ESTRO 36 Abstract Book

S478 ESTRO 36 2017 _______________________________________________________________________________________________

Subsequently, the 2D slices were binned in 10 equidistant bins according to the 1D diaphragm position (amplitude binning). To account for outliers, we developed a strategy that sets the inclusion range such that 95% of the diaphragm positions are included, while the peak-to-peak range is minimized (denoted Min95). We compared this with two frequently used strategies (Fig.1): one that selects the maximum inhale and exhale position as range (MaxIE), not discarding outliers, and one that selects the mean inhale and exhale position as inclusion range (MeanIE). The strategies were evaluated based on the following parameters: • * Data included (DI); the fraction of data used for reconstruction after exclusion of outliers.

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* Reconstruction completeness (RC); the fraction of the 110 (11 slices x 10 bins) bin/slice combinations in the 4D data set that are filled. * Intra-bin variation (IBV); the standard error of the mean diaphragm position inside a bin/slice combination. * Image smoothness (S); assessed by quantifying how well a parabola fits the diaphragm shape in a sagittal plane of the reconstructed 4DMRI, per bin (S = R 2 adj averaged over all bins). S ranges from 0 (discontinuous diaphragm shape; artefacts) to 1 (smooth shape; no artefacts). * Peak-to-peak range (PP);

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A low DI indicates underestimation of motion amplitude. A low IBV indicates high binning precision. Low RC, low S and high IBV result in image artefacts, e.g. discontinuities between reconstructed slices. A paired Wilcoxon’s signed rank test was used to test differences in parameters between binning strategies.

Conclusion For the first time, it has been possible to quantitatively demonstrate that accumulated delivered dose to the rectal wall is more strongly correlated with rectal bleeding and proctitis in prostate radiotherapy than planned dose. The results support the hypothesis that incorporating delivered dose into multi-variable predictive models could improve toxicity outcomes. Poster: Physics track: CT Imaging for treatment preparation PO-0881 4DMRI for RT planning; novel precise amplitude binning in the presence of irregular breathing I. Bones 1 , O.J. Gurney-Champion 2 , A. Van der Horst 1 , A. Bel 1 , T. Alderliesten 1 , G. Van Tienhoven 1 , K. Ziemons 3 , Z. Van Kesteren 1 1 Academic Medical Centre, Radiotherapy, Amsterdam, The Netherlands 2 Academic Medical Centre, Radiotherapy and Radiology, Amsterdam, The Netherlands 3 FH Aachen University of Applied Sciences, Medical Physics, Jülich, Germany Purpose or Objective Irregular breathing, often the case in clinical practice, introduces the need for proper outlier handling for 4DMRI reconstruction. Discarding outliers may lead to underestimation of the respiratory-induced organ motion. Our study aimed to develop and evaluate an amplitude binning strategy that reduces reconstruction artefacts while improving precision in the presence of irregular breathing. Material and Methods Twelve volunteers and 2 abdominal cancer patients were scanned with our 4DMRI sequence. In this 6 minute scan, 11 2D coronal slices were acquired repetitively (60 times) during free breathing, using a T2W TSE sequence (resolution: 1.3x1.6x5.0 mm 3 ). Prior to each slice acquisition, the position of the diaphragm was assessed using a 1D acquisition.

Results Excluding only 5% of images during amplitude binning, the developed Min95 strategy outperformed the MaxIE strategy with a 9.5% higher mean RC, 5.6 mm lower mean PP and virtually the same mean IBV and S (all significant, Table 1). The MeanIE strategy with a mean DI of 76.4%, severely underestimated the motion amplitude even though it had a higher S, higher RC and lower IBV compared to MaxIE. The Min95 strategy outperformed the MeanIE strategy with an 18.6% higher mean DI.