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Commissioning of Mobius3D for Linac-Based SRS and Determination of the Dosimetric-Leaf-Gap

R McCarroll*, P Sabouri , N Lamichhane , Y Poirier , B Yi , S Chen , M Guerrero , University of Maryland School of Medicine, Baltimore, MD


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Several reports describe the commissioning and initial use of independent 3D dose calculation software for second calculation of VMAT/IMRT treatment plans. In contrast, stereotactic LINAC-based radiotherapy requires much tighter passing criteria, and commissioning of independent second calculations has yet to be reported. This study presents the commissioning of Mobius3D®, including determination of the dosimetric-leaf-gap parameter, for a stereotactic radiotherapy 6X-FFF beam.

Methods: A 6X-FFF beam model for Varian TrueBeam Edge (with High-Definition MLC and stereotactic cones) was commissioned in Mobius3D®. The 6X-FFF model in Mobius3D is pre-configured with no user options to change PDDs or off-axis ratios. Only output factors and DLG can be adjusted by users. The commissioning process included three steps: validating configuration parameters, performing water phantom tests with single beams in the Eclipse Treatment Planning System (TPS), and validating patient-specific Mobius3D verification with measurements. For validation, point doses and gamma 3%/1mm passing-rates of absolute dose distributions were compared.

Results: Default output factors from Mobius3D were within 2% of both the measurement and TPS. Similarly, Mobius3D default cone factors were within 2.5% of the measurement. However, the 4mm cone showed only a 93% gamma passing rate (3%/1mm) in single beam phantom tests. Therefore, the cone factor was reduced by 3% to achieve a passing rate of 99%. To improve the gamma index passing-rate for single beam, and patient plans containing single and multiple lesions, the DLG was changed from its default 0 value. A DLG of 1.6mm gave excellent agreement with TPS for conformal arc plans, and represented a good balance between sensitivity and specificity for detection of single-isocenter multi-lesion VMAT plans that failed measurement.

Conclusion: It is possible to commission Mobius3D for linac-based stereotactic beams with excellent dosimetric agreement for conformal arc plans. Single-isocenter multi-lesion VMAT plans required careful optimization of the DLG for acceptable accuracy.


Quality Assurance, Stereotactic Radiosurgery


TH- External beam- photons: Quality Assurance - VMAT

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