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transformation parameters, we created ‘voxel histories’
for the spinal cord relative to the planning CT, and
calculated delivered dose. Maximum planned and
delivered spinal cord dose (D
2%
) were then compared.
Results
A summary of auto-contouring algorithm performance is
shown in Table 1. Auto-contouring performance appeared
comparable to manual segmentation, and we proceeded
to calculate delivered dose. These results are shown in
Figure 1 (A-C). Fig. 1A shows a waterfall plot of planned
D
2%
minus delivered D
2%
for each patient. Mean spinal cord
D
2%
was 35.96Gy (planned) and 36.01Gy (delivered), and
the mean absolute difference between planned and
delivered dose was 1.1Gy (3% of mean planned D
2%
).
Differences between planned and delivered dose were
plotted as a histogram, which appears to be normally
distributed around the mean difference (Fig 1B). The
mean difference (µ, -0.05) and standard deviation (σ,
1.448) were used to approximate a normal distribution to
this data – as shown in Fig.1C. Using this model, a z
statistic can be calculated for a chosen difference (e.g.
Prob. of delivered D
2%
being 4Gy higher than planned is
2.5%).
Conclusion
Differences between planned and delivered D
2%
to the
spinal cord in patients receiving daily IG are small in HNC
patients treated with daily IG on TomoTherapy. Our model
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. Shelley
1,2,3
, J.E. Scaife
1,4
, A.M. Bates
1,4
, J.R.
Forman
1,5
, K. Harrison
1,6
, R. Jena
1,4
, D.J. Noble
1,4
, M.A.
Parker
1,6
, M.R. Romanchikova
1,3
, M.P.F. Sutcliffe
1,2
, S.J.
Thomas
1,3
, N.G. Burnet
1,4
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.