Utilization of Tissue Compensator for Uniform Dose Distribution in Total Body Irradiation

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

Abstract

PURPOSE: This study was performed to verify dose distribution with the tissue compensator which is used for uniform dose distribution in total body irradiation (TBI).
MATERIALS AND METHODS
The compensators were made of lead (0.8mm thickness) and aluminum(1mm or 5mm thickness) plates. The humanoid phantom of adult size was made of paraffin as a real treatment position for bilateral total body technique. The humanoid phantom was set at 360cm of source-axis distance(SAD) and irradiated with geographical field size(FS) 144cm' 144cm2(40 40 cm2 at SAD 100cm) which covered the entire phantom. Irradiation was done with 10MV X-ray(CLINAC 1800, Varian Co., USA) of linear accelerator set at Department of Therapeutic Radiology, Chonnam University Hospital. The midline absorbed dose was checked at the various regions such as head, mouth, mid-neck, sternal notch, mid-mediastinum, xiphoid, umbilicus, pelvis, knee and ankle with or without compensator, respectively. We used exposure/exposure rate meter (model 192, Capintec Inc., USA) with ionization chamber(PR 05) for dosimetry. For the dosimetry of thorax region TLD rods of lxlx6mm3 in volume(LiF, Harshaw Co., Nethrland) was used at the commercially available humanoid phantom.
RESULTS
The absorbed dose of each point without tissue compensator revealed significant difference(from -11.8% to 21.1%) compared with the umbilicus dose which is a dose prescription point in TBI. The absorbed dose without compensator at sternal notch including shoulder was 11.8% less than the dose of umbilicus. With lead compensator the absorbed doses ranged form +1.3% to -5.3% except midneck which revealed over-compensation (-7.9%). In case of aluminum compensator the absorbed doses were measured with less difference (from -2.6% to 5.3%) compared with umbilicus dose.
CONCLUSION
Both of lead and aluminum compensators applied to the skull or lower leg revealed a good compensation effect. It was recognized that boost irradiation or choosing reference point of dose prescription at sternal notch according to the lateral thickness of patient in TBI should be considered.