Evaluating Mobius3D Dose Calculation Accuracy for Small-Field Flattening- Filter-Free Photon Beams

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Flattening-filter-free (FFF) x-ray beams are increasingly used in stereotactic body radiation therapy (SBRT) due to their efficiency.
• Accurate dose calculation is critical for SBRT treatment planning and quality assurance (QA).
• Mobius3D is a treatment planning system used for dose calculation and QA.

Purpose of the Study:

• To evaluate the dose calculation accuracy of Mobius3D for small-field FFF x-ray beams.
• To assess the impact of beam and multileaf collimator (MLC) modeling on Mobius3D's accuracy in SBRT applications.
• To identify specific conditions where Mobius3D's accuracy is compromised for FFF beams.

Main Methods:

• Commissioning of Mobius3D for unflattened 6 MV and 10 MV FFF beams, including beam model adjustment and dosimetric leaf gap (DLG) optimization.
• Experimental evaluation using artificial plans to assess small-field modeling accuracy.
• Retrospective analysis of SBRT patient plans to evaluate dose calculation accuracy against measured data.

Main Results:

• Significant dose calculation differences were observed for fields >5 cm off-axis and <1 cm field size.
• Maximum absolute errors of 9.96% (6 MV) and 9.07% (10 MV) occurred at 1 cm field size in artificial plans.
• For patient plans, mean gamma passing rates (3%/3 mm) were 63.6% (6 MV) and 82.6% (10 MV), with average dose differences up to -19.9% (6 MV) and -10.1% (10 MV).

Conclusions:

• Mobius3D exhibits dose calculation uncertainties for small-field FFF photon beams.
• Current beam and MLC modeling in Mobius3D requires improvement for reliable SBRT pretreatment QA.
• Further development is necessary to enhance Mobius3D's accuracy in clinical SBRT applications.