The use of CT scanners in megavoltage photon-beam therapy planning.

Several problems related to the true value of quantitative information from CT scanners in improving estimates of photon-beam isodose distribution remain unanswered. The authors conclude that two widely used correction factors provide satisfactory isodose corrections behind heterogeneities of simple geometry. Lateral perturbations were found to be small for megavoltage photon beams, indicating that complex computational schemes are uncalled for. It appears doubtful that the use of CT information in megavoltage photon-beam isodose curve generation requires (a) larger computers than those currently available either in existing therapy planning systems or on the scanner itself or (b) direct access to the CT numbers.