The Dosimetric Data of 10 MV Linear Accelerator Photon Beam for Total Body Irradiation

Ahn, Sung Ja; Kang, Wee Saing; Park, Seung Jin; Nam, Taek Keun; Chung, Woong Ki; Nah, Byung Sik

Abstract

PURPOSE: This study was to obtain the basic dosimetric data using the 10 MV X-ray for the total body irradiation.
MATERIALS AND METHODS
A linear accelerator photon beam is planned to be used as a radiation source for total body irradiation (TBI) in Chonnam University Hospital. The planned distance from the target to the midplane of a patient is 360cm and the maximum geometric field size is 144cm' 144cm. Polystyrene phantom sized 30 30 30.2cm3 and consisted of several sheets with various thickness, and a parallel plate ionization chamber were used to measure surface dose and percent depth dose (PDD) at 345cm SSD, and dose profiles. To evaluate whether a beam modifier is necessary for TBI, dosimetry in build up region was made first with no modifier and next with an 1cm thick acryl plate 20cm far from the polystyrene phantom surface. For a fixed source-chamber distance, output factors were measured for various depth.
RESULTS
As any beam modifier was not on the way of radiation of 10MV X-ray the dmax and surface dose was 1.8cm and 61%, respectively, for 345cm SSD. When an 1cm thick acryl plate was put 20cm far from polystyrene phantom for the SSD, the dmax and surface dose were 0.8cm and 94%, respectively. With acryl as a beam spoiler, the PDD at 10cm depth was 78.4% and exit dose was a little higher than expected dose at interface of exit surface. For two-opposing fields for a 30cm phantom thick phantom, the surface dose and maximum dose relative to mid-depth dose in our experiments were 102.5% and 106.3%, respectively. The off-axis distance of that point of 95% of beam axis dose were 70cm on principal axis and 80cm on diagonal axis.
CONCLUSION
1. To increase surface dose for TBI by 10MV X-ray at 360cm SAD, 1cm thick acrylic spoiler was sufficient when distance from phantom surface to spoiler was 20 cm. 2. At 345cm SSD, 10MV X-ray beam of full field produced a satisfiable dose uniformity for TBI within 7% in the phantom of 30cm thickness by two-opposing irradiation technique. 3. The uniform dose distribution region was 67cm on principal axis of the bean and 80cm on diagonal axis from beam axis. 4. The output factors at mid-point of various thickness revealed linear relation with depth, and it could be applicable to practical TBI.