ESTRO 36 Abstract Book

S132 ESTRO 36 _______________________________________________________________________________________________

Hospital, Radiotherapy department, Amsterdam, The Netherlands 2 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiology department, Amsterdam, The Netherlands Purpose or Objective Quantitative MR imaging provides information on tumor physiology and treatment response. For patients treated in a MR-Linac, it is possible to perform daily quantitative MR imaging. Online quantitative imaging provides valuable input for radiomics studies, and can potentially be used for adaptive dose painting. Due to time constraints in an online setting, it is challenging to obtain accurate images that can give detailed information on tumor environment. Therefore we assessed the accuracy of four quantitative imaging sequences on the MR-Linac which are potentially relevant for radiomics and response assessment: T2 mapping, Dynamic Contrast Enhancement (DCE) including T1 mapping as input for pharmacokinetic modeling, and Diffusion Weighted imaging (DWI). Material and Methods We compared phantom measurements on the MR-Linac with two diagnostic scanners: a 3T Philips Achieva dStream and a 1.5T Philips Achieva. We tested T2 mapping using a slow but accurate Carr- Purcell-Meiboom-Gill sequence on the Eurospin T05 phantom (Eurospin TO5, Diagnostic Sonar, Livingston, Scotland). We compared the results of this sequence with an accelerated sequence optimized to clinically feasible acquisition times by reducing the amount of spin echoes. We tested T1 mapping with an Inversion Recovery series, which is more accurate but too slow to be used in clinical practice. We compared this with the fast and clinically applicable Variable Flip Angle (VFA) T1 mapping technique. For DCE we further tested stability of the signal during a 7 min sequence on a phantom containing different contrast agent concentrations. For DWI, we tested the accuracy of Apparent Diffusion Coefficient (ADC) measurements of water at 0 o C with an Echo Planar Imaging (EPI) sequence and with a Turbo Spin Echo (TSE) DWI sequence, which is less susceptible to geometric distortions. Finally, to demonstrate the image quality and clinical applicability on the MR-Linac, we made T1 and T2 maps of the pelvis on a healthy volunteer. Results The phantom results for all four sequences are shown in figure 1. The accelerated T2 mapping is accurate to within 2% standard deviation on all systems. As expected the VFA sequence shows a bias of 10-15%. This sequence has similar precision (within 10% standard deviation) on all three systems. The DCE sequence on the MR-Linac is shown to be stable over a 7min long dynamic series like the regular MR scanners, with a coefficient of variation of < 1% for all contrast agent concentrations. For diffusion, it shows that the literature value ADC of water of 1.13*10-3 mm 2 /s (Holz et al, 2000) can be accurately measured using both EPI and TSE on the MR-Linac. Figure 2 shows a T1 and a T2 map of the prostate of a healthy volunteer, using a scan with 2x2x2 mm voxels.

and without a strong magnetic field. A magnetic field of 1.5T was used for PRESAGE® and FOX and 1.0T for BANG TM (1.5T was not feasible due to size constraints between the pole pieces for BANG TM ). PRESAGE® was irradiated with doses to 5 Gy, FOX to 8 Gy, and BANG TM to 10 Gy. Calibration curves fitting signal read-out and dose were compared between 0T and 1.5/1.0T for each dosimeter type. Results The optical signal was analyzed for PRESAGE® and FOX, and the spin-spin relaxation rate R 2 (=1/T 2 ) MR signal was analyzed for BANG TM . For all three types of 3D dosimeters, the calibration curves were linear. For PRESAGE®, the percent difference between 0T and 1.5T was 1.5% measured at the spectral peak of 632 nm; for FOX, there was a 1.6% difference at 440 nm and 0.5% difference at 585 nm (R 2 = 1.00 for all optical calibration curves). The greatest percent difference for a given point dose was 5.0% at 2 Gy for PRESAGE®, 2.3% at 6 Gy (440 nm) for FOX, and 5.6% at 2 Gy (585 nm) for FOX. For BANG TM , the percent difference between 0T and 1.0T was 0.7% (R 2 = 1.00). The greatest percent difference for a given point dose was 0.3% at 10 Gy for BANG TM .

Conclusion The same doses calculated for 0T were delivered for both 0T and 1.5/1.0T irradiations; the expected dose difference with the strong magnetic field is up to about 0.5%. Considering this potential dose difference and other uncertainties, the percent differences in response with and without strong magnetic field were minimal for all three 3D dosimeter systems, under 1.6% regarding the full dose response curves and up to 5.6% for a single dose point. All three dosimeter systems have already been used for preliminary investigations on the MR-linac, including electron return effect studies with an air cavity and assessing changes in the field edges with and without the strong magnetic field. This study encourages the continued use of all three types of 3D dosimeters for MR- IGRT applications without needing to apply a correction factor for the signal acquired for any of the above 3D dosimeter systems. OC-0259 Online quantitative imaging on the MR-Linac F. Koetsveld 1 , L.C. Ter Beek 2 , P.J. Van Houd t 1 , L.D. Van Buuren 1 , U.A. Van der Heide 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek