Part I | Chapter 2 40 Dosimetric analysis To estimate the effect on the CTV dose for outliers that needed more extensive, potentially clinically relevant adaptations as judged by Observer 3, dose-volume-histogram (DVHs) were calculated for the RTT- and Observer-adapted contours, using the corresponding (RTT contour-based) online dose distribution. Also, CTV D99% (dose to 99% of the volume) was calculated. These analyses were performed using Volumetool®.6 Results The contour adaptation times for the RTTs are presented in Figure S1 (Supplementary A). Mean (SD) contour adaptation time was 12.6 (± 3.8) min. Adaptation results from the two independent observers are presented in Table 1. Observer 1 Observer 2 n % n % Adaptations performed 60 50.0 58 48.3 Location of adaptation Apex 26 21.7 37 30.8 Base 39 32.5 30 25.0 Mid-prostate 5 3.3 2 1.7 Seminal vesicles 3 2.5 4 3.3 Observer 1 and 2 adapted 60 (50.0%) and 58 (48.3%) contours, respectively, while 50 (41.7%) contours were adapted by both. Observer 1 adapted none of the contours in seven patients (23.3%). For Observer 2, this was the case in ten patients (33.3%). Most adaptations were performed in the apex and base region of the prostate and generally consisted of adjusting, adding, and/or removing one to three slices. Figure 2 shows the relative volume differences per patient by observer, showing a median (interquartile range [IQR]) relative volume difference of 9.5% (4.3-13.6) in the RTT group, 9.1% (4.4-12.7) for Observer 1, and 9.3% (4.5-13.0) for Observer 2. Median (IQR) interobserver DSC between RTTs and Observer 1, RTTs and Observer 2, and Observer 1 and 2 was 0.99 (0.98–1.00), 1.00 (0.98–1.00), and 1.00 (0.99–1.00), respectively (Figure 3). RTT contours from fraction 2 to 5 were acceptable for clinical use in 113 (94.2%) fractions as judged by Observer 3. Figure 4 shows the seven remaining ‘outlier’ fractions in which larger adaptations were needed. DVHs for four exemplary outlier cases are displayed in Figure 5 and CTV D99% is presented in Table S1 (Supplementary B). Significant CTV under dosage was observed for one of the seven outliers (patient 4, fraction 4), with a D99% for the adapted CTV contours of 33.5 Gy (Observer 1) and 33.8 Gy (Observer 2) compared to 35.8 Gy for the RTT contour. Table 1 – Total number of Clinical Target Volume (CTV) contours in which adaptations were performed and number of adapted CTV contours per anatomical location, separately for independent Observer 1 and 2 (total number of contours/fractions = 120).
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