Use of molecular imaging to guide and assess radiation therapy

Imaging is intimately associated with radiation therapy (RT). Anatomical imaging is the standard of care for crucial components of the RT process such as tumor localization, treatment planning, and positioning verification. However, as disease progression and treatment response at the molecular and cellular level precede visible structural changes to tissue, applications of functional and molecular imaging are becoming increasingly more important. Use of molecular imaging in RTcan be divided into three phases: (1) Imaging for diagnosis and staging, performed during the initial phases of RT to establish the presence and progression of disease (2) Imaging for target definition, performed prior to RT in order to determine the spatial extent of the tumor and the position of normal tissue (3) Imaging for treatment response assessment, performed during or after RT to establish effectiveness, predict outcome, and potentially modify therapy. Following diagnosis and staging molecular imaging can help to define which type of therapy should be used, as well assess the spatial extent of the tumor, thus providing grounds for more reliable target definition. Molecular imaging has been shown to significantly reduce large inter-observer variability in target definition compared to anatomical imaging. This reduction leads to significant reduction in treatment margin, thereby enabling more accurate and precise tumor targeting. Furthermore, molecular imaging has the potential to characterize biological heterogeneity within tumors, providing foundations for so-called biologically conformal radiotherapy, or dose painting. Early treatment response assessment refers to the use of molecular imaging during the course of therapy, and late treatment response assessment refers to the use of molecular imaging after the therapy has been completed. While late assessment enables prediction of treatment outcome, early assessment, in addition, enables treatment adaptation