Assessment of the Uniformity of Dose Distribution in the Patient’s Body During Total Irradiation

Purpose: To evaluate the uniformity of dose distribution in the target (throughout the patient’s body) for 39 sick children during total body irradiation. Material and methods: Total body irradiation (TBI) of 39 children with lymphoblastic leukemia was performed on the Clinac iX electron accelerator (Varian) with a photon radiation 6 MV. The field size at the diaphragm was 40 × 40 cm. The patient’s position from the isocenter of the accelerator is 550 cm. The single dose is 1,0 Gy, the total dose is 12,0 Gy. The number of fractions is 6. Irradiation was carried out behind plexiglas with protective cerrobend blocks on the lungs. To ensure uniformity of dose distribution, boluses of tissue-equivalent material were used. The dose rate is low and ranges from 0,05 to 0,10 Gy/min, the calculation of irradiation plans is performed on the Eclipse (Varian) planning system. Irradiation is performed with two opposite wide fields with the patient in the lying on the right side position on the therapeutic table. Vacuum mattresses are used as a patient fixation device. A quantitative analysis of patient irradiation plans was carried out using dose– volume histograms with the use of the homogeneity index HI and assessments of dose loads on critical structures (heart, lungs, kidneys, eye lenses). Results: It is shown that for 96 % of patients, doses in all organs are distributed uniformly within a tolerance of 10 %, a similar picture is observed for a tolerance of 15 %. The dose homogeneity of the HI dose distribution within the target volume was estimated with and without taking into account boluses. In the presence of boluses, the average value of the homogeneity index HI is lower than in their absence. This means that the use of tissue-equivalent boluses contributes to an increase in the homogeneity of dose distribution and is absolutely justified.