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Megavoltage irradiation, indispensable in the treatment of some cancers, is considered to be relatively sparing of the skin. Recently, however, a case was reported in which megavoltage irradiation resulted in the appearance of the Koebner phenomenon, which is the appearance of new psoriatic lesions in an area of damaged skin. The most remarkable feature of the response is that the psoriatic area matches the injured area precisely. For this reason, the Koebner phenomenon is also frequently referred to as the 'isomorphic' response. The patient, a 66-year-old male, had been diagnosed with lung cancer, which was surgically treated. He had been treated for psoriasis in the past, but had received no treatment in the five years preceding the operation for lung cancer. Several months after surgery, the patient experienced severe pain in the thigh, and X-ray examination revealed a lesion consistent with a metastatic tumor. A sample removed from the femur confirmed the presence of metastatic cancer, and the patient was treated with 3,000 cGy irradiation of six megavolt gamma rays (a Gy, or Gary, is one Joule of energy absorbed per kilogram of tissue). The patient experienced only a modest reduction in pain. However, he also developed severe itching and crusting of a scaly lesion. An additional radiation dose of 3,000 cGy was given in an attempt to reduce the patient's pain. The reduction in pain resulting from the second dose of radiation was minimal, but the skin lesions evolved to take on the appearance of typical psoriasis. The distribution of psoriatic lesions on the skin matched the irradiated area exactly, and was thus considered to be an example of the Koebner phenomenon. The pathologic process underlying psoriasis is poorly understood, but it is clear that the skin cells forming psoriatic lesions multiply abnormally fast, perhaps seven or eight time faster than normal. The present case would suggest that while megavoltage radiation has little effect on the skin in healthy patients, for patients with underlying skin problems, the radiation may precipitate pathological cell proliferation and psoriasis. (Consumer Summary produced by Reliance Medical Information, Inc.)