The POCS have also increased knowledge of dose effectiveness. Stage B prostate cancer shows greatest sensitivity to doses between 60 and 70 Gy, with no advantage seen in higher doses. The higher risks associated with doses over 70 Gy, however, may be acceptable to patients with stage C prostate cancer.

Collective Dose

Few recent estimates of collective dose to the U.S. population from radiation therapy have been performed. Although such a calculation could be done, it would be of questionable validity and usefulness, for several reasons. Organ weighting factors used to define EDEs are derived for healthy populations exposed to comparatively low (less than a few grays) radiation levels; they do not take into account the effects of fractionated high-level exposure or implications of a population of cancer patients. Thus, collective dose from radiation therapy should not be used to estimate rates of radiation-induced cancer among the patient population that already has cancer.

The incidence of secondary neoplasms following radiation therapy has been extensively reported in the literature. Various types of radiation-induced cancer have been observed to arise both in the treatment area and at distant sites that were irradiated by scattered and leaking radiation. The issue of secondary tumors can be significant in treatment decisions for patients with curable tumors and otherwise long life expectancies. For instance, elevated incidence of in-field sarcoma has been observed 10 years following radiotherapy for breast cancer. Elevated rates of cancer have also been observed in children receiving cranial irradiation for leukemia and in young adults treated for Hodgkin's disease and seminoma. Of course, some chemotherapeutic agents also increase the risk of later malignancy, so choices among therapeutic options are often difficult.

Radiation Regulation and Control

Regulation and control of external beam radiation therapy is analogous to that of diagnostic radiology and nuclear medicine. In NRC states, the federal government regulates cobalt teletherapy (because Co-60 is reactor produced), and the state regulates all other external beam therapy. In Agreement States, the state regulates all external beam therapy.

Typical state laws specify radiation shielding design levels for facility construction, required interlocks, area radiation monitors, warning labels, and access control. Some states may specify qualifications, training, and licensure of equipment operators and (rarely) radiological physicists, as well as content and frequency of accelerator calibrations. The level of oversight varies considerably from state to state, with some states providing inspection by state radiologic health personnel and others simply registering the existence of the facility.

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