A method and apparatus for determining the progressive potential of a disease is disclosed. The forward to backward propagating second harmonic generation signal derived from a second harmonic generation instrument is used to assess the collagen microstructure of imaged body tissue by way of numerical values that are in turn used to determine the progressive or metastatic potential of the disease. The disease may, for example, be a cancer such as breast cancer, lung fibrosis, colorectal adenocarcinoma, or the like. The apparatus may include in vivo instruments or laboratory diagnostic instruments with methods disclosed herein.