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Feasibility of Pseudo 4pi Approach for Stereotactic Radiosurgery On a Linear Accelerator with On-Board MRI

E Omari*, E Melian, S Hoffman, T Thomas, K Stoeckigt, J Ingram, A Sethi, T Refaat, J Roeske, I Rusu, Loyola University Chicago, Maywood, IL


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

Room: AAPM ePoster Library

Purpose: To investigate the feasibility of a pseudo 4pi treatment planning and delivery approach for a MRI-linac with jawless double stacked MLCs, no couch rotational capabilities, and a step-and-shoot delivery technique for fractionated stereotactic radiosurgery (SRS).

Methods: Ten previously treated SRS patients with 1-2 brain lesions (N=15 lesions, 27Gy in 3fxs) were re-planned using a pseudo 4pi technique. The technique was implemented to allow a couch motion in the longitudinal and vertical directions by performing isocenter shifts. VMAT plans were generated for comparison. The SRS plans were evaluated qualitatively and quantitatively, using the following metrics: Paddick's conformity index (CI), Paddick's gradient index (GI), RTOG quality of coverage (Q), and RTOG homogeneity index (HI). Radiation necrosis indicators for normal brain V18Gy and V21Gy, and the maximum dose to the brainstem and optic pathways were also evaluated. Using Lucy 3D QA phantom (Standard Imaging), CT and MRI images were acquired with single and multi-target inserts.Plans were generated using a pseudo 4pi technique and delivered on a MRI-linac.

Results: The pseudo 4pi technique provides clinically acceptable plans compared with the non-coplanar VMAT plans, however, the pseudo 4pi technique shows less rapid dose falloff. Quantitative metrics were as follows for the pseudo 4pi plans and VMAT non-coplanar plans, respectively: CI (0.81-0.92 & 0.85-0.96), GI (3.36-4.6 & 2.23-3.28), Q (0.94-1 & 0.98-1), and HI (1.22-1.26 & 1.23-1.25). Brain V18Gy and V21Gy, optic pathways and brainstem doses all met the planning objectives. The MRI-linac was capable of delivering the phantom plans successfully.

Conclusion: Pseudo 4pi planning generated clinically acceptable SRS plans with Q and HI comparable to the VMAT plans (one-way ANOVA,p<0.01). CI and GI was more favorable with VMAT compared with 4pi plans (p<0.01), however within acceptable clinical variation based on the published SRS metrics. Pseudo 4pi delivery was also feasible.


Treatment Planning, MRI


Not Applicable / None Entered.

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