ESTRO 36 Abstract Book

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S477 ESTRO 36 2017
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permits computation of clinically meaningful differences
in context, but differences in spinal cord dose mandating
ART should be a rare event.
PO-0880 Using accumulated delivered dose to predict
rectal toxicity in prostate radiotherapy
L.E.A. Shelley1,2,3, J.E. Scaife1,4, A.M. Bates1,4, J.R.
Forman1,5, K. Harrison1,6 , R. Jena1,4, D.J. Noble1,4, M.A.
Parker 1,6, M.R. Romanchikova1,3, M.P.F. Sutcliffe1,2, S.J.
Thomas1,3, N.G. Burnet1,4
1 Cambridge University Hospitals NHS Foundation Trust,
Cancer Research UK VoxTox Research Group, Cambridge,
United Kingdom
2 University of Cambridge, Department of Engineering,
Cambridge, United Kingdom
3 Cambridge University Hospitals NHS Foundation Trust,
Department of Medical Physics and Clinical Engineering,
Cambridge, United Kingdom
4 Cambridge University Hospitals NHS Foundation Trust,
Department of Oncology, Cambridge, United Kingdo
5 Cambridge University Hospitals NHS Foundation Trust,
Cambridge Clinical Trials Unit, Cambridge, United
Kingdom
6 University of Cambridge, Department of Physics-
Cavendish Laboratory, Cambridge, United Kingdom
Purpose or Objective
Dose-volume tolerances for organs at risk (OARs) adopted
during radiotherapy planning have been historically
derived from normal tissue complication probability
(NTCP) models linking toxicity with planned dose.
On-treatment image guidance facilitates daily tumour
localisation ensuring target coverage. However, the
positional variation of neighbouring OARs is often
disregarded. Anatomical deviations from the pre-
treatment CT due to interfraction motion can introduce
discrepancies between the planned and delivered dose.
One objective of the VoxTox research programme is to test
the hypothesis that delivered radiation dose can be a
stronger predictor of toxicity than planned dose.
Material and Methods
For 109 prostate cancer patients treated with
TomoTherapy® (74Gy/37#), daily megavoltage CT scans
were acquired. An in-house autocontouring algorithm
determines the rectal position, incorporating the effect of
displacement and deformation, and an independent dose
calculation is performed. Processing is fully automated
within the VoxTox study. Dose surface maps (DSMs) of the
rectal wall were generated following the virtual cutting
and unfolding method of Buettner et al [Phys. Med. Biol,
54, 21 (2009)], allowing conservation of spatial dose
information (Figure 1). Daily delivered DSMs were
summated to produce an accumulated DSM (Figure 1b)
over the whole treatment. Planned and accumulated DSMs
were parametrised by calculating 1) the equivalent
uniform dose (EUD) and 2) the ‘DSM dose-width’, the
lateral extent of an ellipse fitted to the largest isodose
cluster, for 7 discrete dose levels between 30 and 75 Gy.
Receiver-operator characteristic curves were plotted,
linking the extracted dose parameters with six patient-
reported clinical endpoints: rectal bleeding, proctitis,
sphincter control, rectal pain, and “How big a problem are
bowels?” (≥Grade 1, ≥Grade 2). Statistical correlations
between planned and accumulated DSMs were compared
using the calculated area under the curve (AUC) presented
on High-Low plots.
Results
For rectal bleeding, the 30, 40, and 60 Gy accumulated
DSM dose-widths were significant predictors (AUC 0.629,
0.621 and 0.643 respectively), where planned dose was
not (Figure 2a). For DSM dose-widths up to 70 Gy, AUC was
greater for accumulated dose than planned dose. EUD was
the strongest predictor of rectal bleeding from both
accumulated (AUC 0.682) and planned (AUC 0.673) DSMs.
The only significant predictor of proctitis was EUD of the
accumulated DSM (AUC 0.673) (Figure 2b). Neither
planned nor accumulated doses were predictive of the
other endpoints