oa Editorial [Hot topic:Targeted Molecular Radiotherapy (Guest Editor: Veerle Kersemans)]

Cancer is an important health problem in the Developed World where it is the second cause of death, mainly associated with ageing of the population and lifestyle. Along with surgery and chemotherapy, radiation therapy (radiotherapy) is one of the most important tools to combat cancers. More than half of all cancer patients will receive radiotherapy at some stage during the course of their illness [1]. The goal of radiation therapy is to deliver a precisely measured dose of ionizing radiation to a defined tumor area, with as little damage as possible to surrounding healthy, non-cancerous tissue [2]. However, most types of radiation do not attack cancer cells specifically, and therefore cause injury to normal tissues surrounding the tumor. Therefore, a number of patients undergoing radiation therapy will experience a range of side effects, which may lead to an interruption of treatment or limiting the dose of radiation [3]. These adverse effects are a major factor limiting the success of radiation treatment. Considerable research activity is focused on improving radiotherapy outcome and reducing adverse effects. As radiotherapy aims to deliver the necessary therapeutic dose of ionising radiation to tumour tissue whilst minimizing irradiation of normal tissue, precise tumour volume delineation is essential. It is in this setting that the role of PET in the radiotherapy management of patients has been investigated. Indeed, PET offers additional advantages over CT as it can provide both anatomical and biological tumour information. For example, by detailing and quantifying tumour metabolism, hypoxia and perfusion, tumour delineation can be improved and a more refined target can be identified [4]. Although the above strategies can be implemented, radiotherapy is still limited by the tumor volume or the surrounding normal tissue tolerance to radiation. Recent advances in molecular biology has created a novel framework that can improve clinical practice considerably. The identification of cellular receptors, enzymes, and pathways involved in tumorgeneity has resulted in the development of biologically targeted drugs. Also the fact that cancer cells need their microenvironment has opened the door to new therapeutic strategies which can expand radiotherapy practice. Fifty years ago, targeted radiotherapy started with the use of 131I-NaI therapy for thyroid cancer and even up to today, this is the only targeted radiotherapy based approach that is approved by the U.S. Food and Drug Administration [5]. Twenty years later, treatment of metastatic bone pain using bone-seeking radiopharmaceuticals such as 153Sm, 89Sr, and 186Re has become an important tool in clinical practice. Indeed, the incidence of bone metastases is very high and bone secondaries are a common cause of cancer pain [6]. More recently, targeted radiotherapy has become more sophisticated through the use of radiolabelled microspheres and antibodies and by targeting the cell nucleus. This collection of reviews on targeted radionuclide therapy provides a snapshot of the current status of modern clinical applications of therapeutic nuclear medicine.