ESTRO 2022 - Abstract Book

S1257

Abstract book

ESTRO 2022

Conclusion A new metric to define the need of online ART was proposed based on body variation measured along the AP direction through the C3-C4 disc: if such value results > 3 mm the treatment fraction has to be considered needed of ART (92% of probability of not meeting the tolerance criteria). If confirmed on larger cohorts, this strategy could be used as indirect metric to establish the optimal timing of replanning also in center not equipped with online ART technologies

PO-1482 Estimation of inter-fraction motion of pelvic organs in SBRT treatments of prostate oligometastases

M. Pepa 1 , G. Corrao 2 , G. Morigi 3 , F.M. La Fauci 4 , M. Zaffaroni 5 , M.G. Vincini 1 , M. Augugliaro 1 , G.C. Mazzola 2 , S. Comi 6 , F.A. Mistretta 7 , S. Luzzago 7 , S. Gandini 8 , G. Musi 9 , G. Petralia 10 , O. De Cobelli 9 , R. Orecchia 11 , F. Cattani 4 , G. Marvaso 2 , B.A. Jereczek-Fossa 2 1 IEO European Institute of Oncology IRCCS, Division of Radiation Oncology, Milan, Italy; 2 University of Milan; IEO European Institute of Oncology IRCCS, Department of Oncology and Hemato-Oncology; Division of Radiation Oncology, Milan, Italy; 3 University of Milan, Department of Oncology and Hemato-Oncology, Milan, Italy; 4 IEO European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 5 IEO European Institute of Oncology IRCCS, DIvision of Radiation Oncology, Milan, Italy; 6 IEO European Insitute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 7 IEO European Institute of Oncology IRCCS, Division of Urology, Milan, Italy; 8 IEO European Institute of Oncology IRCCS, Department of Experimental Oncology, Milan, Italy; 9 University of Milan; IEO European Institute of Oncology IRCCS, Department of Oncology and Hemato-Oncology; Division of Urology, Milan, Italy; 10 University of Milan; IEO European Institute of Oncology IRCCS, Department of Oncology and Hemato-Oncology; Division of Radiology, Milan, Italy; 11 IEO European Institute of Oncology IRCCS, Scientific Direction, Milan, Italy Purpose or Objective The advent of stereotactic body radiotherapy (SBRT) has led to a high standard of precision and accuracy in dose delivery. However, modifications in the patient anatomy and organ-positioning affect the actual dose distribution to the lesion and to neighboring organs at risk (OARs), with possible impact on oncological and toxicity outcomes. The purpose of this study is to derive geometric and volumetric indexes representative of inter-fraction organ motion. Materials and Methods This study considers a subset of patients enrolled within the prospective phase II study sponsored by AIRC RADIOSA trial (NCT03940235) who treated pelvic lesions. For each patient and treatment fraction, OARs for each patient were contoured by clinicians on the simulation CT (sCT). Successively, the CBCTs were imported in RayStation (Raysearch®) and registered with the sCT and all the relevant OARs inside the field of view were contoured. Organ motion was then estimated in five significant OARs close to the target (right femoral head, left femoral head, bladder, rectum, iliac wing) by (i) means of geometrical evaluations, by computing the Dice similarity coefficient (DSC) and the maximum distance to agreement (MAX_DA), and (ii) volumetric considerations, by evaluating the percentage volume (V%) of an OAR in the sCT that remains outside the union volume of all the possible positions of the same OAR contoured on the CBCTs ( Table 1, right ). The same analysis was repeated for the gross tumour volume (GTV). Results A total of 17 patients were analysed; geometric indexes, namely DSC and MAX_DA, did not show any definitive trend of motion of organs ( Figure 1 ). On the other hand, median V% highlighted bladder and rectum as the OARs more prone to motion ( Figure 2a ). Regarding GTV, median DSC resulted in 0.6065 (IQR 0.48075 – 0.7585); median MAX_DA indices, 0.562 (IQR 0.371 – 0.778), suggested limited levels of displacement, however a median V% of 88.40% (IQR 22.07 – 233.33) was identified as the worst among all structures. Bony structures as iliac wing and femoral heads showed smallest V% values. Figure 1. Dice similarity coefficient (DSC) (1a) and maximum distance to agreement (MAX_DA) (1b) regard five considered organs at risk (OARs).