The role of molecular imaging in precision radiation therapy for target definition, treatment planning optimisation and quality control

Excerpt

During the past decade, remarkable technological developments have led to major improvements in treatment planning, dose delivery and quality assurance in radiation therapy. In particular, radiation therapy planning has evolved considerably through three phases based on the strategies used for tumour targeting. Planning was initially based on the use of clinical judgement and external visible markers (one-dimensional planning). The introduction of the simulator resulted in planning based on the radiographic anatomy, allowing bi-planar isodose distribution (two-dimensional planning), with better beam shaping and normal tissue avoidance and sparing. The recent development of new algorithms for three-dimensional (3-D) reconstructions of anatomy, dose calculation and radiation delivery has made it possible to precisely sculpt the radiation dose to target volumes of almost any shape. This progress has initiated the era of 3-D conformal radiation therapy (CRT), intensity-modulated radiation therapy (IMRT) and intensity-modulated arc therapy (IMAT). These new modalities, along with brain and extracranial stereotactic irradiation (SRT), modern brachytherapy and particle radiotherapy (with hadrons, such as protons and ions), have become forms of high-precision radiation therapy. In 1993, the International Commission on Radiation Units and Measurements (ICRU) published the first report on the definition of target volumes: the gross tumour volume, i.e. the volume that includes the demonstrable extent and location of the primary tumour, regional lymph nodes and distant metastases; the clinical target volume, which includes the gross tumour volume and/or the sites of subclinical disease, and the planning target volume, a volume determined by including any geometric uncertainties and set-up margins [1]. The report was subsequently updated in 1999 [2]. In the 1999 supplement an internal target volume, reflecting the motion of the clinical target volume, was added, and the use of a safety margin around the organ at risk was also recommended, generating the planning organ at risk volume. However, the above concepts were defined at a time when precision radiation therapy techniques were still in a preliminary phase. With the increasing use of modern radiation delivery techniques, which permit a degree of precision in the delivery of the dose to the target that had never been achieved before, a major change in the accuracy of tumour localisation procedures is also essential. Among the various tools needed for such a high-precision radiation procedure, imaging techniques have become crucial for clinical practice. …