Effect of scattered radiation in the total body irradiation technique: evaluation of the spoiler and wall dose component in the depthdose distribution

To determine the additional dose in layers of the body close to the skin during total body irradiation (TBI), due to radiation scattered off the treatment room walls and behind plexiglass spoilers applied to improve dose uniformity within the irradiated body. Large-field 6, 15 and 25 MV photon beams were generated by a Saturn 43 medical accelerator. A solid 30 ´ 30 ´ 30 cm3 PMMA (polymethylmethacrylate) phantom was used to represent radiation scattered from the body of the patient. Dose distributions were measured by a Farmer ionization chamber. The dose component arising from the spoiler was measured 5 mm below the phantom surface, over distances of 5-100 cm between the spoiler and the phantom surface. To measure the contribution of backscattered radiation from the walls, a small lead block was placed between the source and detector. Measurements were carried out in air with the PMMA phantom removed, to eliminate radiation backscattered from the phantom. As measured behind the spoiler, attenuation of the primary photon beam by the spoiler itself was by 8, 5 and 3% for 6, 15 and 25 MV beams, respectively. The highest dose contribution from the spoiler arose at 10 cm separation between the phantom surface and the spoiler. Assessed at a depth of 5 mm in the phantom, at spoiler-phantom separation of 10 cm, relative to case without spoiler and with wall backscatter subtracted, the dose enhancement due to the spoiler was by 8, 13 and 20% at beam energies 6, 15 and 25 MV, respectively. In these measurements, the distance between the source and the phantom surface was 300 cm and that between the source and the spoiler - 290 cm. The dose contributions due to radiation backscattered from the walls, relative to the case without any wall backscatter, estimated over the distal side of the phantom at a distance of 20 cm between the wall and that side of the phantom, were 5, 6 and 8% at beam energies 6, 15 and 25 MV, respectively. The use of a spoiler enhanced the dose in regions close to the phantom surface, compensating for the dose decrease over that area due to build-up effect. Radiation backscattered from the wall enhanced the dose in regions close to the phantom surface facing the wall.