In radiotherapy treatment planning, computer software provides means to draw polygons in sequential planes displayed as overlays over CT scans or MRI scans.  This feature allows radiation oncologists to create wire-frames that correspond to anatomical organs and anatomical features important to the development of the treatment plan.  Drawing these polygons has become know as ‘contouring’.  When any two persons contour the same object, there are differences between the two contours.  Some of these differences have to do with the shape of the contours.  Some have only to do with the sequence by which the contour was drawn.  The purpose of this algorithm is to provide a measure of the differences between two contours that have to do with the shape.

The algorithm must be insensitive to differences between the stored sequence of vertices that have only to do with the order and manner in which the contour was drawn. Suppose we are comparing a contour drawn by a Student who is learning contouring from a Mentor.  The Mentor might have drawn the contour moving clockwise from an initial starting point on the right side of the contour and in so doing create a sequence that is 32 vertices long.  Suppose a Student attempts to contour the same anatomical structure and is able to trace along exactly the same shape as the Mentor.  But suppose the student starts at the bottom left of the contour and traces counter-clockwise picking only 28 points for the vertices.  Sim Metric scoreIn general, the student will not pick vertices that correspond to those chosen by the mentor, even though the Student’s vertices fall along exactly the same curve as those defined by the Mentor.  Given the digitized and stored vertices, what must the algorithm do in order to compare the two tracings?  First the algorithm must resample each contour using the same number of vertices when sampling both the Mentor’s contour and the Student’s contour.  Secondly the algorithm must reorder the Student sequence such that the first vertex in the Student sequence corresponds as nearly as possible to the first vertex in the Mentor’s sequence.  Thirdly, the algorithm must determine the order in which the Student traced the contour from the first point.  If the order is the same as that of the Mentor polygon, then nothing further must be done.  However, if the algorithm determines that the Student drew a contour in the opposite sense as the Mentor, the algorithm must invert the Student sequence so that it runs in the same direction as the Mentor sequence.  Once the sequences are resampled and reodered, they will represent the contour shape and position with the same number of vertices in the same order space in the same increments.  Then the differences between the sequences can be computed using computed metrics such as the distance between the centroids (or centers of gravity) of the two contours as well as the sum of the root mean square differences between corresponding vertices.  If the shapes of the two contours are very similar, the distances will be small.  If the shapes are dissimilar, the distances between corresponding points will be great.