Reduced incidence of bone metastases in irradiated areas after external radiation therapy of prostatic carcinoma.

Fourteen males, out of 380 patients, treated with radiation to the central pelvis and lumbar spine for poorly differentiated prostatic carcinoma were analyzed in retrospect. The dose of radiation to the bones of the target area was 5,000 cGy. The patients showed no signs of metastases at bone scintigraphy performed in connection with the treatment. In an average of 34 months after finishing radiotherapy, the patients developed metastases at bone scintigraphy. The pattern was similar in all patients. The treated target area appeared as a "cold zone" surrounded by more or less homogenously and strongly increased activity of the axial skeleton, characteristic of bone metastases. Radiography, which was performed in 11 patients, confirmed widespread metastatic disease sparing the target area. This was interpretated as bone metastasis being precluded by the irradiation. The most probable explanation of this finding is eradication in situ of distant micrometastases already present in the bone marrow at the time of treatment. An alternate explanation is a reduced implantation of later seeded blood-born metastases as an effect of the irradiation. The characteristic pattern of this phenomenon must be recognized at bone scintigraphy.