Cobalt-60 depth-dose correction as determined by transmission dose measurements.

Abstract

Although the introduction of central-axis depthdose data, measured in tissue-equivalent phantoms, elevated clinical radiation therapy to a semiquantitative discipline, it is generally recognized that in many instances these data, however accurate, fall short of indicating the true dose distribution in the patient because of variations of tissue types within the treatment field. In order to determine the magnitude of the effect that tissues of different densities have upon central-axis depth dose, a technic based upon transit-dose measurements and physically similar to schemes reported earlier, has been developed. In addition to providing factors which correct depth-dose data for tissue variations, a second requirement of this investigation was that the technic evolved should be simple enough to be carried out as a routine procedure by the resident staff. Instrumentation consists of a simple straightbore collimator mounted to the primary beam shield of a rotating cobalt-60 unit. This collimator is designed to carry the standard 25-r Victoreen dosimeter. The patient's transmission is determined by making one measurement during the course of treatment and another, under identical conditions, with the patient removed. Next, the effective absorption coefficient is determined from a family of curves which relate per cent transmission and patient diameters. It is assumed that the contribution of scattered radiation to central-axis depth dose will not change appreciably when the treatment field includes tissues whose densities differ from unity. Centralaxis depth dose will therefore depend mainly upon primary beam absorption. Wilen measured absorption coefficients differ from that for unit density material, depth-dose data should be modified in accordance with the ratio e-µ1t/e-µ1t where µl is the experimentally determined linear absorption coefficient, µ1 is the coefficient for unit density material, and t is the tumor depth. Because of the asymmetrical location of tumors and tissues of different densities within the body, the depth-dose correction factors determined by transit dosimetry are applicable only to opposed portal treatment plans where the effects of asymmetry will average out. This has been experimentally verified with a variety of asymmetrical phantoms. Transit dosimetry applied to patients with lung tumors has resulted in depth-dose correction factors ranging from 1.00 to 1.18. It was not possible to correlate these data with the appearance of the lung on a diagnostic x-ray film. Measurements made for mediastinal fields resulted in correction factors between 0.90 and 1.00.