You can use vtkvmtkPolyDataBoundaryExtractor to create a polydata where point ids are contiguous. The input is a polydata with co-planar points. You can next easily loop through the points of the output to tally up the distance between successive points, the result is the perimeter.
It works with complex input also, like below. The yellow outline is not produced by that class, it’s just a visualisation model from the output.
What clinical inferences can you draw from this quantity?