Dose Homogeneity of the Total Body Irradiation in vivo and in vitro confirmed with Thermoluminescent Dosimeter

INTRODUCTION Although radiotherapy is generally used as a local modality, it also can be used as systemic modality in number of occasions. Indications of total body irradiation (TBI) or whole body irradiation are immune suppression prior to bone marrow transplantation (immunologic diseases, aplastic anemia), eradication of malignant cells (leukemia, lymphoma and some solid tumor), and eradication of genetic diseases (Fanconi’s anemia, thalasemia major). Though low single dose irradiation of 2-3 Gy has been used for immune suppression of immune diseases and in combination with chemotherapetic agents, such as cyclophosphamides for aplastic anemia, higher doses of fractionated irradiation is required to eradicate malignant cells as well as to overcome graft injection in allogenic bone marrow transplantation. TBI also plays role in providing grafting space in bone marrow, which heightens successful transplantation rate. Single high dose therapy of 9-10 Gy was initially which was later replaced with fractionated and/or low dose rate therapy to lessen the treatment limiting complications, such as radiation pneumonitis. In cases of fractionated regimen, adjustments could be done in following fraction for more homogenous dose delivery by accurately estimating the exposed dose to organ of interest (1,2). Methods of TBI can be divided into parallel-opposed lateral technique and parallel opposed anteroposterior/posteroranterior (AP/PA) technique, although there are many other different methods. Advantages of parallel-opposed lateral technique are natural lung compensation with arms and more comfortable patient position. Although parallel opposed anteroposterior/posteroranterior (AP/PA) technique does not require additional compensator as patient thickness along long axis is minimized, additional lung block to reduce lung dose and additional irradiation to increase the doses to chest wall are necessary and also requires long source-to –patient distance (1). TBI setup represents a very irregular and extended field. It is essential to deliver homogenous radiation dose over whole body, which requires careful setup design to minimize the possible errors. Verification of total body irradiation dosimetry techniques at many institutions in United States revealed that quality control and assurance is essential for corresponding institute, facility, and treatment setup (3). Reported range of error of dose homogeneity with methods incorporating water phantom and entrance dose was ±7%, and with methods using humanoid phantom was ±10% (4-6). American Association of Physicists in Medicine (AAPM) recommended error range of –10 ~ +5% for dose homogeneity in case of total body irradiation in the report 17 (7). Frequently used in vivo dosimeters are thermoluminescent dosimeter (TLD) and diode. TLD was used more frequently in multiple dosimetry due to low cost, but with development of multi-channel diode electrometer, use of diodes is increasing (4). Small water phantom was frequently used in confirmation of dose distribution. But, as systemic error in range of ±4% was reported in case of TBI, use of humanoid phantom is recommended (3). Dose distribution and homogeneity for Co-60 teletherapy has been reported previously from this department (8). But, dose homogeneity and quality assurance program using linear accelerator has not been settled. Recently, some institution in Korea have reported dose profiles in TBI setup (9-11). The objective of this study was to analyze and confirm the accuracy and the homogeneity of the treatment setup, the parallel opposed lateral technique, currently used in Seoul National University Hospital.