Validation of Mobius3D Used in the Cone Beam-CT Based Adaptive Radiotherapy Workflow

Abstract

AbstractIntroduction: Adaptive Radiation Therapy (ART) enables real-time treatment modifications based on patient-specific anatomical changes, with online ART (oART) allowing daily plan adaptations using in-room imaging such as CBCT. The Varian Ethos system streamlines this process with automated contouring and plan re-optimization, necessitating robust secondary dose verification to ensure treatment plan calculation accuracy. Mobius3D and SciMoCa, two independent verification tools, employ different algorithms for dose calculation, with the 3%/2mm Gamma criterion commonly used for evaluation. This study aims to assess the accuracy of Mobius3D in oART by comparing it with SciMoCa and evaluating its sensitivity to intentional errors, ultimately refining quality assurance protocols for CBCT-based adaptive radiotherapy. Materials and Methods: 17 treatment plans, representing diverse anatomical sites and treatment techniques, were selected from Duke Health to ensure comprehensive validation. These plans were adapted from TrueBeam to Ethos in Eclipse Treatment Planning System (TPS) while maintaining field geometry and similar optimization objectives. Following re-optimization and quality assessment, the plans were exported to SciMoCa and Mobius3D for secondary dose calculations. Dose metrics (Dmean, D99%, D95%, and D1%) and gamma passing rates (3%/2mm) were evaluated. Additionally, phantom-based measurements were performed using Mobius Verification Phantom (MVP) and Delta4 to validate secondary calculations. Cases with large dose discrepancies between TPS and Mobius3D were further investigated by identifying the contributing factors in dose calculation accuracy, which included tissue heterogeneity and field off-centricity. To establish comparable plan, check criteria with other software and phantom measurements, different gamma criteria (1%/1mm, 2%/2mm, 3%/3mm, 5%/3mm) were also tested in Mobius3D and SciMoCa. To assess error sensitivity, treatment plans were modified by introducing isocenter shifts (2mm, 4mm) iv and collimator angle rotation (3°, 5°, 10°), followed by secondary dose verification using Mobius3D and SciMoCa. Statistical comparisons were performed to determine appropriate evaluation criteria and establish a consistent validation framework. Results: Mobius3D demonstrated an overall strong agreement with TPS calculations, achieving an average gamma passing rate of 91.7% at 3%/2mm across 17 cases. However, relatively large deviations were observed in breast cancer case, with the lowest passing rate of 70.1%, indicating potential limitations in breast cases calculations. Phantom-based measurements confirmed that both Mobius3D and Eclipse TPS slightly underestimated doses, with Mobius3D exhibiting larger deviations but remaining within clinically acceptable thresholds. Comparative analysis with Delta4 and SciMoCa revealed that Mobius3D consistently had lower gamma passing rates, with SciMoCa achieving an average of 98.6%, highlighting differences in algorithmic accuracy. Tissue heterogeneity and off-center field dose distribution were both identified as the contributing factors of dose discrepancy. Setting the entire body density to water equivalent improved Mobius3D’s gamma passing rates, confirming its sensitivity to inhomogeneous tissue effects. Additionally, iso-center adjustments to center the fields on the target in breast cases resulted in a significant increase in gamma passing rates, suggesting off-center fields as a contributing factor to lower dose calculation accuracy. Gamma criteria refinement showed that Mobius3D’s 5%/3mm criterion with a 90% threshold aligns most closely with SciMoCa and Delta4’s 3%/2mm standard, achieving a 94.12% passing rate, while the stricter 3%/2mm criterion resulted in only 64.71%. Dose metric analysis revealed that Mobius3D exhibited larger deviations in D99% (12 cases) and D95% (7 cases) compared to SciMoCa. The Dmean, with the smallest average difference (1.31%) and for 17 cases all within the 5% threshold compare with TPS, is the relatively stable matrix calculation in Mobius3D. Sensitivity testing showed that Mobius3D was highly responsive to geometric shifts, v with passing rates decreasing significantly for isocenter shifts (4mm reduced passing rate by 20%) and collimator rotations (10° shift reduced passing rate by 30.4%). The D99% and D95% values also decreased accordingly. These findings highlight Mobius3D’s ability to detect significant dose deviations but also its potential overestimation of minor errors compared to SciMoCa, reinforcing the need for refined acceptance criteria and optimization for adaptive radiotherapy applications. Conclusion: Mobius3D shows sufficient calculation accuracy overall but has limitations in inhomogeneous structures and off-center fields, where deviations are more pronounced. Dmean and the 5%/3mm 90% gamma criteria in Mobius 3D are more comparable with the results for SciMoCa and Delta4. Mobius3D is sensitive to error-introduced plans with iso-shift and collimator rotation, making it capable of detecting these deviations.