The influence of x-ray energy on lung dose uniformity in total-body irradiation

In this study we examine the influence of x-ray energy on the uniformity of the dose within the lung in total-body irradiation treatments in which partial transmission blocks are used to control the lung dose.

A solid water phantom with a cork insert 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 cork insert was either 10 cm wide or 6 cm wide. Partial transmission blocks with transmission factors of 50% were placed anterior to the cork insert. The blocks were either 8 or 4 cm in width. Kodak XV-2 film was placed in the midline of the phantom to record the dose. Midplane dose profiles were measured with a densitometer.

For the 10 cm wide cork insert the uniformity of the dose over 80% of the block width 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 three x-ray energies, but for 18 MV the central dose increased by 9.4% compared to the 10 cm wide insert.

Many factors must be considered in optimizing the dose for total-body irradiation. This study suggests that for AP/PA techniques lung dose uniformity is superior with 6 MV irradiation. The blanket recommendation that the highest x-ray energy be used in TBI is not valid for all situations.