THE INFLUENCE OF X-RAY-ENERGY ON LUNG DOSE UNIFORMITY IN TOTAL-BODY IRRADIATION

Purpose: In this study we examine the influence of x-ray energy on the uniformity of the dose within the lung in total-body irradiation trea tments in which partial transmission blocks are used to control the lu ng dose, Methods and Materials: A solid water phantom with a cork inse rt to simulate a lung was irradiated by x-rays with energies of either 6, 10, or 18 MV, The source to phantom distance was 3.9 meters. The c ork insert was either 10 cm wide or 6 cm wide, Partial transmission bl ocks with transmission factors of 50 % were placed anterior to the cor k insert, The blocks were either 8 or 4 cm in width, Kodak XV-2 film w as placed in the midline of the phantom to record the dose, Midplane d ose profiles were measured with a densitometer, Results: For the 10 cm wide cork insert the uniformity of the dose over 80% of the block wid th varied from 6.6% for the 6 MV x-rays to 12.2% for the 18 MV x-rays. For the 6 cm wide cork insert the uniformity was comparable for all t hree x-ray energies, but for 18 MV the central dose increased by 9.4% compared to the 10 cm wide insert, Conclusion: Many factors must be co nsidered in optimizing the dose for total-body irradiation, This study suggests that for AP/PA techniques lung dose uniformity is superior w ith 6 MV irradiation, The blanket recommendation that the highest x-ra y energy be used in TBI is not valid for all situations. (C) 1997 Else vier Science Inc.