ESTRO 2022 - Abstract Book

S1256

Abstract book

ESTRO 2022

couch translations were thereafter used to rigidly register the CBCT images to the pCT. The pCT was deformably registered to all the daily CBCT images (using the ‘Adapt-To-Anatomy’ functionality in Monaco), and the reference heart contour was subsequently propagated to each CBCT to generate 15 ‘heart-of-the-day’ contours. The daily heart contours were then rigidly copied to the pCT. Thus, for each patient we had 16 heart contours segmented in the pCT: 1 reference heart contour and 15 representing the day-to-day variation in heart position and shape (Figure 1). To estimate the heart dose for each fraction, we re-calculated the treatment plan 15 times. For each re-calculation, a relative electron density (ED) of 1 (relative to water) was assigned to the heart-of-the-day contour and for the reference heart contour, the relative ED was set to 0.01. The body contour of the pCT was assumed to be representative for the geometry of each fraction. For each treatment fraction, the mean heart dose was registered and the differential DVH of the heart was exported and used as input in the calculation of the NTCP (risk for late cardiac morbidity), using the relative seriality model. Results For all patients, the mean heart dose calculated on the pCT did not exceed our planning constraint of 4.005 Gy. The mean heart dose calculated for the different treatment fractions, using the heart contour transferred from the daily CBCT, ranged from -21.1 % to 58.7 %, relative to the mean heart dose determined for the reference heart contour. A summary of the results is presented in Figure 2. The NTCP for late cardiac morbidity for the reference heart was 0 % for all patients. However, for the heart contour taken from the CBCT images, the NTCP varied between 0 % and 0.33 %. Conclusion The variation in the heart position and shape between fractions could lead to higher doses being delivered to the heart, compared to the dose calculated on the pCT. This could lead to an underestimation of the expected heart NTCP due to RT. These variations should be considered in the plan evaluation for left-sided breast cancer patients receiving RT in DIBH. F. Catucci 1 , D. Cusumano 2 , S. Menna 2 , A. D'Aviero 1 , C. Di Dio 1 , A. Re 1 , M. Iezzi 3 , D. Piccari 1 , F. Quaranta 1 , A. Boschetti 1 , M. Marras 1 , D. Piro 1 , C. Votta 1 , E. Sanna 1 , C. Flore 1 , G.C. Mattiucci 4 , V. Valentini 4 1 Mater Olbia Hospital, Radiation Oncology, Olbia, Italy; 2 Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Department of Radiation Oncology, Rome, Italy; 3 Università Cattolica del Sacro Cuore, Radiation Oncology, Roma, Italy; 4 Fondazione Policlinico Universitario Agostino Gemelli IRCCS, Radiation Oncology, Roma, Italy Purpose or Objective Several experiences have demonstrated the benefit of adaptive radiotherapy (ART) in patients affected by head and neck squamous cell carcinoma (HNSCC), both considering offline and online strategies. As a new discipline, online ART requires new indicators to quantify the impact of inter-fraction variations on dose distribution, thus allowing identification of the optimal time to switch towards online ART approaches. In this experience, a predictive model was proposed to early identify treatment fractions where unacceptable dose variations may be present Materials and Methods Patients affected by HNSCC were treated using an Artificial Intelligence-based linac (Varian Ethos) with 12 IMRT beams, acquiring a daily positioning CBCT image without online adaptation, prescribing 70 Gy in 35 fractions with normalisation at median dose. For each patient, all CBCT images acquired for patient positioning were rigidly matched to the planning CT (pCT), excluding rotational shifts according to Ethos clinical workflow. Daily CBCT images were automatically recontoured and treatment plan were recalculated on the corresponding synthetic CT. The variation of V95% of PTV1 and max dose of spinal cord from the original values reported on pCT were collected along the treatment: fractions where PTV V95% decreased of 3% and spinal cord Dmax increased of 5% were considered as needed of ART. The following radiological parameters were measured on each daily CBCT aligned with pCT to quantify the inter-fraction variability present in each RT fraction once compensated for couch shifts: the absolute body variation along AP and LR directions measured in proximity of the plans passing through different vertebrae (C1, C2, C3 C4) and the corresponding discs (C1-C2, C2-C3, C3-C4). The correlation between such parameters and the fractions needed of adaptation was investigated using the Wilcoxon Mann Whitney test. A logistic regression was calculated considering the most significant radiological parameter and the predictive performance were quantified considering the Receiver Operating Characteristic (ROC) curve. Results On the basis of the predefined criteria, 61/104 fractions analysed required online adaptation. At the univariate analysis the most significant parameter was the body variation along the AP direction measured through the C3-C4 disc (p=0.0002). The developed predictive model (ROC curve in fig.1) showed an AUC of 0.69 (0.59-0.79 as 95% CI) with sensitivity of 77.1% and specificity of 58.2% at the best threshold, which was 3 mm PO-1481 Radiological measurements to predict dose variation due to inter-fraction variability in H&N

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