ESTRO 2023 - Abstract Book

S2040

Digital Posters

ESTRO 2023

PO-2268 comparison of the pelvic movement with cone beam CT (CBCT), SGRT, and the CyberKnife x-ray cameras

R. Nachtigall 1 , S. Exner 2 , O. Bislich 1 , F. Behrens 2 , N. Seeto 3 , B. Peters 3 , F. Fehlauer 2

1 Strahlenzentrum Hamburg MVZ GmbH, Medical Physics, Hamburg, Germany; 2 Strahlenzentrum Hamburg MVZ GmbH, Oncologists, Hamburg, Germany; 3 Strahlenzentrum Hamburg MVZ GmbH, Radiotherapists, Hamburg, Germany Purpose or Objective Since the installation of the SGRT-system AlignRT by VisionRT at the “Strahlenzentrum Hamburg MVZ” there is a unique possibility to evaluate the patient movement during the treatments, and the therapy as a whole. Furthermore, the stereoscopic x-ray cameras of the CyberKnife (CK) allow a comparison between three different systems (AlignRT, CBCT, CK cam) with qualitative conclusions about the accuracy of the patient positioning. Materials and Methods This analysis uses finished treatments of the pelvic region of 40 patients per system independently of their prescriptions. The evaluation comprises all treatments as a whole, and is focused on the three translation dimensions: superior-inferior, left-right, and anterior-posterior. Results The mean deviation over all fractions per system is within a millimetre tolerance (sigma_CK(lng/lat/vrt) = 0.83/0.2/0.32; sigma_CB(lng/lat/vrt) = 0.04/0.03/0.01; sigma_SGRT(lng/lat/vrt) = 1.3/0.6/1.0; all in mm). These qualities of accuracy are displayed with the magnitude of the deviations in all the three dimensions mag = √ (lng ² +lat ² +vrt ² ); mag_CK = 0.91mm, mag_CB = 0.05, mag_SGRT = 1.75. Conclusion The combination of SGRT and CBCT is a benefit for the gold standard CBCT since the SGRT assures the CT positioning during the treatment within a 2mm tolerance. It also supports less frequent CBCTs to reduce the radiation load. The influence of the bladder and rectum on the anterior patient surface, as well as possible abdominal motion, has yet to be investigated further. The camera system accuracy of the CyberKnife is between the accuracies of the CBCT and the SGRT. Therefore, it benefits from a constant tracking on bony structures or fiducials during the treatment, and is probably the best choice for its purpose of a low number and a long duration of fractions compared to the conventional linacs. 1 Ente Ospedaliero Cantonale EOC, Radiation Oncology Clinic, Bellinzona, Switzerland; 2 Ente Ospedaliero Cantonale EOC, Medical Physics Division, Bellinzona, Switzerland Purpose or Objective About 5-10% of men undergoing curative radiotherapy for prostate cancer develop radiation cystitis. Adequate bladder filling at simulation and during treatments can reduce bladder irradiation and the consequent risk to develop treatment- related side effects. Bladder scanners can be easily implemented in the treatment workflow to ensure correct bladder filling and treatment reproducibility. Aim of the present study is to assess the accuracy in bladder volume measurements of a commercial available bladder scan. Materials and Methods Data of sixteen consecutive patients with prostate cancer undergoing definitive (13 = 81.3%) or salvage (3 = 18.7%) radiotherapy were analyzed. All men were treated with Volumetric Modulated Arc Therapy with a total dose of 78Gy and 64-66Gy in 2-Gy per fraction for primary and salvage treatments, respectively. Bladder scan measurements were performed immediately prior to the CT simulation or the pre-treatment CBCT scan. The bladder structure was contoured on the acquired images by a radiation oncologist. Bladder volumes estimated with the bladder scan were compared with volumes measured using the planning CT or CBCT scans. Twenty-seven pairs of data were collected and analyzed. Bland-Altman plot and Lin’s correlation concordance coefficient were used to evaluate the accuracy and precision of the bladder scan i10®. The results were compared with the contouring inter-operator variability. The bladder was contoured on 16 images by a radiation oncologist, a medical dosimetrist and a radiotherapy technologist. For each image, the standard deviation of the bladder volumes was calculated. Inter-operator variability was estimated as the mean value of the standard deviations. Results The image-based bladder volumes ranged from 83 to 364 ml. The Bland-Altman plot (figure a) showed a mean difference of -8.6 ml (range: -61.1 - 44.0 ml) in bladder volumes measured with the two modalities. These differences were independent from the bladder volume. The results were confirmed by the Lin’s concordance correlation coefficient that showed bias correction factor Cb= 0.99 (high accuracy) and Pearson correlation coefficient ρ =0.95 (moderate precision). The inter-operator variability was 10.7 ml (1 standard deviation), while the precision of the bladder scan was 26.8 ml (1 standard deviation). The larger range of variability of the measurements could be due to the bladder wall volume that is included in the images but not in the scanner measurements. PO-2269 Bladder filling verification with bladder scan for prostate radiotherapy: how reproducible we are? D.G. Bosetti 1 , G.A. Pesce 1 , S. Leva 1 , F.M. Castronovo 1 , T. Zilli 1 , M. Coppotelli 2 , S. Presilla 2 , M.A. Piliero 2