ESTRO 2023 - Abstract Book

S1530

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ESTRO 2023

Figure 2: Comparison of the LETd distributions for step and shoot and continuously rotating delivery.

Conclusion This work presents for the first time the dosimetric quality of 4D (3D + continuous rotation) C-Arc delivery. With robust optimization, dosimetric accuracy for C-Arc delivery with continuous patient/gantry rotation can be preserved, making this a valuable delivery strategy for efficient future clinical C-Arc. As a next step, the framework will be expanded to 5D (4D + anatomical motion) dose calculation. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101008548.

PO-1805 Clinical impact of moving from type-b to type-c algorithms in lung SBRT treatment planning.

G. Reggiori 1,2 , L. Dominici 1 , D. Franceschini 1 , A. Bresolin 1 , P. Gallo 1 , F. La Fauci 1 , F. Lobefalo 1 , L. Paganini 1 , P. Mancosu 1 , S. Parabicoli 1 , M. Pelizzoli 1 , M. Scorsetti 1,2 , S. Tomatis 1 1 IRCCS Humanitas Research Hospital, Radiotherapy and Radiosurgery Department, Rozzano (MI), Italy; 2 Humanitas University, Department of Biomedical Sciences, Pieve Emanuele (MI), Italy Purpose or Objective Moving from a type-b to a type-c algorithm introduces significant dosimetric differences especially in low density materials as is the case of lung SBRT planning. The aim of this work was to evaluate whether these differences affect the patient outcome in terms of local response, early and late toxicities. Materials and Methods 145 patients with lung metastases treated in our department from 2014 to 2021 were selected. All patients received a VMAT treatment with a 10 MV FFF beam. 38 patients received 50 Gy in 5 fractions while 108 received 48 Gy in 4 fractions. For 105 patients the algorithm used was AAA (type-b) and Acuros (type-c) was used for the remaining 40. All plans were recalculated (fixing MUs) with the algorithm not used in the clinical plan and dose differences were evaluated in terms of coverage (PTV mean dose) and OARs sparing (Lungs-CTV V20Gy). Mean PTV density was also extracted. Local response and toxicities (early <90 days and late >90 days) were evaluated for all patients according to the CTCAE V.5.0 classification. A univariate analysis on the algorithm and density parameters was performed with Fisher exact test, considering response and toxicity as dependent variables. Multivariate logistic regression was performed to discriminate the impact of the algorithm on the selected endpoints. These analyses were performed both stratifying for toxicity grade (G1, G2 and G3) and with a binary variable (toxicity yes or no). Results The median follow-up time was 19.2 months (range 2-78 months), 25.8 and 9.3 months for type-b and type-c sets, respectively. Mean PTV density was -578±139 HU and mean PTV dose difference was 5±4%. Correlation coefficient between PTV dose difference and mean density (HU) was -0.744 (p<0.01) (Fig 1). No significant differences were observed on OARs (<0.1%). The univariate analysis on the algorithm showed no significant differences with p-values of 0.86, 1.00 and 0.17 for response, late and early toxicity, respectively. In the multivariate analysis none of the considered variables was significant,