Comparison of dose calculations between pencil-beam and Monte Carlo algorithms of the iPlan RT in arc therapy using a homogenous phantom with 3DVH software

Abstract

BackgroundTo create an arc therapy plan, certain current general calculation algorithms such as pencil-beam calculation (PBC) are based on discretizing the continuous arc into multiple fields to simulate an arc. The iPlan RT™ treatment planning system incorporates not only a PBC algorithm, but also a more recent Monte Carlo calculation (MCC) algorithm that does not need beam discretization. The objective of this study is to evaluate the dose differences in a homogenous phantom between PBC and MCC by using a three-dimensional (3D) diode array detector (ArcCHECK™) and 3DVH software.MethodsA cylindrically shaped ‘target’ region of interest (ROI) and a ‘periphery ROI’ surrounding the target were designed. An arc therapy plan was created to deliver 600 cGy to the target within a 350° rotation angle, calculated using the PBC and MCC algorithms. The radiation doses were measured by the ArcCHECK, and reproduced by the 3DVH software. Through this process, we could compare the accuracy of both algorithms with regard to the 3D gamma passing rate (for the entire area and for each ROI).ResultsComparing the PBC and MCC planned dose distributions directly, the 3D gamma passing rates for the entire area were 97.7% with the gamma 3%/3 mm criterion. Comparing the planned dose to the measured dose, the 3D gamma passing rates were 98.8% under the PBC algorithm and 100% under the MCC algorithm. The difference was statistically significant (p = 0.034). Furthermore the gamma passing rate decreases 7.5% in the PBC when using the 2%/2 mm criterion compared to only a 0.4% decrease under the MCC. Each ROI as well as the entire area showed statistically significant higher gamma passing rates under the MCC algorithm. The failure points that did not satisfy the gamma criteria showed a regular pattern repeated every 10°.ConclusionsMCC showed better accuracy than the PBC of the iPlan RT in calculating the dose distribution in arc therapy, which was validated with the ArcCHECK and the 3DVH software. This may suggest that the arc step of 10° is too large in the PBC algorithm in the iPlan RT.

Feasibility and accuracy of UF/NCI phantoms and Monte Carlo retrospective dosimetry in children treated on National Wilms Tumor Study protocols

“…Accurate dose estimation for infield and out‐of‐field organs is crucial for evaluating dose–response relationship for both tumor and organs at risk. Although the modern RT TPS is fairly accurate for estimating doses both infield and near‐field locations, it is not accurate for out‐of‐field dose calculations . Howell et al showed that commercial RT TPS (Eclipse) underestimated out‐of‐field doses by an average of 40% compared with water phantom measurements, and this error was greater (up to 55%) at distances >11 cm from edge of RT fields .…”

“…Howell et al showed that commercial RT TPS (Eclipse) underestimated out‐of‐field doses by an average of 40% compared with water phantom measurements, and this error was greater (up to 55%) at distances >11 cm from edge of RT fields . Others have also reported that MC dose calculation algorithm measures out‐of‐field doses accurately . The main dose contributors for out‐of‐field doses include machine head leakage, scatter through machine components, and internal tissue scatter which are not fully accounted for in TPS calculations.…”

“…These NWTS patients received the following RT fields: flank (2 patients), whole abdomen (WA) (6 patients), whole lungs (1 patient), and combination of fields (11 patients). The RT doses were as follows: flank (10-30 Gy), WA (10-40 Gy), and whole lung (12)(13)(14)(15) (Tables 1 and 2). These patient-matched CT scans and Phantom DICOM files were imported into the Pinnacle RT TPS v9.10 at Northwestern University.…”

“…This computational human phantom series is freely available for research purpose (please contact Dr. Choonsik Lee (choonsik.lee@nih.gov) . Many reports have shown that Monte Carlo (MC) dosimetry is more accurate than standard treatment planning system (TPS) dosimetry, especially for out‐of‐field organs and heterogeneous regions . This pilot study was done to determine the feasibility and accuracy of UF/NCI phantoms and MC retrospective dosimetry and had two aims: (1) to determine the anatomic accuracy of UF/NCI phantoms by comparing 3D organ doses in NWTS patient‐matched UF/NCI phantoms to organ doses in corresponding patient‐matched CT scans and (2) to compare infield and out‐of‐field organ dosimetry using two dosimetry methods—standard RT TPS and MC dosimetry in these two anatomic models.…”

“…10 Many reports have shown that Monte Carlo (MC) dosimetry is more accurate than standard treatment planning system (TPS) dosimetry, especially for out-of-field organs and heterogeneous regions. [11][12][13][14][15][16] This pilot study was done to determine the feasibility and accuracy of UF/NCI phantoms and MC retrospective dosimetry and had two aims:…”