ESTRO 2023 - Abstract Book

S1793

Digital Posters

ESTRO 2023

advantages though with a trade-off in PTV coverage. MCS and GAI were better for the original plan. Therefore, considering a complexity index in the evaluation of a KBP model could help in obtaining high quality dose distributions while assuring their deliverability.

PO-2018 Service Development for Adaptive Stereotactic Ablative Radiotherapy on 0.35 Tesla MR Linac

D. Yucel 1 , D. Murray 2 , E. Chinherende 2 , B. George 2 , H. Harford-Wright 1 , J. Drabble 2 , E. Ilsley 2 , P. Maungwe 2 , A. Huynh 2 , P. Laycock 1 1 GenesisCare UK, Radiation Oncology, London, United Kingdom; 2 GenesisCare UK, Radiation Oncology, Oxford, United Kingdom Purpose or Objective Daily adaptive MR-guided stereotactic ablative radiotherapy (MRgRT SABR) provides several advantages: ablative doses to the most challenging targets; reduced treatment margins and improved organ at risk (OAR) sparing; and improved tumour target control. Treatment planning for complex SABR targets often involves a compromise between conflicting objectives such as plan deliverability and treatment time. This is especially critical in MRgRT SABR where on-bed times can exceed one hour. This retrospective study includes twelve prostate, liver, and thorax MRgRT cases, aiming to find the optimal balance between the target coverage, treatment time, dose spillage and calculation accuracy. Materials and Methods Previously applied MRgRT treatments avoided the couch edges due to high density and positioned the arms either on the chest or by side for patient comfort. In stage 1, the ability of the treatment planning system (TPS) to accurately account for high-density regions in the couch edges was assessed using patient-specific QA measurements (Sun Nuclear MR-ArcCheck, LAP RadCalc) in pelvic cases. In stage 2, the impact of arms-up immobilisation on treatment dosimetry and delivery time was assessed for thorax and abdominal cases. In stage 3, the optimal dose calculation parameters were analysed for plan deliverability, target coverage, dose spillage, treatment time and total monitor units for all cases. Results For stage 1, the ArcCheck QA results were 97.9% (SD=0.9) for 2 mm and 2% gamma analysis. The RadCalc study was also within 2% when the densities were corrected for each case. For stage 2, the ArcCheck QA results were 97.1% (SD=1.1) for 2 mm and 2% gamma analysis, where the low dose (25% of the prescription) spillage was reduced by 31.0% (SD=16.9%) and the mean liver dose reduced by 9.9% (SD=5.8). The direction in required MU was not evident in Stage 2 of -7.4% (SD=15.1%). In stage 3, changes in the required number of histories, histories per area, segments, accuracy, bixel and efficiency reduced the median treatment time and the number of beams by 20% and 26%, respectively, where the coverage for the prescription dose increased by 4%. Conclusion MRgRT is relatively novel and open for development. The results validated that using any required beam angle for 0.35 T MRgRT adaptive SABR cases is robust, and changes in the default computation parameters can remarkably profit dose distribution and treatment time. S. Cilla 1 , C. Romano 1 , G. Macchia 2 , M. Boccardi 2 , D. Pezzulla 2 , M. Buwenge 3 , A. Arcelli 3 , A. Zamagni 4 , S. Cammelli 5 , L. Indovina 6 , V. Valentini 7 , F. Deodato 2 , A.G. Morganti 5 1 Gemelli Molise Hospital – Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy; 2 Gemelli Molise Hospital – Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy; 3 IRCCS Azienda Ospedaliero- Universitaria di Bologna, Radiation Oncology, Bologna, Italy; 4 IRCCS Azienda Ospedaliero-Universitaria di Bologna, adiation Oncology, Bologna, Italy; 5 Alma Mater Studiorum Bologna University, Department of Experimental, Diagnostic, and Specialty Medicine - DIMES, Bologna, Italy; 6 Fondazione Policlinico Universitario A Gemelli IRCCS, Medical Physics Unit, Roma, Italy; 7 Fondazione Policlinico Universitario A Gemelli IRCCS, UOC di Radioterapia Oncologica, Dipartimento di Diagnostica per Immagini, Radioterapia Oncologica ed Ematologia, Roma, Italy Purpose or Objective We evaluated the excess absolute risk (EAR) for secondary cancer in various organs after radiation treatment for breast cancer comparing in light of the continuous development of planning techniques, from 3D-CRT to VMAT approaches Materials and Methods Using CT data set of 25 patients, we generated four different radiation treatment plans of different complexity, including three-dimensional conformal radiotherapy with wedge (3D-CRT) or field-in-field (FinF) technique, hybrid-IMRT (HMRT) and hybrid-VMAT (HVMAT). The last two techniques were optimized using an automated-based approach implemented in the Autoplanning module of Pinnacle TPS. Prescribed doses were 50Gy to the breast (PTVB) in 2Gy per fraction and a simultaneous integrated boost of 2.4Gy per fraction to the tumor bed (PTVTB). The organ-equivalent dose (OED) was calculated from differential dose–volume histograms on the basis of the “linear–exponential,” “plateau,” and “full mechanistic” dose–response models and was used to evaluate the EAR for secondary cancer in the contralateral breast (CB), contralateral lung (CL), and ipsilateral lung (IL). Statistical comparisons of data were performed by a Kruskal–Wallis analysis of variance followed by a Bonferroni–Dunn post hoc non-parametric test in order to correct for multiple comparisons. PO-2019 Does VMAT increase the risk of secondary malignancies after radiotherapy of left-breast cancer?

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