Application of Electronic Portal Imaging Device-based Portal Dosimetry for Evaluating Treatment Planning after Volume Changes

Abstract

During the clinical course of radiation therapy, the volume of the treated area may increase or decrease depending on the patient’s condition during the treatment period. When this occurs, the treatment plan must be reconsidered. The purpose of this study was to investigate whether treatment plan quality can be assessed based on dose distribution in the treatment field using electronic portal imaging devices (EPIDs) in cases wherein the volume of the treatment area has changed. Human phantom and bolus were used to develop a treatment plan for the volumetric modulated arc therapy of the neck and breast. The two-dimensional (2D) planar dose distribution acquired from the field of treatment of the neck (with 15 mm bolus) and breast (without bolus) using a linear accelerator and EPID were set up as a constant check baseline image. Using a 3-mm thickness bolus, the dose distributions obtained from each thickness from 0 to 15 mm bolus on human phantom and the baseline image were compared in terms of area gamma quantity, average gamma, and average dose difference in the 3% dose difference and 3-mm distance-to-agreement criteria, respectively. We performed this technique by gradually decreasing the bolus thickness for the neck and gradually increasing it for the breast. For the neck, the area gamma showed a maximum difference of 72.2 ± 0.05%, and the maximum difference in the average gamma was 0.91 ± 0.00. For the breast, these values were 41.7 ± 0.05% and 1.27 ± 0.00, respectively. The minimum and maximum calibration units (CUs) of the average dose difference based on bolus thickness were 0.05 ± 0.01 CU and 6.04 ± 0.01 CU, respectively (p < 0.05). Results are shown in Table. We could compare the 2D planar dose distributions using the EPID assuming a volume variation in the treatment area. We could also confirm the relative difference in dose and dose distribution using the EPID during volume changes. Although our study was performed using a phantom, we demonstrate that similar to other clinical indicators, EPIDs can be helpful in objectively assessing treatment plan quality. These devices can thus help in making decisions regarding the need for treatment plan changes in cases of volumetric variation in the treatment area in real-world clinical settings.Tabled 1Abstract 2564; Table; Results of the portal dosimetry constancy checkBolus (mm)Area gamma < 1 (%)Average gammaAverage dose difference (CU)Neck15100.0 ± 0.000.01 ± 0.000.05 ± 0.0112100.0 ± 0.000.11 ± 0.000.51 ± 0.07994.6 ± 0.060.27 ± 0.001.09 ± 0.01689.2 ± 0.070.44 ± 0.011.75 ± 0.01383.1 ± 0.060.62 ± 0.002.39 ± 0.01072.2 ± 0.050.91 ± 0.003.46 ± 0.00p-value1)0.0000.0000.000Breast0100.0 ± 0.000.02 ± 0.000.09 ± 0.01399.0 ± 0.050.30 ± 0.011.40 ± 0.02683.2 ± 0.230.60 ± 0.012.81 ± 0.02967.3 ± 0.330.85 ± 0.003.97 ± 0.021250.4 ± 0.181.08 ± 0.005.10 ± 0.011541.7 ± 0.051.27 ± 0.006.04 ± 0.01p- value1)0.0000.0000.0001) Statistical significance test was done by Kruskal Wallis test. CU = calibration unit Open table in a new tab