Half body radiotherapy: The use of computed tomography to determine the dose to lung

Abstract

The complication of radiation pneumonitis is of major concern in upper half body irradiation. A CT-aided treatment planning system was utilized to study the variation in lung geometry and lung density and to determine the resultant dose variation within the corresponding treatment volumes. Twenty-three patients who were scheduled for upper half body irradiation, were studied with this CT-aided treatment planning system and a number of parameters were analyzed to determine the factors affecting the dose to lung. For healthy lungs, electron densities, relative to water, varied from 0.084 to 0.40; the average lung density was approximately 0.25. Using the equivalent tissue-air ratio method for dose calculation, the dose within lung increased from 10% to 24% in comparison with doses calculated on the basis of a homogeneous water like patient. This dose increase is dependant on patient thickness, lung thickness and lung density. The calculated doses were verified experimentally by thermoluminescent dosimetry in a human-like phantom for photon beam energies ranging from cobalt-60 to 25 MV x-rays. Agreement between the measured and calculated data was better than 3% in lung. A number of other dose computation procedures were found to be in error by more than ±10% in the determination of dose to the middle of lung. Some simplified procedures, which do not require a CT-aided treatment planning system, also were evaluated. The previously published dose complication curve for radiation pneumonitis now can be considered in terms of absolute dose to lung rather than the dose prescribed to unit density tissues.