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A Multi-Target SRS QA Method for Fast and Accurate 3D Dose Verification

B Rosen

B Rosen*, J Mikell , I El Naqa , K Lam , J Moran , Univ Michigan Medical Center, Ann Arbor, MI


WE-RAM3-GePD-IT-1 (Wednesday, August 2, 2017) 10:30 AM - 11:00 AM Room: Imaging ePoster Theater

Purpose: To present a novel QA method for multi-target SRS treatments to assess the accuracy in absolute 3D dose distributions and geometry for multiple targets in a single treatment delivery.

Methods: Four single-isocenter VMAT SRS plans (2-6 targets, prescription range: 15-22 Gy) and a stereotactic spine plan were delivered to cylindrical ClearView gel phantoms (Modus QA, London, ON). For each gel, a 2 x 2 cm² static reference field was delivered to a low-dose region. Optical CT scans of the gels provided 1 mm isotropic resolution optical attenuation matrices, which were registered to the computed dose distribution by automatic alignment of the measured and computed reference fields. The central depth dose portion was used to determine a second-order polynomial conversion function. Dose profile comparisons and 3D gamma analysis of the plan distributions were performed. To quantify geometric alignment, the plan distribution alignment was further optimized to obtain optimal residual shifts. Additionally, high-resolution radiochromic film dosimetry of each target was performed for inter-comparison of the two methods.

Results: A baseline 1 mm geometric accuracy and 3% absolute dosimetric accuracy of the method was established using the relatively large-field spine plan. For the small SRS targets, gamma passing rates (4%/2 mm) ranged from 82-99% for the multi-target plans prior to optimal dose alignment and from 90-99% after optimal residual shifts (<3 mm) were applied. The geometrical agreement and dose distributions were consistent with film measurements. Finally, significant time savings on the order of 1 hour per QA plan was demonstrated for gel compared to film, since the gel supported simultaneous measurement of multiple targets in a single delivery.

Conclusion: A fast, high-resolution QA method for complex multi-target SRS treatments has been presented that meets the accuracy goals necessary for routine clinical use.

Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by NIH grant P01CA059827. The authors would also like to thank Modus Medical Devices Inc. (London, ON) for providing the gel dosimeters and optical readouts used in this work.

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