ESTRO 36 Abstract Book

S197 ESTRO 36 2017 _______________________________________________________________________________________________

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.

Poster Viewing : Session 8: Radiobiology

PV-0369 The potential of hyperpolarized ¹³C MRS to monitor the effect of vascular disrupting agents A. Iversen 1 , M. Busk 1 , L. Bertelsen 2 , C. Laustsen 2 , O. Munch 3 , T. Nielsen 4 , T. Wittenborn 1 , J. Bussink 5 , J. Lok 5 , H. Stødkilde-Jørgensen 2 , M. Horsman 1 1 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus C, Denmark 2 Aarhus University Hospital, Institute for Clinical Medicine- MR Research Centre, Aarhus C, Denmark 3 Aarhus University Hospital, PET Centre, Aarhus C, Denmark 4 Aarhus University Hospital, Center of Functionally Integrative Neuroscience, Aarhus C, Denmark 5 Radboud 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 13 C-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.

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.