ESTRO 2020 Abstract Book

S1056 ESTRO 2020

(53ml = 20%) and P5 (63ml = 20%) respectively. The mean volume of 5 patients on ultrasound each day was 347 ml (range 34–682 ml, SD 111 ml), the mean CBCT volume was 366 ml (range 83–726 ml, SD 127 ml) and the mean V2CBCT volume was 299 ml (range 51–626 ml, SD 116 ml) shown in table 1. Significant correlation between V2CBCT and Mean UiCT bladder measurements on ultrasound was found, Pearson Correlation coefficient r= 0.80 and P-Value is < 0.0001. The result is significant at p <0.05. The mean overestimation of the bladder volume comparing ultrasound was 23% and underestimation 23% where compared to the V2CBCT. The mean error in measurement done by RTT was 15% (P1= 12%, P2=26%, P3= 15%, P4= 3% and P5= 18%).

was also investigated by comparing with the patient’s shifts that did not require center couch. In both groups shift from second CBCT and final CBCT were analyzed separately. Two-tailed T-test was performed to evaluate statistical significance. In SRS group patient´s second CBCT and final CBCT shifts were compared to the maximum allowed shift. Results In SBRT group with center couch shifts both second and final CBCT mean shifts were smaller than 1 mm. Standard deviation of shifts is also £ 1 mm. We have found a statistically significant difference between second CBCT vertical and lateral shifts between these fractions where center couch was applied (69) and those where it is not (29). In group where additional immobilization were used all mean shifts were under 1 mm, also standard deviation was £ 1 mm. For second CBCT statistically significant difference was observed for lateral shift between treatment fractions for the patients with customized immobilization (82) and those without (16). In SRS group 176 fraction shifts were analyzed. 25 cases (14,2%) during second CBCT and 49 fractions (27,8%) during final CBCT had shifts larger than tolerance. However, only 3 fraction (1,7%) for second CBCT and 15 (8,5%) for final CBCT had shifts larger than 1 mm or 1 ⁰ . Conclusion Additional immobilization for lung SBRT patients did not improved significantly neither patient position reproducibility, nor position stability during treatment. Although, there was statistical difference between patient positioning with center couch and without, the actual differences were smaller than tolerance. Strict criteria used for SRS treatment resulted in a number of patients being outside of tolerance. Further investigation and analyses is needed. PO-1893 Average body diameter as a predictive factor for the usability of inline CBCT for spine SBRT J. Stam 1 , G. Lim 1 , T. Wiersma 1 , F. Koetsveld 1 1 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Stereotactic body radiation therapy (SBRT) for spine metastasis needs accurate treatment delivery. Our imaging protocol applies CBCT acquired just before (online), and during treatment delivery (inline) for patient positioning and monitoring of intrafraction motion. If the quality of the inline CBCT appears to be insufficient, an extra CBCT needs to be made between arcs and post- treatment. The aim of this study was to investigate if the average body diameter (AD) of the patient can be used to predict the usability of inline CBCT during SBRT for lumbal and sacral spine metastasis. Material and Methods Retrospectively 20 patients with lumbal or sacral spine metastasis were selected. All patients were treated on an Elekta linear accelerator with 10 MV Flattening Filter Free (FFF) dual arc Volumetric Modulated Arc Therapy (VMAT) technique, which had a minimum rotation of 356 degrees. The patients were positioned in a vacuum fixation (VacFix®) and a thoracic support. For all patients at least one inline CBCT was performed. For each patient the quality of the first inline CBCT is categorized as “sufficient” or “insufficient”. Sufficiency is jugded by the Radiation Therapist Technologists, such that the automatic registration algorithm correctly performs the registration. We defined image quality by the parameter contrast-noise ratio (CNR) of a scan as | µ B - µ S |/ σ S , where µ B is the mean voxel value of the body part of the central vertebra, and µ S and σ S are the mean and the standard deviation of the voxel values in the soft tissue surrounding the central vertebra. We determined up to

Conclusion Although the study was based on five patients’ data, results that were obtained are very promising. Results show strong correlation between bladder volume measured on CBCT imaging and urine volume as determined by ultrasound – R = 0.99 and p < 0,0001. The user error was only 15% compared to the actual filling of the bladder. There is no statistical differences between CBCT and USG bladder volume. The measurement made by ultrasound coincided with the decisions made daily by RTT to irradiate the patient. Nevertheless, in order to obtain full and reliable data on the usefulness and use of ultrasound to assess bladder filling in irradiation of prostate cancer patients, it is necessary to extend the study group with new patients. PO-1892 Stereotactic Radiation Therapy – how accurate we are? J. Ter-Minasjan 1 1 North Estonia Medical Centre Foundation, Radiation Therapy Department, Tallinn, Estonia Purpose or Objective The purpose of this study was to evaluate accuracy of stereotactic treatment delivery. Material and Methods Specialised IGRT protocol is used for every type of stereotactic procedure in our clinic. First CBCT evaluates the position of the patient. If position differs by more than 2 mm/2 ⁰ , the patient is repositioned. If position differs by less than 2 mm/2 ⁰ , shifts are applied and second CBCT performed. Second CBCT is performed to check that the shifts were applied correctly and we expect the differences close to 0. According to IGRT protocols allowed differences on second CBCT for lung SBRT is ‹2 mm and for SRS ≤ 0,6 mm and ≤ 0,5 ⁰ . Same criteria applies to the final CBCT which is performed after treatment delivery. 64 patient were included in the study, 19 of whom were lung SBRT patients and remaining brain SRS patients. For SBRT patient group the data was divided in two main groups according to the use of immobilization and need for center couch during CBCT. In immobilization group the patients had either customized vacuum bag or 5 point mask and those who did not have a customized immobilization and being treated using standard lung- board. Some of the patient had to undergo center couch shift before CBCT to safely rotate the gantry and it impact