been proven to be an effective treatment for arteriovenous malformations, and it is being evaluated for treatment of primary and metastatic brain tumors. Dynamic conformal radiation therapy uses a computer to shape the radiation field continuously as the linear accelerator rotates about a patient. In combination with three-dimensional treatment planning, this technology facilitates the treatment of tumors with complex geometric shapes and allows radiation therapy clinicians to increase tumor dose and simultaneously reduce normal tissue dose.

Clinical Uses in Illustrative Medical Conditions

Cancer can be treated in three principal ways: surgery, radiation therapy, and chemotherapy. Surgery and radiation therapy are localized treatments, whereas chemotherapy provides systemic treatment for both local and disseminated disease. The choice of treatment depends upon which modality or combination of modalities offers the patient the greatest chance of cure, the best preservation of normal function and appearance, and the least chance of harmful side effects. Radiation therapy alone, or combined with conservative surgery, generally affords the cancer patient the greatest opportunity for curative treatment with functional preservation. This combination is widely employed for a variety of human neoplasms. Breast cancer and prostate cancer illustrate the points.

Breast cancer. Detection and treatment of early breast cancer is a significant health care issue for the United States. With the advent of effective breast cancer screening programs, more breast cancers are being detected at an earlier stage. The American Cancer Society estimates that about 183,000 new cases of breast cancer will be diagnosed in 1995 (Steele et al., 1994). The functional and cosmetic results of treatment are of significant concern to patients with early breast cancer. Clinical trials have established that breast-conserving treatment, consisting of local tumor excision and radiation therapy, results in a 10-year survival rate equal to that of radical surgery (masectomy). Following limited surgical excision of the tumor (lumpectomy), all remaining breast tissue is treated with radiation to a uniform dose of 45 to 50 Gy. The surgical site is then "boosted" with an additional 10 to 20 Gy delivered with either electrons or an interstitial radioactive seed implant (see discussion of brachytherapy, below). As more clinicians have become aware of these results, in part through the publication of national consensus panel recommendations, the proportion of breast cancer patients treated with this approach has increased. For more advanced breast cancer, radiation therapy also is used to irradiate the chest wall after mastectomy to reduce the rate of local recurrence.

Prostate cancer. Radiation therapy also plays a major role in the treatment of prostate cancer. Screening programs, particularly prostate specific antigen testing, have resulted in a dramatic increase in the number of new cases diagnosed each year. Early-stage prostate cancer can be successfully treated with



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