Abstract: This invention is a real-time non-invasive cancer diagnosis method, which is based on detecting morphological alteration of cancer cells in-vivo using elastic light scattering spectrum. An apparatus and method is developed for recording back-scattered light in a small angle range limited by numerical apparatus of a single fiber optical probe. The same optical probe is used to illuminate a tissue and to collect the light scattered back from the tissue. To test our system, we used five Balb/c mice that were injected EMT-6 mammary cells in breast region. We took spectra of the light scattered back from tumors and breast epithelial tissue on the mice. Average size of scatterers in tissue is estimated by fitting spectra of the back-reflected light to Mie theory.