Molly: I went to NCI (Natl Cancer Institute) and did some research. I had never seen or heard that particular term either. Some of it was way over my head (which doesn't take a lot these days). I couldn't find anything that I understood regarding this word and the stats that you were referring to. Here is the definition that they gave:
Occurring in various distinct forms. In terms of cells, having variation in the size and shape of cells or their nuclei.