ESTRO 37 Abstract book

ESTRO 37

S538

ADC map. These data were also acquired for 9 distinct patients on a conventional diagnostic 3T Achieva scanner (Philips Healthcare, Best, The Netherlands) for comparison. Test-retest data was registered using deformable registration. The within-subject voxel-by- voxel coefficient of variation (wCV) was calculated for the set of T 2 - and ADC maps for each patient. A log transform was performed when the variation between the two maps depended on the mean value. The images were smoothed with an increasing kernel size and the wCV was calculated to assess the dependence of the wCV on ROI size. At voxel level, the wCV is mainly influenced by imaging noise, which is reduced by smoothing, leaving the effect of daily variations in the patient and machine. Results The quality of MR-Linac images is represented in Figure 1, showing test and re-test data of both the T 2 - weighted, the T 2 - and the ADC map of the prostate. The wCV values of the T 2 and ADC map are shown in Figure 2 for the MR- Linac and 3T scanner. The wCV at the MR-Linac at voxel- level are 17.5% for the patient and 14.5% for the volunteer for the T 2 maps, and 23.2% and 13.4% for the ADC maps. A reduction in this variation is found with an increase in the smoothing kernel size. With a kernel size of 0.5 mm 3 , the wCV for the T 2 maps are 4.6% and 4.5%, and 7.5% and 4.9% for the ADC maps. The repeatability of the T 2 map is similar in the patients scanned at the 3T scanner, with a median wCV of 18.9% at voxel level and 3.9% for a kernel size of 0.5 mm 3 . For ADC the repeatability at 3T is better with 10.3% at voxel level and 3.5% for 0.5 mm 3 . Conclusion In this study, we showed the feasibility of quantitative imaging on the MR-Linac in prostate cancer patients. When compared to the conventional diagnostic scanner, the MR-Linac produces comparable results for T 2 mapping. For ADC mapping, the MR-Linac shows a greater variability between the two sessions compared to the conventional diagnostic scanner. The voxel-level differences can be explained by the fact that the protocols were not matched to have exactly the same SNR.

Conclusion Iterative segmentation based on SUVmax and SUVpeak provide similarly accurate tumour volumes compared to pathology, indicating that both methods can be used to calculate the lung tumour volume, and that contrary to expectation no benefit was shown for the SUVpeak method. PO-0974 Repeatability of Quantitative Imaging on the MR-Linac for Treatment Response Monitoring E.S. Kooreman 1 , P.J. Van Houdt 1 , M.E. Nowee 1 , V. Van Pelt 1 , F. Koetsveld 1 , L.C. Ter Beek 1 , J.M. Peeters 2 , U.A. Van der Heide 1 1 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands 2 Philips, MR Clinical Science, Best, The Netherlands Purpose or Objective With the introduction of the MR-Linac, daily MR imaging of the tumor opens the possibility for accurate treatment response monitoring as a typical treatment takes place over the course of several weeks. A promising technique for this purpose is quantitative MR imaging. However, the adjusted design of the MR-Linac compared to conventional diagnostic scanners could have influence on the repeatability of quantitative imaging. In this study, we analyzed this repeatability by test-retest scans of the prostate on the MR-Linac and compared the results to repeatability on a conventional diagnostic scanner. Material and Methods Test-retest scan data of a patient with prostate cancer and a healthy volunteer were acquired on the 1.5T scanner of the MR-Linac (Elekta Unity, Elekta AB, Stockholm, Sweden) before radiation treatment, with more than one day in between. The acquired data included a T 2 -weighted image and a quantitative T 2 - and