ESTRO 36 Abstract Book

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S197 ESTRO 36 2017
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vagina to a depth of 3 mm from the mucosal surface. To
account for anisotropy in the longitudinal direction of the
source two points (A1 and A3) were defined at 5 mm from
the cranial applicator surface and additional points (A4-6)
were added (fig 1). The average dose between A1 and A3
should be approximately 100%, with A1 minimal 90% and
A3 maximal 110%. Central evaluation of contours and
treatment plans took place and in case of deviations from
the protocol, feedback was provided and necessary steps
in the dummy-run repeated.
Results
Fifteen centers participated, 12 centers used CT planning,
2 used MRI planning and 1 both. For 11 plans some
adjustments were required, and in 6 cases a second
revision was requested. Main reasons for adjustments
were: delineation (N=8), dose planning (N=7),
reconstruction (N=2). Three different commercially
available treatment planning systems and HDR sources
were used. Table 1 summarizes dose to points A1-6, CTV
and OAR’s of the final submissions. Consistency with the
protocol improved and interobserver differences
significantly decreased with the revisions.
Conclusion
Interobserver variation in delineation resulted in the
largest dose deviations, most pronounced for bowel on
postoperative CT. The use of a second point (A3) at the
apex was most useful for controlling the anisotropy of the
source and should be recommended for dose reporting in
routine clinical practice.
Poster Viewing : Session 8: Radiobiology
PV-0369 The potential of hyperpolarized ¹³C MRS to
monitor the effect of vascular disrupting agents
A. Iversen1, M. Busk 1, L. Bertelsen 2, C. Laustsen2, O.
Munch3, T. Nielsen4, T. Wittenborn1, J. Bussink5, J. Lok5,
H. Stødkilde-Jørgensen2, M. Horsman1
1Aarhus University Hospital, Department of Experimental
Clinical Oncology, Aarhus C, Denmark
2Aarhus University Hospital, Institute for Clinical
Medicine- MR Research Centre, Aarhus C, Denmark
3Aarhus University Hospital, PET Centre, Aarhus C,
Denmark
4Aarhus University Hospital, Center of Functionally
Integrative Neuroscience, Aarhus C, Denmark
5Radboud University Medical Center, Department of
Radiation Oncology, Nijmegen, The Netherlands
Purpose or Objective
Targeting tumor vasculature with vascular disrupting
agents (VDAs) is attractive. Since treatment effects
precedes tumor shrinkage, ways of detecting metabolic
changes to assess treatment efficacy are warranted.
Positron emission tomography (PET) using
fluorodeoxyglucose (FDG) is currently a first-choice
imaging approach for early assessment of metabolic
changes during treatment. However, hyperpolarized ¹³C
magnetic resonance spectroscopy (MRS) is more refined
since it allows dynamic measurements of the metabolism
of 13C-labeled substrates in vivo. The aim of this study is
to investigate the potential of hyperpolarized ¹³C MRS to
monitor the vascular changes induced by combretastatin-
A4-phosphate and it structural analogue OXi4503.
Material and Methods
The VDAs combretastatin-A4-phosphate (CA4P) and
OXi4503 were tested in mice bearing subcutaneous C3H
mammary carcinomas. Hyperpolarized [1-¹³C]pyruvate
was intravenously injected while hyperpolarized ¹³C MRS
was performed with a 9.4 T MRI scanner and parameters
of interest was calculated. Other, similarly treated, mice
were PET scanned using a nanoScan Mediso PET/MRI
scanner following administration of FDG. Ultimately,
metabolic imaging results were compared to direct
measures of vascular damage derived from dynamic
contrast-agent enhanced magnetic resonance imaging
(DCE-MRI) and histological analysis and to the clinical
relevant endpoint tumor regrowth delay.
Results
Treatment efficacy was confirmed by DCE-MRI, tissue and
tumor growth analysis, which revealed profound vascular
damage and associated changes in blood-flow-related
parameters, cell death and slowed tumor growth. FDG-
PET revealed early detectable changes in signal, which
may reflect true changes in glucose metabolism, impaired
FDG delivery or a mixture of both. Nonetheless, the ratio
of [1-¹³C]lactate/[1-¹³C]pyruvate area under the curve
(AUC ratio) and the lactate time-to-peak (TTP), calculated
from hyperpolarized ¹³C MRS, was unaffected by
treatment.