ESTRO 2023 - Abstract Book

S1662

Digital Posters

ESTRO 2023

The slope of the relationship could be predicted pre-treatment based on sigmoid volume at planning, difference in bladder volume between full and empty CTs, tumour volume on MRI, and change in the superior extent of the CTVLR contour between full and empty planning CTs (linear regression, R2 0.77). Rectal size generally had weaker per-patient correlations with U, C and CoM. When bladder and rectal size were combined as explanatory variables, U motion could be predicted with confidence (R2 > 0.7) in 10 patients, C motion in 7 patients, and CoM motion in 8 patients. Conclusion Interfraction target motion during cervix cancer radiotherapy is dependent on bladder volume and rectal diameter, but the strength of correlation and slopes vary by patient. We have demonstrated that several features that influence this relationship can be extracted from the planning CT. Further efforts to develop a predictive motion model could provide valuable prior knowledge for adaptive radiotherapy strategies including automatic plan of the day selection and online replanning. V. Panni 1 , E. Galofaro 1 , C. Di Carlo 1 , M.V. Agbaje Olufemi 1 , F. Fenu 1 , L. Vicenzi 1 , L. Alticozzi 2 , F. Cucciarelli 1 , M. Valenti 2 , G. Mantello 1 1 Azienda Ospedaliero Universitaria delle Marche, Radiotherapy Department, Ancona, Italy; 2 Azienda Ospedaliero Universitaria delle Marche, Medical Physics, Ancona, Italy Purpose or Objective In liver stereotactic radiotherapy (SRT), breath control methods and image-guided radiotherapy (IGRT) are recommended techniques to online correct the uncertainties that characterize this treatment. Liver deformability is a liver feature, mainly influenced by neighboring OARs filling and position, that represents an important issue in the setting of liver SRT, since it is not online correctable by IGRT. Aim of this study was to assess and offline quantify liver deformability as residual error after online IGRT match in patients treated with liver SRT in our Department. Materials and Methods The CBCTs from a sample of 8 patients with liver metastases were analyzed. CT simulation was performed with abdominal compression for 1 patient and breath hold (BH) for 7 patients. The radiotherapy treatment was provided through 3 or 5 fractions. Whole liver silhouette was offline delineated on each daily CBCT (acquired in BH in 7 out of 8 patients) and compared to liver contour on planning CT. On the basis of liver/liver match between CT planning and each daily CBCT, liver center of mass shift on 3 axes (x, y and z) was calculated. Dice Similarity Coefficient (DSC), as deformability index, was estimated. Results Breath hold technique was well tolerated and reproduced. Liver deformability was analyzed for all the sample. The standard deviation for the center of mass shift was evaluated for the 3 axes in all the patients, as shown in Table 1. According to the results, the liver deformability is bigger in the – X axe, probably due to the interfraction filling variation of the abdominal hollow organs (stomach, bowel). This was considered as a preliminary result of interfraction Internal Margin (IM) to extend PTV margin. The mean DSC was 0.87. PO-1912 Liver deformability as residual error in stereotactic radiotherapy of hepatic metastases