Quality assurance (QA) for every patient plan is a major part of the radiation treatment process to ensure that the prescribed dose is accurately delivered to the planned volume. QA techniques have evolved over time following the development of radiation treatment techniques. The conventional patient-specific QA process is typically performed by using a phantom-based system; during the process, specific point dose(s) and dose distribution are measured using a mapped plan from a patient treatment plan onto a phantom, and the measurement data were compared with those of planned data [1–7]. However, during the mapping process in the phantom-based QA system, the phantom used in the QA cannot represent the inhomogeneity of a patient’s anatomy .
Recently, the Mobius3D® QA platform (Mobius Medical Systems, Houston, TX, USA) has been released for the evaluation of patient-specific QA. Mobius3D can calculate a dose distribution for a patient CT dataset directly by using an independent dose calculation algorithm from the primary radiation treatment planning system. Mobius3D has been widely implemented in various institutions and has been applied as a primary- and secondary-checking QA tool to evaluate not only conventional radiotherapy techniques but also advanced ones, such as intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) [9–15].
To mimic the characteristics of a LINAC beam, the Mobius3D beam must be modelled by the user; however the vendor suggests a minimum configuration for beam commissioning because Mobius3D was designed to perform accurate dose calculations by using reference beam data. One of the major procedures is to adjust the dosimetric leaf gap (DLG) correction factor. The DLG correction factor is defined to account for additional beam transmission from the leaf shape of a multi-leaf collimator used in advanced treatments. During Mobius3D beam commissioning, the vendor recommends modifying the DLG value such that the mean dose difference between measurement and calculation is within 2%.
For commissioning, Mobius3D users commonly utilize the Mobius Verification Phantom (MVP) supplied by Mobius Medical Systems and a phantom verification section included in Mobius3D. The section provides the expected doses at the pre-defined chamber locations (i.e., a total of seven ion camber holes) automatically calculated by a separate plan mapped from a patient plan onto an image dataset of the MVP. Although MVP-based DLG correction is a comparatively simple method, the expected doses corrected by DLG parameter cannot be reflected by the effect of a couch top. Note that, in accordance with AAPM TG-176 , beam attenuation through couch tops might have a range of approximately 2% and several studies have already reported the effect of the couch top on dose difference [17–21]. Unfortunately, in Mobius3D, the DLG correction factor affected by the couch top has not been evaluated quantitatively to date when evaluating the DLG parameter by using the MVP automatic dose calculation system.
In this study, the commissioning procedure for the DLG correction factors of Mobius3D was evaluated based on the effect of the couch top. For this, 3 LINAC machines and a total of 30 patients VMAT plans (i.e., 10 plans / LINAC) were selected, and volume-averaged doses in the same chamber position were calculated using Mobius3D and MVP with and without the couch top, respectively. Finally, the correlation factors between DLG and couch top were derived for each LINAC.