Abstract Book

S1186

ESTRO 37

board cone-beam CT (CBCT) open possibilities for strategies such as online monitoring, re-calculation, adaptation and even re-optimisation of dose distributions. However, CBCT reconstructions are influenced by scatter and other artifacts that distort the density values away from their real values. A scatter correction method based on the a priori information of the planning CT has previously shown potential to improve dose calculation accuracy in CBCTs of phantoms. In this first clinical study we apply the a priori scatter correction algorithm on pelvic CBCTs and evaluate the basis for all on-line applications, the correctness of the Hounsfield Unit (HU) values. Material and Methods Image data sets of four prostate cancer patients were used, including planning CTs (pCTs) with organ delineations and CBCT projections from a Varian on-board imager (OBI) along with the clinical reconstruction (VarianCBCT). The CBCT projections were first reconstructed using a regular Feldkamp-Davis-Kress (FDK) algorithm (rawCBCT). We then registered the pCT to the rawCBCT rigidly (rigidCT) and deformably (deformCT). The deformCT was forward projected to the same angles as the CBCT projections followed by a linear intensity correction. The difference between the two projection sets were smoothed to create a scatter map, which was then subtracted from the CBCT projections before a final FDK (corrCBCT). To evaluate the correctness of the HU values in the CBCT reconstructions we propagated all organ delineations from the pCT using rigid registration. The values inside the structures were then plotted against those at the same geometrical position in the deformCT and histograms were created of all values in the CBCTs and CTs. The correctness was scored as the similarity to the histogram of the deformCT using the Euclidean Earth Movers Distance (EMD). As we expected the HU distribution of the bladder and the femur heads to be the least influenced by the physiological changes between CT and CBCT, these structures were used for comparison. Results The EMD of the bladder were similar for the corrCBCT and the VarianCBCT (<5% difference) while the rawCBCT had an EMD between 18 and 62 percent points higher than corrCBCT and VarianCBCT. For the head of the femur (left and right) the corrCBCT had the lowest EMD in four out of six cases, and were 31% lower than rawCBCT and 33% lower than VarianCBCT across the scans.

Conclusion In this study we found that the scatter correction algorithm overall improved the HU correctness compared to the clinical/commercial reconstruction. However, in a few cases, both the raw reconstruction and the clinical/commercial reconstruction performed better than the scatter correction. The scatter correction may be further improved by taking air and weight-loss better into account. EP-2150 Can contouring probability maps be a predictor for prostate cancer treatment outcome and toxicity? D. Roach 1,2 , J.A. Dowling 1,3,4,5 , A. Kennedy 6 , M. Jameson 2,7 , P. Greer 3,8 , M. Ebert 5,6,9 , L. Holloway 1,2,5,7 1 University of New South Wales, Faculty of Medicine, Sydney, Australia 2 Ingham Institute for Applied Medical Research, Medical Physics, Liverpool, Australia 3 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia 4 Royal Brisbane Hospital, Australian e-Health Research Centre- CSIRO, Herston, Australia 5 University of Wollongong, Centre for Medical Radiation