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Challenges in Implementation of MPGG5.a. for Non-Standard Delivery Systems

J Smilowitz*, P Hill, P Yadav, J Bayouth, University of Wisconsin, Madison, WI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

As part of our clinic-wide MPPG5.a validation, we found implementation on TomoTherapy (Tomo) and ViewRay (VR) treatment delivery systems (TDS) presented unique challenges: (1) Static field testing is neither practical nor relevant for TomoTherapy, (2) 1D MR-compatible and 2D Tomo water tanks precluded many profile acquisitions suggested for static fields and (3) validation focus necessarily differs based on vendor supplied model and/or reference beam data.

Tomo: Standard modulated “tomophant” plans (measured with ion chamber, film and volumetric array detector) replaced measurements of static fields in water tank for output factor and basic commissioning and validation. Couch model validation was performed. Heterogeneity correction was validated with a simple modulated small field. VR: array detector and film measurements were obtained at several depths for static beam validation. A comprehensive suite of representative clinical plans for modulated validation was generated.

Ion chamber and film results confirmed array and/or volumetric detector for “basic” validation with measurement and calculation agreeing within MPPG5.a. tolerance criteria. Use of a detector array and simplified plans allowed us to decrease the number of static field validation plans by validating multiple parameters with one plan. All clinical IMRT/VMAT plans passed the clinical criteria and were evaluated at the 2%/2mm evaluation criteria.

MPPG5a was applicable to a non-standard gantry TDS. By adapting plan design and measurement devices, the spirit of the practice guideline was maintained. The convolution-superposition (Tomo) and Monte Carlo (VR) photon dose algorithms were validated within the recommended tolerance and action levels. For these systems, time and energy for validation should be focused on dynamic delivery to assess the MLC for beam models based on golden beam data. Aspects of each system not adequately addressed in the current guidelines (e.g.: effect of magnetic field and couch model) should be addressed in the subsequent versions.


Validation, Commissioning, Dose


TH- External Beam- Photons: Quality Assurance - Linear accelerator

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