Error Detectability of Isodose Volumes as ROIs in Prostate Intensity-modulated RT QA

Abstract

Background/Aim: To evaluate the quality of error detectability with a three-dimensional verification system using isodose volumes as regions of interest (ROIs) in quality assurance (QA) of intensity-modulated radiation therapy. Patients and Methods: Treatment plans with four types of intentional errors were created from the data of 20 patients with localized prostate cancer. These plans underwent QA using the three-dimensional verification system. The datasets of another 30 cases without inserted errors were assessed as controls. The ROIs used in the evaluations were those used in our conventional method (planning target volume, rectum, and bladder). The isodose volume method (5%, 50% and 95% isodose volume) and the error detection rates (measurement above the tolerance values, as set from the other 30 cases) were assessed and compared. Results: There was significantly higher multileaf collimator systematic closed error detectability with the isodose volume method compared to the conventional method (A-side 0.2 mm: p=0.005, A-side 0.35 mm: p=0.002, B-side 0.2 mm: p=0.001 and B-side 0.35 mm: p=0.010). There were no error types for which the error detection rate of the isodose volume method was lower than that of the conventional method. Conclusion: The isodose volume method was able to evaluate the irradiated ROIs that could be delineated, and improved error detectability. This method has the potential to provide a wider margin of safety in intensity-modulated radiation therapy.Intensity modulated radiation therapy (IMRT) is an irradiation technique that can create a steep dose gradient to deliver a high dose to the target while avoiding normal tissue (1, 2). Volumetric-modulated arc therapy (VMAT) is an IMRT technique that uses a dynamic multileaf collimator (MLC) and varies the dose rate while rotating the gantry to rapidly deliver IMRT (3,4). In order to ensure safe delivery of VMAT, it is recommended that each treatment plan created by a treatment planning system (TPS) be verified to detect inherent errors and to verify dose accuracy (5). The IMRT verification methods include absolute dose verification using a dosimeter (6) and dose distribution verification using a two-dimensional detector or radiochromic film (7-11). In absolute dose verification, a phantom is used in a three-dimensional treatment plan. The absolute dose is verified by comparing the results calculated with the phantom's CT scan data with those doses measured using the phantom and a dosimeter. The verification of dose distribution is evaluated comparing distribution recalculated by the TPS with those obtained using radiochromic film or twodimensional detectors. A gamma analysis is often used as the evaluation method (12-14).A recently developed three-dimensional dose verification system can predict the dose distribution in the patient body from the phantom measurement results obtained using a twodimensional detector (15,16). It can be evaluated using a dose volume histogram (DVH), which can predict the dose delivered to organs at risk (OARs)...