ESTRO 2020 Abstract Book

S1055 ESTRO 2020

kV CBCT are equipped in TomoTherapy and linear accelerator respectively. Recently, the latest generation of TomoTherapy named Radixact ® X9 has been introduced to Hong Kong. However, the comparison between the novel MVCT and kV CBCT has not been conducted. The purpose of this study was to compare the verification accuracy, feasibility to be used for adaptive radiation therapy and imaging dose between MVCT and kV CBCT. Material and Methods Planning computed tomography (pCT) images of a respiratory motion phantom (QUASAR™) were acquired with and without motion. Known deviations from the pCT reference position were introduced and drawn onto the phantom in three translational and three rotational directions. The assigned deviated positions were automatically verified with MVCT, 3D kV CBCT and Symmetry TM 4D kV CBCT images. The discrepancies between the detected and actual positional shifts (Absolute Mean Detection Errors (δD )) were analyzed statistically. CATPHAN ® 700 was used to evaluate image quality while anthropomorphic phantom was utilized to compare the calculated dose agreement to pCT based on a nasopharyngeal treatment plan between two verification images. MOSFET dosimeters were placed in the anthropomorphic phantom to measure the dose to selected depths by the two verification systems.

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

Results Regardless of motion, verification by 3D or 4D kV CBCT showed significantly smaller mean δDs than MVCT in all directions except yaw rotation. The image quality of both verification modalities was not optimal enough for adaptive radiation therapy (ART) use. Various dosimetric parameters and dose-volume histograms (DVHs) showed that the doses calculated with MVCT and kV CBCT were overestimated and underestimated respectively as compared to that with pCT. Imaging dose was found to depend on the scanning protocol or pitch-reconstruction interval combination selected. For MVCT, dose increased with depth since MV radiation is more penetrating than kV.