Molecular and Functional Imaging in Radiation Oncology

Abstract

Imaging has always played a very important role in radiation oncology. It started with the invention of X-rays over a century ago, through development of computed tomography (CT) imaging which significantly improved tumor localization and sensitivity [1–4] as well as enabled CT-based treatment planning of radiation therapy. Implementation of nuclear magnetic resonance techniques in medicine quickly led to an expanding field of magnetic resonance imaging (MRI) [5–7], which has further revolutionized clinical practice due to the improved soft-tissue contrast. Both, CT and MRI are primarily modalities for determining anatomical information. However, for complete assessment of disease status, monitoring tissue function is of a great importance because the disease processes typically start at the molecular and cellular level that only later lead to visible structural changes to the tissue. Similarly, anatomical changes in response to therapy come after the response on the molecular and cellular level, which becomes important in treatment outcome prediction as well as treatment efficacy assessment. Functional imaging refers to either imaging of physiologic processes, such as blood flow to an organ or diseased tissue; visualizing ongoing biochemical and metabolic activities of normal and abnormal tissues; or using established pharmacologic methods to assess disease processes and develop new drugs [8]. Functional imaging is typically performed by the mainstream clinical radiology techniques like ultrasound, CT and MRI. On the other hand, the term molecular imaging typically refers to imaging specific molecular interactions and pathways, which reflect biochemical, physiological and pharmacological processes. The term molecular imaging was defined by the Commission on Molecular Imaging of the American College of Radiology as “the spatially localized and/or temporally resolved sensing of molecular and cellular processes in vivo.” The main modalities for molecular imaging are positron emission tomography (PET), single photon emission tomography (SPECT), magnetic resonance spectroscopy (MRS) and optical imaging. Although they are based on quite different imaging processes, the terms functional and molecular imaging are often used interchangeably, due to a lack of consensus regarding their definitions. 4 Molecular and Functional Imaging in Radiation Oncology