ESTRO 36 Abstract Book

S493 ESTRO 36 2017 _______________________________________________________________________________________________

images. Plans optimized on the realistically distorted data and undistorted data were compared based on their DVH and the two one- sided equivalence test (TOST).

Ten patients who were referred for brain metastasis radiosurgery were analysed in this study. A planning CT (1 mm slice thickness), a contrast-enhanced T1 3D MRI scan (1.5T, 1 mm isotropic voxel size, surface coils) with patient immobilized in a 3-point thermoplastic shell (mask-MR) and a contrast-enhanced T1 3D MRI scan (1.5T, 1 mm isotropic voxel size, multi-channel head coil) without immobilization mask (no mask-MR) were acquired. First, a clinician stated which of the MRI scans had superior quality, to assure that the no-mask MR had at least the same image quality compared to the clinically used mask- MR. Then, the two MRIs were registered independently to the planning CT by a normalized mutual information algorithm which was restricted to rigid registration. The GTV was delineated by 3 clinicians on 1) mask-MR and 2) no mask-MR. The brain stem, chiasm and right eye were delineated by one clinician. Furthermore, 8 well-defined landmarks were marked by an observer in both scans. Residual registration errors were estimated for both MRIs by measuring the absolute coordinate differences in the three orthogonal directions between the set of landmarks on both imaging series after registration. Moreover, the absolute differences in the centres-of-gravity coordinates of GTV (median of 3 observers), brain stem, chiasm and right eye on mask-MR and no mask-MR were compared. Results The no mask-MR image quality was found to be superior in 9 of the 10 patients. The average coordinate difference between mask-MR and no mask-MR for all landmarks along the three orthogonal directions were within 0.5 mm (table 1). Similar results were found for the coordinates of the centre-of-gravity of all delineated OARs and GTV. Deviations in OAR registration > 1mm could be attributed to variations in delineation (figure 1). Only in one case, a registration error was observed. All GTV deviations were within 1mm. Conclusion The registration of MRIs obtained with or without immobilization mask to a planning-CT generally differs less than the MRI resolution (1 mm isotropic). Therefore, immobilization of the head during MRI for patients undergoing radiotherapy of brain metastasis is not necessary. However, to guarantee high accuracy of image registration when omitting an immobilization device during MRI, more attention should be paid to the quality of MR-CT fusion. Furthermore, consecutive MR images should be matched separately to CT, to correct for intra-scan motion. We foresee two benefits of scanning without mask. Firstly, the patient comfort during the MRI scan sessions will be improved. Secondly, omission of the immobilization mask permits the use of a multi-channel head coil which results in higher image quality. Moreover, using a head coil allows for introduction of MRI techniques which require high signal-to-noise ratios or acceleration (e.g. DWI and FLAIR).

Results Increasing the bandwidth reduced the distortions. Moving from 122 to 244 Hz/Pixel decreased the maximum distortions by 43% and reduced the absolute difference in doses to the PTV between dCT and CT plans from 0.417 ± 0.241 Gy to 0.129 ± 0.286 Gy in the R/L gradient readout direction. However, this increase in bandwidth did not significantly affect the difference in doses in the A/P readout direction: 0.347 ± 0.150 Gy and 0.362 ± 0.240 Gy respectively. We found a difference of 1.2% and 1.9% between dCT and undistorted plans for gradient readout in R/L and A/P directions for the rectal volume receiving more than 69 Gy. The equivalence test on the two plans showed the 90% Confidence Interval all lied within the equivalence intervals (-0.6, 0.6) Gy for difference in PTV mean doses and (-1, 1) % for difference in the relative volume of the PTV and Rectum with a 0.05 significance. Conclusion By combining measured Machine-specific and si mulating Patient-induced Susceptibility effects w e have successfully investigated their combined effect on dose distributions for Prostate cancer treatment plans. Our results showed that dose errors due to disturbed Patient outline and shifts due to Patient-induced Susceptibility effects at Prostate/Rectum interfaces caused by gas in the Rectum were small. The smallest effect was found for high bandwidth and readout in the R/L direction. Equivalence tests showed equivalence within our investigated equivalence intervals at 0.05 alpha level for all studied dose distribution quality indicators. PO-0901 Is MRI in immobilization mask nec essary for brain metastasis patients? A. Van Lier 1 , A. De Boer 1 , M. Kramer 1 , G. Fa netti 2 , W. Eppinga 1 , J.J.C. Verhoeff 1 , M. Philippens 1 , E. Seravalli 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands 2 European Institute of Oncology, Department of Radiation Oncology, Milan, Italy Purpose or Objective To investigate the necessity of performing MRI in treatment position (ie. with immobilization mask) for brain metastasis patients. Material and Methods

PO-0902 Identifying the dominant prostate cancer focal lesion using 3D image texture analysis