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Session Item

Tuesday
December 01
08:30 - 09:10
Interdisciplinary Stream 1
Spatially fractionated GRID radiotherapy - rationale and promise
Teaching Lecture
Interdisciplinary
10:30 - 10:38
Dosimetric impact of central OAR review on rectal and bladder constraint attainment in PACE-B trial
PH-0602

Abstract

Dosimetric impact of central OAR review on rectal and bladder constraint attainment in PACE-B trial
Authors: Brand|, Douglas(1)*[dhbrand@gmail.com];Tree|, Alison(1);Fernandez|, Katie(2);Naismith|, Olivia(3);Brueningk|, Sarah(2);Hall|, Emma(4);Gulliford|, Sarah(5);van As|, Nicholas(1);On behalf of the PACE Trial Management Group|, x(4);
(1)The Royal Marsden NHS Trust & The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom;(2)The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom;(3)The Royal Marsden NHS Trust & The National Radiotherapy Trials Quality Assurance RTTQA group, Radiotherapy, London, United Kingdom;(4)The Institute of Cancer Research, Clinical Trials and Statistics Unit, London, United Kingdom;(5)The Institute of Cancer Research & University College London Hospital, Radiotherapy, London, United Kingdom;
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Purpose or Objective

PACE-B (NCT01584258) is a randomised phase III trial comparing SBRT, 36.25 Gy in 5 fractions (Fr), to conventionally or moderately hypofractionated radiotherapy (CFMHRT), either 78 Gy in 39 Fr or 62 Gy in 20Fr for localised prostate cancer. Most dose volume constraints specified in the trial are based on relative OAR volume. We hypothesised that delineation of OARs by participating centres may differ from per protocol definition and that this may influence dose constraint attainment.

Material and Methods

305/874 PACE-B participants with DICOM data available for centralised review at the time of the study were included. The bladder and rectum contours (original OARs) were checked and edited to protocol definition (reviewed OARs) by a single observer [KF], who was quality assured (QA) by two experienced clinicians [DB, AT].  Both organs were solid structures; the rectum was from anus to rectosigmoid junction. DVHs for original and reviewed OARs were calculated.  The proportions of patients with any major dose constraint variation (separately for bladder and rectum), were compared for original versus reviewed OARs by exact McNemar test. Major dose constraint variation thresholds were: CFMHRT rectum (Vconventional/Vmoderatehypo): V50/V40Gy>60%, V60/V48Gy>50%, V65/V52Gy>30%, V70/V56Gy>25%, V75/V60Gy>5%. CFMHRT bladder: V50/V40Gy>50%, V60/V48Gy>25%, V74/V59Gy>15%; SBRT rectum: V18.1Gy>50%, V29Gy>20%, V36Gy>2cc; for bladder: V18.1Gy>40%, V37Gy>20cc. OAR volumes were compared by Wilcoxon signed-rank. Differences between original and reviewed OARs across the cumulative dose-volume curve were visualised by: i) rescaling dose so 100% = prescribed PTV dose [x-axis]; ii) subtracting dose bin volumes for original OAR from those of the reviewed OAR [y-axis] (forming the difference between the two cumulative DVH lines).

Results

For the rectum, significantly fewer original contours (12/305, 3.9%) had major variations compared to reviewed (45/305, 14.8%)  (p<0.001). This was not seen for bladder original (12/305, 3.9%) vs reviewed (10/305, 3.3%) contours (p=0.50). Rectal volumes were larger for original (60.2cm3 IQR 50.4-72.2cm3) vs reviewed (53.2cm3, IQR 45.2-64cm3) contours (p<0.001). Bladder volumes were slightly larger for original (272cm3, IQR 180-391cm3) vs reviewed (271cm3, IQR 179-393cm3) contours (p<0.001). Larger differences were seen across the dose-normalised DVH difference curve for rectum (Figure 1.A) than bladder (Figure 1.B).

Figure 1.A Rectum


Figure 1.B Bladder

Figure Caption 
Difference between original and reviewed OAR relative cumulative DVHs. Shown for rectum (A) and bladder (B). Dose is rescaled so prescription dose = 100%. Legend shows relation to dose variations. Thick black line = mean difference.

Conclusion

Central review of OAR contours significantly increased the number of treatment plans which failed relative cumulative rectal dose-volume constraints. Centres may benefit from more detailed contouring guidelines and ongoing QA during trial conduct, to ensure OAR consistency.