Collimator misalignment in high energy therapy machines.
Measurements of the intensity distribution of a 60-Co beam revealed a significant field distortion caused  by minor misalignment of the collimator assembly which was not detected by routine methods. Accurate monitoring of radiation intensity distribution and periodic visual inspection of collimators are suggested for early detection of this problem.
D015502:Absorptiometry, Photon; D003037:Cobalt Radioisotopes; D011874:Radiometry; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*
D003037:Cobalt Radioisotopes