A Axford^1,2*, N Dikaios¹, C H Clark², D Roberts³, P Evans^1,2, (1) The Centre for Vision Speech and Signal Processing (CVSSP), University of Surrey, Guildford, Surrey, UK (2) Metrology for Medical Physics(MEMPHYS), National Physical Laboratory, Teddington, UK(3) Elekta Ltd, Linac House, Crawley, UK

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  A Axford

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

Room: AAPM ePoster Library

Purpose:
To develop an end-to-end phantom and framework for assessing the accuracy of adaptive radiotherapy in MR guided systems to changes in target shape and position.

Methods:
An MR and CT visible end to end audit phantom was designed and built through additive and subtractive manufacturing. This contains 3D prostate volumes based on MR patient data, and 3 surrounding organs at risk, allowing for user-speci?ed changes in the shape and position of the prostate to be made using interchangeable inserts.
The Elekta Unity system was used to plan, adapt and deliver the treatment plans to the phantom using the adapt to position and adapt to shape workflows. Treatment plan adaptations were made to the reference plan using online MR images, with the resultant delivered dose distributions measured using gafchromic ?lm for two central slices in the sagittal plane.
Five measurement configurations were used to investigate both adapt to shape and position, looking at whole body displacements, target displacements of 15 and 40mm, and shape deformations from a realistic to a spherical prostate of the same volume with both no displacement, and with 40mm displacement.
Verification of the accuracy was performed through gamma evaluation, comparing planar dose distributions calculated by the treatment planning system, with the measured dose distribution from the gafchromic ?lm.


Results:
Gamma analysis evaluations for each type of treatment plan adaptation investigated showed a very high agreement with an average pass rate of 99% and 95%, for a criteria of 3%/3 mm and 2%/2mm respectively. These pass rates were consistent for both shape and position changes.

Conclusion:
The high agreement and consistency of results shown for all position and shape changes in this investigation demonstrates the accuracy of MR guided adapted treatment, in addition to the sufficient quality of the phantom and framework for adaptive radiotherapy assessment.

Funding Support, Disclosures, and Conflict of Interest: This research has been partially funded by Elekta and Royal Society

Keywords

Image-guided Therapy, Phantoms

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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