ESTRO 2020 Abstract book

S945 ESTRO 2020

Material and Methods Seventeen patients of various tumor sites treated on Halcyon were retrospectively selected. Their planning CTs were deformed to the corresponding 469 fractional MVCBCTs using Velocity software, yielding adapted MVCBCTs representing actual anatomy of the treatment day and corresponding HU values from planning CT. Reconstructed dose on fractional MVCBCT was accumulated (sum dose 1f). To simulate the weekly image guidance, the reconstructed fractional dose was resampled on every 5 MVCBCT, and was accumulated for 5 times to evaluate the uncertainties of weekly imaging ignoring the anatomic varieties in the remaining days (sum dose 5f). Target dose-volume parameters were compared. Results CTV_D98%, PTV_D95%, GTV_D98%, and PGTV_D95% were evaluated (Dx% indicates the minimal dose received by x% volume). Pairwise comparisons were made between sum dose 1f and sum dose 5f (difference1), and between sum dose 1f and planned dose (difference2). The median values of dose-volume parameters of difference1 and difference2 are shown in the table. The median values of difference1 are all negative, indicating that sum dose 5f is lower than sum dose 1f, suggesting that the target dose of a five-day image reconstruction underestimates the actual target dose by 0.05% on average. The maximum difference1 of CTV_D98% and PTV_D95% reached -10.421% and -6.446%, respectively, suggesting the unreliability of weekly image guidance. It was also observed that the reconstructed target dose on daily images are slightly lower than planned dose (difference2), but is clinically acceptable. Table. Median (ranges) of dose-volume parameters. Unit: % difference 1 difference 2 CTV_D98% -0.118 (-10.421 ~3.314) -0.262 (-5.245 ~0.356) PTV_D95% -0.024 (-6.446 ~2.732) -1.397 (-4.924 ~0.138) GTV_D98% -0.048 (-1.750 ~3.065) -0.226 (-5.070 ~0.645) PGTV_D95% -0.008 (-3.074 ~3.267) -1.392 (-5.767 ~0.156) Difference1 = (sum dose 5f – sum dose 1f) /sum dose 1f; Difference2 = (sum dose 1f – planned dose) Conclusion Reconstructed dose on weekly image may introduce uncertainties up to 10% relative to the dose reconstruction on daily images. Fractional image guidance on Halcyon provides full records for dose reconstruction and anatomic management. PO-1634 Cone-beam CT-based adaptive planning or filling protocol for neoadjuvant gastric cancer radiotherapy H. Schuster 1 , J. Boda-Heggemann 1 , L. Jahnke 1 , D. Bürgy 1 , F. Wenz 2 , K. Siebenlist 1 , F.A. Giordano 1 , A. Jahnke 1 , G. Sarria 1 1 University Medical Center Mannheim- Medical Faculty Mannheim- Heidelberg University, Department of Radiation Oncology, Mannheim, Germany ; 2 University Medical Center Freiburg- Medical Faculty Freiburg- Freiburg University, Department of Radiation Oncology, Freiburg, Germany Purpose or Objective To investigate, in the setting of neoadjuvant gastric irradiation with integrated boost, whether cone-beam CT (CBCT)-based adaptive radiotherapy compared to defined- filling protocol would be of benefit in terms of achieving a daily reproducible dose volume indexes (DVI) of target volumes (TV) and organs at risk (OAR). Material and Methods

Conclusion Gating can significantly reduce the PTV size and the treated healthy lung volume. Also for central tumors doses to healthy tissue could be significantly reduced making dose escalated gated SBRT a considerable solution for central lung Tumors. PO-1633 Impact of IGRT frequency on the dose reconstruction based on deformable registration of Halcyon CBCT H. Wang 1 , H. Wu 1 , W. Wang 1 , Y. Zhang 1 1 Beijing Cancer Hospital & Institute- Peking University Cancer Hospital & Institute, Department of Radiotherapy, Beijing, China Purpose or Objective Fractional image guided radiation therapy (IGRT) is beneficial for reducing setup errors and can be used for monitoring dose delivery based on deformable image registration (DIR). However, frequency of imaging guidance is sometimes sacrificed concerning the concomitant dose and secondary cancer risk, without tracking of its dosimetric influence. Using identical beams for treatment and MV cone beam CT (MVCBCT), the integrated dose can be optimized collectively by Eclipse hence fractional guidance has been made mandatory on Halcyon system. To investigate the impact of imaging frequency on the accuracy of target dose reconstruction using DIR, this work resampled the fractional Halcyon MVCBCT images to simulate various frequencies.