ESTRO 36 Abstract Book

S497 ESTRO 36 _______________________________________________________________________________________________

by the 3 observers showed a median dice similarity coefficient of 0.71, 0.69 and 0.72 respectively. For all 3 observers the mean hausdorff distance was small with median (range) distances between PET and DW of 2.3 (1.5 – 6.8), 2.5 (1.6 – 6.9) and 2.0 (1.35 – 7.6) mm respectively. Over all patients, the median 95 th percentile distances were 6.0 (3.0 – 13.4), 6.6 (4.0 – 24.0) and 5.3 (3.4 – 26.0) mm.

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).

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.

Conclusion Diffusion weighted imaging optimized for geometric accuracy resulted in target volume delineation with good interobserver agreement and a large similarity with PET. PO-0900 Quantifying the Effect of MRI Geometrical Distortions on Radiotherapy Treatment Planning Doses. M. Adjeiwaah 1 , M. Bylund 1 , J. Lundman 1 , J. Jonsson 1 , T. Nyholm 1 1 Umeå University, Radiation Sciences, Umea, Sweden Purpose or Objective The use of MRI for Radiotherapy Treatment Planning (RTP) is increasing and the proposed MR-only workflow could be beneficial. One worry of an MR-only RTP is geometrical distortions. There are at present few studies focusing on the effect of MR geometrical distortions on planned doses in an MR-only treatment planning and to our knowledge, none fully takes into account both gradient non-linearities and Patient-induced Susceptibility effects. This study focused on quantifying the effect of gradient non- linearities and Patient–induced Susceptibility effects on dose distributions for Prostate Cancers. Material and Methods The deformation field was generated by adding measured machine-specific and simulated patient-induced susceptibility effect deformation fields for a 3T scanner as shown in Fig. 1. Different bandwidths and simulated gradient readouts in the anterior/posterior (A/P) and right/left (R/L) directions were used. To isolate the effect of the distortions, the deformation fields were applied to 17 Prostate Patient CT images and their corresponding clinically delineated structures, giving a distorted CT (dCT). VMAT optimized plans were generated for all distorted cases and recalculated on the undistorted CT