Semiautomatic segmentation for prostate brachytherapy: dosimetric evaluation

To demonstrate that manual prostate segmentation in transrectal ultrasound images can be replaced with semiautomatic segmentation.

Semiautomatic segmentation using a tapered ellipsoid model was applied to transrectal ultrasound images. Region-based volumetric evaluation was performed between original and physician-reviewed semiautomatic contours. For dosimetric assessment, treatment plans generated on semiautomatic contours were overlaid on physician-reviewed semiautomatic contours and dose parameters were computed. To establish a threshold for the acceptable amount of dosimetric degradation below which the adoption of semiautomatic planning is unacceptable, the range of variability in dosimetric quality attributed to manual variability was obtained and compared with that of semiautomatic contours.

An average volume error (1-Dice similarity coefficient) of less than 7% between semiautomatic and manual volumes (140 cases) was obtained. The difference between the mean V(100) of plans created for semiautomatic contours then overlaid on physician-reviewed semiautomatic contours and the original V(100) values, that is, before overlaying on the physician-reviewed contours (41 cases) was lower than 5%. An average total duration of 2-4min, which includes algorithm initialization, 11.67±3.57s algorithm time, and contour modification is required per case. This algorithm is being used at the British Columbia Cancer Agency and to this date has been applied for the treatment of more than 600 patients.

In terms of volumetric and dosimetric accuracy, the proposed algorithm is a suitable replacement for manual segmentation in the context of our planning technique. The benefits are shorter segmentation times; greater consistency; less reliance on user experience; and smooth, symmetric contours.