Optical biopsy: the noninvasive diagnosis of cancer with optical spectroscopy

ABSTRACT In recent years, researchers have demonstrated that various types of optical spectroscopy can be used to perform noninvasive,real-time and in-situ diagnosis oftissue pathologies, especially cancer. In this overview ofthe topic, various spectroscopicmethods are described. The promises and hopes, as well as the difficulties ofthese developing techniques are discussed. UV-induced fluorescence spectroscopy and elastic-scattering spectroscopy are addressed at some length, and infrared-absorptionspectroscopy and Raman spectroscopy are more briefly discussed.Keywords: Optical biopsy, tissue spectroscopy, optical diagnosis 1. INTRODUCTION In recent years optical spectroscopies have become the basis for a high level of research activity directed toward thedevelopment of novel, noninvasive technologies for tissue diagnostics, frequently dubbed "optical biopsy," which is perhapssomething of an oxymoron since "biopsy" refers specifically to the removal of tissue, whereas the implication of "optical" isthat tissue is not removed. The motivation is to eliminate the need for surgical removal of biopsy tissue samples; rather. someform of spectral analysis of the tissue is recorded in vivo by an imaging system or with an optical probe placed on or near thesurface ofthe tissue in question. A diagnosis ofthe tissue is then attempted based on the optical measurements. The intent ofthese systems is to provide diagnostic signatures, in situ, noninvasively and in real time. Additional motivation is provided bythe potential for reduced health-care costs as a consequence of eliminating histology and, in many cases, eliminating the needfor the surgical environment required to take biopsy samples. Moreover, the immediacy of diagnostic information can reducethe emotional trauma to the patient awaiting an answer. While the ultimate goal is the elimination of the need to removetissue samples, during early phases of the clinical implementation of these optical technologies the intermediate goal, ratherthan replacement ofbiopsy and histology, may be to provide additional guidance in locating the optimum sites for biopsy.For diseases of the gastrointestinal (GI) tract, for example, the potential benefits of optical tissue diagnosis can besignificant. Several disorders of the GI tract are correlated with a predisposition for cancer, including colitis, colon polyps,and Barrett's esophagus. Typically these diseases are followed with annual (or more frequent) endoscopic examinationaccompanied by tissue biopsies. As many as 20-30 biopsies may be taken in one session. This is a time consuming (andtherefore expensive) procedure, which entails some degree of risk for the patient. For each conventional biopsy, the biopsytool must be withdrawn from the endoscope and the specimen removed before the tool can be reinserted for the next biopsy.In contrast, an optical diagnostic probe could be moved from site to site in succession, with each measurement being recordedin a fraction of a second, by simply moving the location of the probe tip. In summary, optical diagnostic techniques offer thepotential to improve disease management, with reduced risks for the patient, and the potential for earlier diagnosis andimmediate treatment.A range of spectroscopies have been investigated for optical diagnosis, all of which have one basic principle in common.The specific optical spectrum of a tissue sample contains information about the biochemical composition andlor the structureof the tissue. The biochemical information can be obtained by measuring absorption, fluorescence, or Raman scattering