ESTRO 2020 Abstract book

S1109 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%).

Conclusion Kilovoltage CBCT showed generally better verification performance with its higher contrast resolution. HU adjustment techniques or algorithms may be applied on kV CBCT and MVCT in future studies to investigate any improvement in HU calibration and dose agreement to pCT. Special attention on the additional accumulated imaging doses is suggested to certain organs-at-risk (OARs), especially to those with lower dose tolerance. It is noted that a balance between the scan time, image quality and imaging dose have to be achieved and the intent of treatment has to be concerned before selecting which scanning mode to be used in clinical situations. PO‐1891 Bladder filling assessment in prostate cancer patients using ultrasound bladder measurements K. Kaczmarek 1 , B. Bak 1 , D. Radola 1 , T. Winiecki 1 , T. Bajon 1 1 Greater Poland Cancer Centre, Radiotherapy Department II, Poznan, Poland Purpose or Objective Preliminary analysis of bladder filling assessment in prostate cancer patients using the Echoson® device for automatic measurement of the amount of urine in the patient's bladder. Evaluation of the suitability of the device and compatibility between the measured value on ultrasound and bladder filling on CBCT scans during RT. Material and Methods The first validation measurements were performed on a group of 5 patients (P1-P5) treated for prostate cancer. All patient received TD= 50 Gy in 2Gy fraction (25fr) using VMAT (volumetric arc therapy) radiotherapy between August and October 2019. Before iCT (initial computed tomography) scans and during RT (radiotherapy), the patients were asked to drink 3 cups of water (500-600 ml). Three ultrasound measurements were performed, and the mean volume UiCT (ultrasound initial computed tomography) was calculated daily during the whole treatment period on a therapeutic device just before CBCT (cone beam computed tomography) imaging. Then the radiation therapist (RTT) contoured the bladder on CBCT scans and cut off a 2mm margin from the outer bladder wall (V 2 CBCT) to determine the actual amount of urine in the bladder. Results Total of 375 (mean 125) ultrasound bladder filling measurement was performed. On 125 CBCT scans bladder was contoured and V2CBCT from the outer bladder wall was calculated. Pearson Correlation between CBCT bladder contour and V2CBCT was significant, R = 0.99 and P-Value is < 00001. The result is significant at p <05. Mean differences between CBCT bladder contour and V2CBCT was P1 (66 ml = 17%), P2 (85ml = 26%), P3 (69ml = 16%), P4

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/2⁰, the patient is repositioned. If position differs by less than 2 mm/2⁰, 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