ESTRO 36 Abstract Book

S85 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Kilovoltage cone-beam computed tomography (kVCBCT) has often been regarded as the preferred imaging modality for the visualisation of soft tissues and verification of treatment position due to its superior spatial resolution [1-3]. Transperineal ultrasound (TPUS) is an alternative imaging tool that can be employed for pre-treatment verification and in-treatment monitoring as it is non- invasive and does not involve additional imaging dose [4, 5]. This study aimed to compare the daily inter-modality derived setup shifts using TPUS versus kVCBCT (gold standard) for prostate radiotherapy. Material and Methods A total of 1927 paired datasets (TPUS versus kVCBCT) from 55 patients were compared in three directions (i.e. x,y,z shifts representing left/right, anterior/posterior and superior/inferior directions respectively). The derived setup shifts were reported to the nearest mm. Data were analysed using PASW for windows, version 20.0 (SPSS Inc, Chicago, IL). Observed differences in the derived shifts for each imaging modality were reported. Statistical tests were conducted under a two-tailed significance level, at a minimum 95% confidence interval. Results A Shapiro-Wilk test revealed that the data was not normally distributed (p<0.05). A non-parametric Wilcoxon Signed Ranks test demonstrated no statistically significant difference between the derived setup shifts from TPUS and kVCBCT for all planes; x (p= 0.376), y (p=0.244) and z (p=0.253). The proportion (%) of datasets where the difference in the derived shifts between the two imaging modalities were within 5/4/3mm in the x, y and z directions are reported in Table 1. Spearman’s rank correlation coefficients of the derived shifts were moderate (0.612-0.671) for all three directions (p<0.005), signifying that the accuracy of TPUS-derived setup shifts was comparable to kVCBCT. Table 1: Proportion of datasets where the difference in the derived shifts between the two imaging modalities were within 5/4/3mm in the x, y and z directions.

OC-0166 Fast 3D CBCT imaging for Lung SBRT: Is image quality preserved ? B. De Rijcke 1 , R. Van Geeteruyen 1 , E. De Rijcke 1 , Y. Lievens 1 , E. Bogaert 1 1 Ghent University Hospital, Radiation Oncology, Gent, Belgium Purpose or Objective Irradiation of Early Stage Non-Small Cell Lung Cancer (ES- NSCLC), through Stereotactic Body Radiotherapy (SBRT) requires image guidance. At our institute double pre- treatment CBCT, with manual registration is performed at every fraction. Speeding up CBCT gantry rotation and implementation of automated registration allows for faster decision taking. It also offers the possibility of intrafraction CBCT, without severe prolongation of treatment time. In a first step we investigated the image quality and performance of a CBCT protocol with lower dose and faster acquisition time. Material and Methods Standard (S) and Fast (F) scan protocols only differed in gantry speed (180°/min (S) and 360°/min (F)) and were performed on XVI Elekta ® CBCT. For six patients receiving lung SBRT (60Gy in 3 or 4 fractions) for upper lobe ES- NSCLC, dual pre-treatment imaging consisted of a S scan followed by a F scan. This resulted in 17 useful S and F image sets. Tumor movement amplitude stayed below 1cm (1) , removing the necessity for 4D-CBCT. All CBCT images were retrospectively exported to Raystation ® (RaySearch Laboratories, Sweden) for easy and blended side-by-side evaluation. The resolution was 1x1x1mm 3 for all scans. All CBCT images were matched to planning CT. WW/WL was set fixed per patient. Zooming was allowed. Visual Grading Analysis (VGA) comprised well defined criteria over the three planes (T, C, S), categorized in three Image Quality (IQ) Focus groups: bony anatomy (N=11), tumor characteristics (N=3) and anatomical landmarks (N=7). Examples are: visualization of corpus vertebrae (C, S plane), tumor edge (3 planes); carina bifurcation (C, T plane). Scoring was done independently by 3 routined RTTs. Possible answers were: equal, better or worse for ‘upper’ scan (randomly assigned to F or S). Data were analyzed using SPSS software v24 (IBM Corp., New York, NY). Results In 73.7 % of all cases, visualization of anatomical structures was appreciated equally on S and F scans. When differences emerged, visualization on F scan was appreciated more in 71.3 % of the cases (71.8 % for bony anatomy, 75.0 % for tumor characteristics and 67.2 % for anatomical landmarks). Binary Logistic Regression in these cases did not reveal significant dependence on patient (for which BMI or tumor location are most relevant; however not evaluated separately) (p=0,638), not on IQ focus group (p=0,540) and not on reader (p=0,883). Thus, in 92.4 % of all cases, image quality was scored equal or better for fast imaging protocol compared to the standard protocol (Figure 1).

Conclusion Measured differences were acceptable considering the planning target volume (PTV) margin expansion was 10mm in all directions, except posteriorly (6mm). Findings were in agreement with the recent report by Trivali et al. [6] who found no significance difference in the x, y and z coordinates between TPUS and fiducial-based CT localisation of the prostate gland. With specialised training and user experience, TPUS is a promising imaging modality in treatment setup and verification for prostate radiotherapy without the need for additional exposure to ionising radiation.