ESTRO 2023 - Abstract Book

S214

Saturday 13 May

ESTRO 2023

3D target translations only, the other structures remain in the original position. An overview of these methods is shown in Figure 1.

Monaco (v5.51.10) was used to analyze the dose of ten prostate patients, using drift correction and replanning, both mid- treatment. To compare five DVH metrics for both intra-fraction adaptations, the dose was recalculated on the same scan. The fractions of ten prostate patients, treated with intra-fraction replanning with a PTV margin of 2, 2, and 3 mm (LR SI and AP), were included. From 52 treated patients, these ten patients showed the lowest and highest translations in x, y, and z directions, to cover the full range of possible translations. The 3D translations ranged up to 6.2 mm. Results Figure 2 shows the DVH metrics of ten prostate patients. Significant differences were found for the PTV ( p <0.01), rectum ( p <0.01), bladder ( p <0.01) and sphincter metrics ( p =0.03), using a paired Student’s t-test. However, the dose differences are clinically irrelevant, and the OAR doses did not meet the criterion for only two fractions. Mean CTV values are similar for both intra-fraction adaptation techniques ( p =0.36).

Conclusion The in silico study showed good DVH outcomes for the intra-fraction drift correction and the intra-fraction replanning. The dose outcomes between the two intra-fraction methods are comparable, as the dose differences are clinically irrelevant. Intra-fraction drift corrections can be performed multiple times during a treatment, within one minute and with similar dose results compared to intra-fraction replanning. Therefore, this novel drift correction could be promising for intra- fraction adaptations for Unity MR-linac users. OC-0280 MR-guided SBRT of infra-diaphragmatic soft tissue metastases – early toxicity and dosimetric results M. Felter 1 , P. Krause Møller 2 , M. Josipovic 3,4 , S. Bekke 1 , U. Bernchou 2 , E. Serup-Hansen 1 , P. Parikh 5 , K. Joshua 5 , P. Geertsen 6 , C. Behrens 1,7 , I. Vogelius 3,4 , M. Pøhl 3 , T. Schytte 2,8 , G. Persson 1,4 1 Herlev and Gentofte Hospital, Department of oncology, Herlev, Denmark; 2 Odense University Hospital, Department of oncology, Odense, Denmark; 3 Rigshospitalet, Department of oncology, Copenhagen, Denmark; 4 University of Copenhagen, Department of Clinical Medicine, Faculty of Health, Copenhagen, Denmark; 5 Henry Ford Hospital, Department of oncology, Detroit, USA; 6 Herlev and Gentofte Hospital, Department of oncology, Herlev, USA; 7 Technical University of Denmark,