ESTRO 2022 - Abstract Book

S1504

Abstract book

ESTRO 2022

the patient in one fraction was assessed by applying different rotational systematic errors to the dose matrices. Intrafractional variations of these rotations were also applied based on data from our patient population. For each treatment, errors from -10 to 10 degrees in pitch and from -5 to 5 degrees in both roll and yaw were tested. Thus, for each of these pitch/roll/yaw combinations, the D98 on both prostate and SV CTVs were obtained, while D1cc and Dmax of the urethra was also included in the analysis. The mean values of the 17 treatments at each point were fitted to a second grade 3D polynomial. Results A 2D representation of fitted values of prostate D98 and urethra D1cc at each plan are shown in Figures 1 and 2. All fits yielded R2 values of 0.97, 0.97, 0.99 and 0.93 for D98 of prostate, D98 of SV, D1cc of urethra and Dmax of urethra respectively. Results depended on the volume analysed, prostate D98 was above the prescription dose of 36.26 Gy for all the rotations analysed, with dose being almost independent of roll rotation. In the case of SV D98, pitch and yaw rotations of ±5 degrees implied D98 less than 36.25 Gy but it was still over 34.4 Gy (90% of prescription dose) for rotations within ±7.5 degrees. Again, SV D98 showed less dependency with roll rotations for which ±10 degrees still assure a D98 of 36.25 Gy. Regarding the urethra, Dmax showed little variation with rotational errors, with increases of up to 0.5 Gy for all rotations analysed, while D1cc of the urethra could be higher than its constraint (38.78 Gy) for pitch and yaw rotations of ±7.5 degrees. Nevertheless, for pitch values of more than 10 degrees D1cc was still below 39 Gy.

Conclusion We have developed a useful tool to decide whether it is safe to treat a prostate SBRT without rotational corrections for a given fraction with Cyberknife. Nevertheless, if large errors appear systematically for more than one fraction, an investigation of the reasons should be performed and replanning should be considered.

PO-1705 Characterization of injectable marker in a low tesla MR-Linac for online tracking of liver tumour

L. Placidi 1 , M. Nardini 1 , D. Calatayud 2 , A. Capotosti 1 , D. Cusumano 1 , R. Moretti 1 , M.A. Gambacorta 1 , V. Valentini 1 , L. Indovina 1 , L. Boldrini 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, Radiation Oncology, Rome, Italy; 2 NANOVI A/S, Diplomvej 378, NANOVI, Lyngby, Denmark Purpose or Objective Magnetic Resonance guided Radiotherapy (MRgRT) allows an online gating of the tumor during treatment delivery. It can occur that due to image resolution and low contrast, lesions are not always visible and online gating could be compromised. A novel liquid injectable fiducial marker has been characterized in a low tesla MR-Linac system. Materials and Methods The evaluated fiducial marker (BioXmark®) is a liquid, sticky and soft iodine-based marker enabling an implantation of size customizable markers in one injection. Multi-imaging modalities marker evaluation has been performed implanting eleven markers (3 of 0.05 ml, 4 of 0.10 ml, 3 of 0.20 ml and 1 of 0.30 ml) in a cow liver. Different imaging modalities have been acquired: orthogonal kV, cone beam CT (CBCT), CT and 0.35T TRUFI MRI. Additionally, a cine MRI (8 frames per second) has been acquired. Spatial resolution accuracy has been evaluated. Marker suitability for MRgRT gating system has been performed placing different marker sizes (0.10,0.20 and 0.30 ml) in different vials, filled with hand gel (65% ethanol) and minced beef, as example of respectively high and low contrast. The vials have been placed on the belly of a volunteer mimicking a breath hold treatment, to evaluate the visibility of the marker using online MR imaging. Tracking confidence values (TCV) was used to quantify the tracking quality of the marker, once the marker was completely inside the tracking boundary.