ESTRO 2021 Abstract Book

S1285

ESTRO 2021

F. La Fauci 1 , G. Marvaso 1,2 , M. Augugliaro 1 , S. Comi 1 , M. Pepa 1 , M. Zaffaroni 1 , G. Corrao 1,3 , F.A. Mistretta 4 , S. Gandini 5 , G. Petralia 6,3 , F. Cattani 1 , O. De Cobelli 7,3 , R. Orecchia 8 , B.A. Jereczek-Fossa 1,3 1 IEO, European Institute of Oncology IRCCS, Radiation Oncology, Milan, Italy; 2 University of Milan, Departement of Oncology and Hemato-Oncology, Milan, Italy; 3 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy; 4 IEO, European Institute of Oncology IRCCS, Urological Surgery division, Milan, Italy; 5 IEO, European Institute of Oncology IRCCS, Department of Epidemiology and Biostatistics, Milan, Italy; 6 IEO, European Institute of Oncology IRCCS, Department od Radiology, Milan, Italy; 7 IEO, European Institute of Oncology IRCCS, Urological Surgery Department, Milan, Italy; 8 IEO, European Institute of Oncology IRCCS, Scientific Directorate, Milan, Italy Purpose or Objective One of the most important features of radiotherapy is the evaluation of the accuracy of the treatment delivery through quantification of the error due to set up inaccuracies, reproducibility of patient positioning and intra- or inter-fraction uncertainties due to organ motion. These modifications can affect the actual dose distribution in patient’s treatment plans, altering the clinical outcome and having different clinical effects in terms of toxicity on OARs. Geometric and anatomical variations have a major impact on SBRT treatments, where high gradient dose in few fractions and small fields are applied, so better accuracy of the delivery treatment is required. The purpose of this research is to evaluate the OARs displacement by simulating inter- fraction variability in SBRT treatments. Materials and Methods Patients with lymph nodes oligorecurrent prostate cancer treated with SBRT technique between May 2012 and October 2015 on the VERO system and planned with Iplannet (version 4.5.3 by BrainLab®) were included in the study. To evaluate inter-fraction variability, OARs were delineated both on planning CT and on rigidly co- registered CBCTs using RayStation (RaySearch) and the union volume for each one was computed (V U ). For each OAR, a new contour was created by sequentially applying a margin of 3, 5, 8, 10, 15, 20 mm (V E ) and the percentage volume (V % ) of the intersection between V E and V U with respect to V U was computed. The evaluation of organ motion or deformation between the OARs contoured on planning CT and on CBCTs was performed on RayStation using Dice similarity coefficient (DSC) and maximum distance to agreement (Max_DA).

Results A total of 35 patients were included in the study. Among the considered OARs, the highest agreement was reported for the femoral head (DSC = 0.77; Max_DA = 1 cm) and the lowest for colon (DSC = 0.34 and Max_DA = 1.49). All the other OARs showed an intermediate behaviour ( Table 1 ). The minimum value of margin, expressed in mm, which ensured a V% higher than 95%, for femoral head, cauda, bladder, ileum, colon and rectum was 3 (97.1%), 5 (97.2%), 8 (97.0%), 8 (95.2%), 15 (96.1%) and 8 (95.4%), respectively.

Conclusion Considering DSC values, colon and ileum resulted the most critical OARs considering the DSC and Max_DA similarity indexes. Considering the V%, they need a margin of 15 and 8 mm respectively to reach the 95% threshold. This can be ascribed not only to organ motion but also to the different shape of the organs, as showed by the lower DSC. Anatomical differences concerning bladder and rectum can be mainly related to their filling status and hence to patient’s preparation. Future prospective foreseen the plans to be recalculated with higher dose prescription per fraction to evaluate constrains compliance. The present study is an ancillary study of RADIOSA clinical trial (AIRC IG-22159) and this preliminary investigation will be used as a benchmark for analyzing prospective results from the trial.