy

Purpose/Objective: This course will review the radiobiology of malignant melanoma (MM) and the clinical use of radiation therapy for metastatic melanoma and selected primary sites. The course will emphasize the scientific principles underlying the clinical treatment of MM. Introduction: The incidence of malignant melanoma has one of the fastest growth rates in the world. In 1991, there were 32,000 cases and 7,000 deaths from MM in the United States. By the year 2000, one of every 90 Americans will develop MM. Wide local excision is the treatment of choice for Stage I-II cutaneous MM. Five-year survival rates depend on (a) sex: female-63%, male-40%; (b) tumor thickness: t < 0.75 mm-90%, t > 4 mm-25%; (c) location: extremity-60%, trunk-41%; and (d) regional lymph node status: negative-77%, positive-31%. Despite adequate surgery, 45-50% of all MM patients will develop metastatic disease. Radiobiology: Both the multi-target model: S = 1-(1-e-D/Do)n and the linear quadratic mode: -In(S) = alpha x D + beta x D2 predict a possible benefit for high dose per fraction (> 400 cGy) radiation therapy for some MM cell lines. The extrapolation number (n) varies from 1-100 for MM compared to other mammalian cells with n=2-4. The alpha/beta ratios for a variety of MM cell lines vary from 1 to 33. Other radiobiologic factors (repair of potentially lethal damage, hypoxia, reoxygenation, and repopulation) predict a wide variety of clinical responses to different time-dose prescriptions including high dose per fraction (> 400 cGy), low dose per fraction (200-300 cGy), or b.i.d. therapy. Based on a review of the radiobiology of MM, no single therapeutic strategy emerges which could be expected to be successful for all tumors. Time-Dose Prescriptions: A review of the retrospective and prospective clinical trials evaluating various time-dose prescriptions for MM reveals: (1) MM is a radiosensitive tumor over a wide range of diverse time-dose prescriptions; and (2) The high clinical response rates to a diverse range of time-dose prescriptions are consistent with the wide diversity and complexity of the radiobiological data for MM. RT for Metastatic Melanoma: RT is the single most effective therapy for local palliation of metastatic disease. Complete response (CR) rates range from 24-75% and overall response (OR) rates are 59-98% with RT compared with CR rates of 3-10% and OR rates of only 15-36% with either chemotherapy or immunotherapy. Achieving a CR is important even for patients with metastatic disease. Five-year survival for patients with metastatic disease who achieve a CR is 49% compared to a 5-year survival of only 3% for patients who do not achieve a CR. RT for Primary Tumors: RT is effective therapy for selected primary sites including uveal melanoma and non-cutaneous, non-ocular mucosal MM. The RT results for ocular melanoma and non-cutaneous, non-ocular mucosal MM are superior to surgery. In addition, post-operative RT for residual microscopic/macroscopic disease following primary surgery produces long term local control rates of 75-88%. It is likely that radiation therapy is being under-utilized in current oncology practice