ESTRO 2023 - Abstract Book

S1568

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ESTRO 2023

conformity and lower normal tissue dose. Proton minibeam FLASH conformal planning reduced D2 brain dose up to 50%; and to 0 Gy for other OARs (including Chiasm, Brainstem, Cochleas) compared to standard photon plans. Local increase in brain dose at D2/1 level was minimalised by allowing higher dose inside the tumour and appropriate spot reduction and spot charge optimisation. (a) (b) (c)

Figure 1. Minibeam FLASH plan evaluation showing in (a) Plan Dose distribution, (b) Percentage of dose within FLASH dose rate cut-offs, (b) Plan FLASH Dose rate. Table 1. Summary of Dose to Targets and OARs

Conclusion It is possible to plan clinically relevant conformal proton linac (standard and minibeam) FLASH plans for SRS brain metastasis. In studied cases, the UHDR averaged over all brain targets was in the order of 70 Gy/s for standard proton linac FLASH and 60 Gy/s for FLASH minibeams. Target conformity is comparable to nominal photon plans, with lower OARs dose, indicating that SRS brain metastasis are potential candidates for conformal FLASH therapy using a proton linac, with additional benefit of the Flash Effect.

PO-1835 Scanning Mode Parameters to Improve Image Fidelity on an Ultrafast Ring-Gantry Linac kVCBCT System

A. Haertter 1 , B. Koger 1 , M. Salerno 1 , C. Kennedy 1 , B.K. Teo 1 , M. Alonso-Basanta 1 , L. Dong 1 , T. Li 1

1 University of Pennsylvania, Department of Radiation Oncology, Philadelphia, USA

Purpose or Objective A recently developed CBCT acquisition mode from Varian, HyperSight™ kVCBCT system, was designed to facilitate breath hold imaging and reduce free-breathing motion artifacts. This study aims to recommend best practices for free-breathing fast CBCTs by assessing target contouring and localization robustness under various respiratory amplitudes and speeds using a motion phantom. Materials and Methods CBCT and CT images were taken of the QUASAR respiratory motion lung tumor insert phantom set to several breathing periods (0, 3, 6, 9s) and amplitudes (0, 5, 10, 15, 20mm). HyperSight CBCT thorax imaging protocols include an ultrafast 6s acquisition (“fast”) and a longer 60s acquisition (“slow”), equivalent to the TrueBeam™ full-fan gantry rotation. 4DCT average scans from a standard CT simulator were used as reference images. For each breathing period and amplitude combination, the random interplay effect between breathing motion and image acquisition were investigated by acquiring two fast scans (6s) consecutively. Motion capturing robustness was evaluated by comparing the fast to slow scans for each period and amplitude combination. Robustness was evaluated using target volume (defined by HU thresholding from -500 to 0 HU), and measurements of target dimensions.