AbstractID: 8929 Title: Automatic image registration for the daily CT localization of the prostate
A number of cancer centers and other facilities have recently installed CT scanners inside radiation therapy treatment rooms, thus introducing the possibility
of daily CT localization for prostate patients. Manual registration of the original treatment plan with the daily CT images can be slow, and can introduce
significant inter-user variations. An automatic alignment registration technique has been developed which is not adversely influenced by the changes in
prostate shape, rectal gas, and bladder filling. The cost function used in the registration is the absolute difference in CT numbers voxel-by-voxel between the
daily CT image and the original planning CT image for a volume extracted from the planning CT images using the original physician-drawn contours. To
enhance soft tissue contrast in the prostate region, voxels with CT numbers which represent gas or bone are filtered out from the calculation. The results of
the automatic algorithm agreed with the average results of seven human observers with standard deviations of 0.5-0.6mm, 0.5-1.4mm and 1.0-1.9mm in the
lateral, anterior/posterior and superior/inferior directions respectively. The range is due to the differences in anatomy of different patients. The results are
independent of the shape of contours in the original treatment plan, reducing the impact of inter-observer variations in contouring the prostate. The
algorithm is fast and reliable: In 120 CT image sets from 7 patients, the failure rate is less than 1%.