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Validation of a Non-Isocentric Positioning of the ArcCHECK System for Patient-Specific QA On a 0.35 T MR-Linac

O Lorenzo, A Petitfils, I Bessieres*, Centre Georges-francois Leclerc, Dijon, FR

Presentations

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

Room: AAPM ePoster Library

Purpose: This work aims to evaluate the use of the ArcCHECK (AC) system for a non-isocentric and lateralized positioning. Because of the limited size of the tunnel of our MR-linac, the centering of target volumes on the isocentre of the machine is not always possible. In this context, the lateral shifting of the AC is needed to maximize the detection of significant and relevant levels of dose. This positioning is not standard and not recommended by the manufacturer. Consequently it has to be investigated and validated.

Methods: An algorithm included in the AC analysis software allows to deduce the beam angle at any time of the irradiation. According to the angles detected, correction factors are applied to the signal measured with the AC diodes. The main issue by shifting laterally the AC is to impact the correct detection of the beam angles and corrupt the results with non-adapted correction factors.
For different lateral AC position, simple squared fields and clinical plans have been tested in order to successively evaluate the beam angle detected versus the planned one, the difference of correction induced and the final impact on gamma index pass rate.

Results: The difference in the beam angle detection in comparison with the planned one increases with the lateral shift of the AC (up to 4° at 6 cm shift). In most cases, the difference of the applied correction remains lower than 1%. Consequently, at the clinical scale, the gamma index pass rate is mostly not significantly impacted. In parallel, we identified extreme and non-clinical cases in which the result were more clearly impacted and not usable.

Conclusion: In this study, we succeed in defining conditions in which the AC system can be used laterally shifted for non-isocentric treatment that are regularly performed on our MR-linac.

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