Abstract
More than 1,000,000 women were diagnosed with breast cancer in the last decade worldwide. The prognosis for this disease improves greatly when it is detected early. Detecting the efficacy of treatments in-vivo can not only expedite the development of drugs but also enable treatments to be more individually tailored. Drugs can induce cell apoptosis and can be a biomarker for monitoring the effectiveness of, for example, chemotherapy. It is preferable to undertake this detection using non-invasive techniques and with non-ionising radiation when possible to minimize discomfort and risks to the patient.

Techniques currently used to diagnose or monitor lesions in the breast often require ionising radiation (e.g. x-ray mammography) or are expensive and time-consuming (e.g. MRI). A number of non-ionising imaging techniques have been investigated for the imaging of the breasts including: near-infrared spectroscopy, electrical impedance spectroscopy and tomography, microwave imaging spectroscopy and photoacoustic and thermoacoustic imaging. Optical imaging can detect suspicious lesions and cell apoptosis, and in some cases can be used as an alternative to imaging using ionising radiation. Optical signals in tissues can however be dependent on a number of factors including vascularity, cellularity, oxygen consumption, water concentration, lipid content, oedema, fibrosis and remodelling.

A key challenge to detecting cancer utilizing light has been the ability to differentiate between absorption and scattering. The breast has a high level of scattering and absorption to optical wavelengths. Most of the important physiological information is contained in the absorption processes. However, scattering processes tend to dominate in breast tissue to the extent that signal requiring propagation over more than just a few millimetres will be dominated by diffusely scattered light. This article reviews recent advances in optical techniques as well as combinations with an advanced molecular imaging method for the early detection of breast cancer and assessing the efficacy of response to therapeutic regime(s).