A preliminary study of test and clinical application of a physical model based on the three-dimensional dose verification system using electronic portal imaging device

Objective To establish a physical model and optimize its physical parameters using the electronic portable imaging device (EPID) three-dimensional dose verification system, and to prepare it for clinical application. Methods EPID was used to acquire images of 3, 5, 10, 15, 20, and 25 cm square fields for construction of a physical model. The parameters of the physical model were optimized based on the percentage depth dose, total scatter factor, and off-axis ratio at a depth of 10 cm in a homogeneous water phantom. A thimble ionization chamber and radiochromic films were used to measure the point and planar doses for single fields, combined fields, and IMRT plans in a homogenous phantom and a human phantom. The results were compared with those in the three-dimensional reconstruction. In the human phantom and 10 intensity-modulated radiotherapy (IMRT) plans for tumors in different sites, the passing rates under the criteria of 5%/3 mm and 3%/3 mm were compared between three-dimensional reconstruction and treatment planning system (TPS) calculation. The dose and volume analyses were performed on target volume and organs at risk (OARs) in patients. Results For the single fields, combined fields, and IMRT plans, the mean deviations of point dose were less than 0.5% between three-dimensional reconstruction and ionization chamber measurement, and less than 2.0% between three-dimensional reconstruction and TPS calculation. In the homogenous phantom, the human phantom, and patients, the mean passing rates of both two-and three-dimensional doses were higher than 95% under the criteria of 5%/3 mm and 3%/3 mm. In patients, however, OARs with small volume had relatively large dose deviations. Conclusions A series of pre-clinical tests show that the three-dimensional dose verification system is an effective approach for clinical dose verification and holds promise for clinical application. Key words: Electronic portal imaging device; Three-dimensional dose verification; Physical model; Intensity modulated radiation therapy