Tehran University Medical Journal (Oct 2020)
Evaluation of dose uniformity in total body irradiation (TBI) technique in parallel-opposed anterior/posterior geometry using Varian Clinac 2100C/D linear accelerator
Abstract
Background: Total body irradiation (TBI) is a technique that is commonly used as a part of the patient conditioning regimen before the bone marrow transplant (BMT). The purpose of this study is to introduce and implement a reasonable TBI technique on the human-like phantom in Imam Khomeini Hospital in Tehran. Methods: The present experimental study was conducted from October 2016 to November 2017 to implement the TBI technique at the Cancer Institute of Imam Khomeini Hospital in Tehran. For this purpose, percentage depth dose, and dose rate were measured in TBI condition (i.e. SSD=310 cm, field size=40×40 cm2, gantry angle=90°, and collimator angle 45°) in homogeneous phantom. Gafchromic EBT3 films were used to measure the absorbed dose in different areas of the human like phantom at the levels of head, neck, thyroid, lung, umbilicus, pelvic, thigh, knee and leg. Phantom irradiation was performed in parallel opposed anterior-posterior geometry using an 18MV photon beam produced by Varian 2100C/D. Cerrobend blocks were used for lung protection. After analyzing the exposed films with Image J software, the dose uniformity was calculated. Results: Dose distribution uniformity was acquired in the order of -1.01% to +11.82% relative to the prescribed dose at the umbilicus. The difference between the calculated and measured dose at the umbilicus level was -2.73%. The radiation absorbed dose to the lung with blocks was 127.53cGy in one fraction which resulted in 756.18cGy in six fractions. Conclusion: The implemented technique, obtained the acceptable ±10% dose uniformity in most of the body regions. The dosing accuracy was within the acceptable range. The lungs¢ dose was reduced to the desired level using lung shields. This technique is a simple and cost-effective method that does not require complicated dosimetric techniques. Regarding the obtained results, the proposed technique has the necessary conditions for implementation in Imam Khomeini hospital in Tehran.
