Conformity analysis to demonstrate reproducibility of target volumes for Margin-Intense Stereotactic Radiotherapy for borderline-resectable pancreatic cancer

Daniel L P Holyoake, Maxwell Robinson, Derek Grose, David McIntosh, David Sebag-Montefiore, Ganesh Radhakrishna, Neel Patel, Mike Partridge, Somnath Mukherjee, Maria A Hawkins, Daniel L P Holyoake, Maxwell Robinson, Derek Grose, David McIntosh, David Sebag-Montefiore, Ganesh Radhakrishna, Neel Patel, Mike Partridge, Somnath Mukherjee, Maria A Hawkins

Abstract

Background and purpose: Margin-directed neoadjuvant radiotherapy for borderline-resectable pancreatic cancer (BRPC) aims to facilitate clear surgical margins. A systematic method was developed for definition of a boost target volume prior to a formal phase-I study.

Material and methods: Reference structures were defined by two oncologists and one radiologist, target structures were submitted by eight oncologist investigators and compared using conformity indices. Resultant risk of duodenal bleed (NTCP) was modelled.

Results: For GTV, reference volume was 2.1cm3 and investigator mean was 6.03cm3 (95% CI 3.92-8.13cm3), for boost volume 1.1cm3 and 1.25cm3 (1.02-1.48cm3). Mean Dice conformity coefficient for GTV was 0.47 (0.38-0.56), and for boost volume was significantly higher at 0.61 (0.52-0.70, p=0.01). Discordance index (DI) for GTV was 0.65 (0.56-0.75) and for boost volume was significantly lower at 0.39 (0.28-0.49, p=0.001). NTCP using reference contours was 2.95%, with mean for investigator contour plans 3.93% (3.63-4.22%). Correlations were seen between NTCP and GTV volume (p=0.02) and NTCP and DI (correlation coefficient 0.83 (0.29-0.97), p=0.01).

Conclusions: Better conformity with reference was shown for boost volume compared with GTV. Investigator GTV volumes were larger than reference, had higher DI scores and modelled toxicity risk. A consistent method of target structure definition for margin-directed pancreatic radiotherapy is demonstrated.

Trial registration: ClinicalTrials.gov NCT02308722.

Keywords: Borderline-resectable; Pancreatic cancer; Radiation therapy quality assurance; SBRT; Target volume definition.

Copyright © 2016 The Authors. Published by Elsevier Ireland Ltd.. All rights reserved.

Figures

Fig. 1
Fig. 1
Definition of target volumes for SPARC trial. Orange = GTV, Green = VesselContact, Red = Boost volume.
Fig. 2
Fig. 2
Reference (orange = GTV, red = boost volume) and investigator contours (green) of boost volume (images a and c) and GTV (image b) on contrast-enhanced axial CT of patient with borderline-resectable pancreatic carcinoma.
Fig. 3
Fig. 3
Investigator target structure volumes compared to reference.
Fig. 4
Fig. 4
Mean conformity indices for investigator target structures when assessed against reference (error bars show 95% confidence interval).
Fig. 5
Fig. 5
NTCP (risk of duodenal bleed) according to Discordance Index.

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