The Dosimetry of 60Co Using p-Type Diodes and Human Phantom for Total Body Irradiation

ntroduction: total body irradiation is a type of external radiotherapy which allows the delivery of a homogenous dose to the entire body while the critical organs such as lung, etc. can be shielded, if needed. The role of TBI is to suppress the patient's immune system prior to bone marrow transplantation in order to avoid the rejection of the donor bone marrow. in this study, some basic dosimetric parameters (percentage depth dose and dose rate) were measured under TBI condition for 60Co machine. A Perspex water phantom having dimensions of 30´30´30 cm and a farmer type ionization chamber (0.6 cc) were used to measure some parameters such as percentage depth dose.For the measurement of surface dose, a parallel plate ionization chamber was used. All the chambers in this study were used with a UNIDOS electrometer. A human phantom consisting of 3 sections (head and neck, trunk and hip) was used to check the accuracy of the dose delivered. The dose in the midpoint of the phantom was measured by a farmer type ionization chamber (0.3 cc) which was placed at the umbilicus level. The dose to be delivered was calculated by two 1. using the measured percentage depth dose and the dose rate under TBI condition 2. using the calculated percentage depth dose based on Mayneord factor and the dose rate for the TBI condition which was derived from the inverse square law.P-type Silicon diodes while being connected to a multi channel electrometer was used to investigate the dose uniformity throughout the whole body. For this purpose, the diodes were taped at several locations on the entrance and the exit side of the human phantom. there is an agreement between the measured and the calculated dose within 2.6 and 3% for the first and second method, respectively. The mean difference between the calculated and measured percentage depth dose was 0.71% with standard deviation of 0.96%. The dose uniformity through the whole body at the anterior and posterior side was within 6 and 5%, respectively. The measured dose in the midlobe of the lung shows that the delivered dose to the lungs was 14.14 Gy which is higher than the prescribed dose of 13.5 Gy. Discussion and the results of the dose delivered by both methods show that the difference between the calculated and measured values are within the acceptable level (±5%).The second method is recommended for the use in TBI calculation than the first one because it is simpler to use. Although the obtained result from the first method is in a better agreement with the measured data than the one from the second method.One of the main objectives in TBI is to achieve dose uniformity throughout the whole body. The results of this work on the human phantom show that dose uniformity was achieved. The measured dose in the midlobe of the lung shows that the delivered dose to the lung was higher than the prescribed dose, thus appropriate shielding should be used to reduce the lung dose. Based on the dose uniformity obtained in this work it is recommended that TBI treatment can be implemented in the radiotherapy centers throughout the country using Cobalt-60, provided proper lung shield is used.