ESTRO 2020 Abstract Book

S894 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

Planning and cone-beam CT data (PCT and CBCT) of a gastric lymphoma patient were used and the neoadjuvant therapy of gastric carcinoma was simulated. PTVs and OARs were defined according to the TOPGEAR study protocol (TV 45 Gy/1.8 Gy) with an additional integrated boost (GTV 50.4 Gy/2.016 Gy). For each fraction, patients needed to follow a defined gastric filling regime consisting of a fasting period of 12h followed by the intake of 200 ml (2 glasses of water) immediately before irradiation. On each daily CBCT, OARs and TVs were contoured. Using an image registration platform, non-rigid matching of the PCT and CBCT scans was performed. The treatment plan without adaptation was recalculated on each CBCT (R- CBCT). For analyzing the DVIs of TVs and OARs, each fraction was evaluated for various DVH parameters PTV, GTV and OARs (kidneys, liver, heart) and gastric volumes. Furthermore, an adapted treatment plan was created and optimized for the new anatomy (A-CBCT). The data on gastric volume and dose parameters in the PTV, GTV and OARs were compared.

Results The mean gastric volumes were 277.32 (± 54.40) cm 3 in CBCTs and 519.2 cm 3 in PCT. Mean doses to the PTV did not differ significantly, with a volume variation of 1.54% in average. In parallel, the D95 was improved in the GTV boost volume coverage by 5.26% compared to the R-CBCT plan, as well as the D min with 6.35% better coverage. The mean dose to the heart, liver and right kidney could be reduced with the A-CBTC plan by -35.74%, -10.71% and - 29.47%, respectively. No differences were found in the left kidney or overdosing (V>110%) parameters.

PTV

GTV

Dmean [Gy]

Dmin [Gy]

Dmean [Gy]

D95 [Gy]

DVH-Parameter

CBCTs recalculated (R) (Mean value) CBCTs adapted (A) (Mean value)

45.79

45.30 49.47

46.79

45.09

48.17 50.30

49.26

Difference R to A [%] -1.54

6.35 1.68

5.26