ESTRO 2022 - Abstract Book

S1311

Abstract book

ESTRO 2022

In case, where the magnetic field is parallel to the chamber axis ( B x ), k B of the SNC 600c and the SNC 125c changes in dependence of the magnetic field strength B x up to 1% and 0.5% respectively. In this case the Lorenz force directs the secondary electrons perpendicular to the chamber axis, as a result the correction factor k B is symmetrical around B x = 0 T. If the magnetic field is perpendicular to the chamber axis ( B y ), the change is up to 7% and 2.3% respectively, and the Lorenz force directs the secondary electrons in the direction of the stem or the chamber tip. The variation of k B as a function of B is somewhat larger applying the TRS-398 protocol than applying the German DIN protocol. Conclusion In this study, the k B values for two different ion chambers and two different Codes of Practice in different magnetic field strengths were determined. In case where the external magnetic field B is parallel the chamber axis, the variation of k B as a function of the magnetic field is within 1.5% even for the large-volume SNC 600c chamber.

PO-1530 Dosimetric comparison of Intraoperative Radiotherapy and SRS for liver metastases

D. Scafa 1 , G.R. Sarria 1 , F.A. Giordano 1 , D. Koch 1 , T. Muedder 1 , J.A. Holz 1 , Y. Nour 1 , S. Garbe 1 , M.A. Gonzalez-Carmona 2 , G. Feldmann 3 , T.O. Vilz 4 , M. Köksal 1 , L.C. Schmeel 1 1 University Hospital Bonn, Department of Radiation Oncology, Bonn, Germany; 2 University Hospital Bonn, Department of Internal Medicine I, Bonn, Germany; 3 University Hospital Bonn, Department of Internal Medicine III, Bonn, Germany; 4 University Hospital Bonn, Department of Surgery, Bonn, Germany

Purpose or Objective Purpose/Objectives

To perform a dosimetric comparison between kilovoltage intraoperative radiotherapy (IORT) and stereotactic radiosurgery (SRS) simulating both deep-inspiration breath-hold (DIBH) and free-breathing (FB) modalities for patients with liver metastases.

Materials and Methods Methods/Materials

Diagnostic computed-tomographies (CT) of patients carrying one or two lesions < 4 cm and who underwent surgery were retrospectively screened and randomly selected for the study. For DIBH-SRS, a gross target volume (GTV) plus planning target volume (PTV) were delineated. For FB-SRS, a GTV plus an internal target volume (ITV) and PTV were defined. Accounting for the maximal GTV diameters, a modified GTV (GTV-IORT) was expanded circumferentially to simulate a resection cavity. The best suitable round-applicator size was thereafter selected. All treatment plans were calculated homogeneously to deliver 40 Gy. Doses delivered to organs at risk (OAR) and target volumes were compared for IORT vs. both SRS modalities. Eight patients encompassing 10 lesions were included in the study. The mean liver volume was 2,050.97 cm 3 (SD 650.82) and mean GTV volume 12.23 cm 3 (SD 12.62). As for target structures, GTV-IORT (19.44 cm 3 [SD 17.26]) were significantly smaller than both PTV DIBH-SRS (30.74 cm 3 [SD 24.64], p=0.002) and PTV FB-SRS (75.82 cm 3 [SD 45.65], p=0.002). The median applicator size was 3 cm (1.5 – 4.5) and the mean IORT simulated delivery time was 45.45 min (SD 19.88). All constraints were met in all modalities. Liver V 9.1 showed significantly smaller volumes with IORT (63.39 cm 3 [SD 35.67]) when compared to DIBH-SRS (150.12 cm 3 [SD 81.43], p=0.002) or FB-SRS (306.13 cm 3 [SD 128.75], p=0.002). No other statistical or dosimetrically relevant difference was observed for stomach, spinal cord or biliary tract. Mean IORT D 90 was 85.3% (SD 6.05), whereas D 95 for DIBH-SRS and FB-SRS were 99.03% (SD 1.71, p=0.042) and 98.04% (SD 3.46) p=0.036), respectively. Results Results

Conclusion Conclusion

Kilovoltage IORT bears the potential as novel add-on treatment for resectable liver metastases, significantly reducing healthy liver exposure to radiation in comparison to SRS. Prospective clinical evidence is required to confirm this hypothesis.

PO-1531 Evaluating Electronic Portal Imaging Device for small field measurements

A.A. Sait 1,2 , T. Patel 2 , N. Rastogi 1 , S. Yoganathan 3 , S. Mani 4 , J. Berilgen 5 , R. Boopathy 6

1 IFTM University, Physics, Moradabad, India; 2 Advanced Medical Physics Inc., Medical Physics, Houston, United States Minor Outlying Island; 3 NCCCR, Hamad Medical Corporation, Radiation Oncology, Doha, Qatar; 4 Advanced Medical Physics Inc. , Medical Physics, Houston, United States Minor Outlying Island; 5 Millennium Physicians, Radiation Oncology, The Woodlands, United States Minor Outlying Island; 6 School of Medicine, OHSU, Radiation Medicine, Portland, United States Minor Outlying Island Purpose or Objective Small field measurement is a challenging and time-consuming task. Although, many high-resolution detectors are commercially available, the performance of Electronic Portal Imaging Devices (EPID) is yet to be explored thoroughly for small field dosimetry. Therefore, the objective of this study is to evaluate the EPID for small field dosimetry commissioning and verification.