cancers Review Radioresistance of Brain Tumors Kevin Kelley 1, Jonathan Knisely 1, Marc Symons 2,* and Rosamaria Ruggieri 1,2,* 1 Radiation Medicine Department, Hofstra Northwell School of Medicine, Northwell Health, Manhasset, NY 11030, USA;
[email protected] (K.K.);
[email protected] (J.K.) 2 The Feinstein Institute for Molecular Medicine, Hofstra Northwell School of Medicine, Northwell Health, Manhasset, NY 11030, USA * Correspondence:
[email protected] (M.S.);
[email protected] (R.R.); Tel.: +1-516-562-1193 (M.S.); +1-516-562-3410 (R.R.) Academic Editor: Zhe-Sheng (Jason) Chen Received: 17 January 2016; Accepted: 24 March 2016; Published: 30 March 2016 Abstract: Radiation therapy (RT) is frequently used as part of the standard of care treatment of the majority of brain tumors. The efficacy of RT is limited by radioresistance and by normal tissue radiation tolerance. This is highlighted in pediatric brain tumors where the use of radiation is limited by the excessive toxicity to the developing brain. For these reasons, radiosensitization of tumor cells would be beneficial. In this review, we focus on radioresistance mechanisms intrinsic to tumor cells. We also evaluate existing approaches to induce radiosensitization and explore future avenues of investigation. Keywords: radiation therapy; radioresistance; brain tumors 1. Introduction 1.1. Radiotherapy and Radioresistance of Brain Tumors Radiation therapy is a mainstay in the treatment of the majority of primary tumors of the central nervous system (CNS). However, the efficacy of this therapeutic approach is significantly limited by resistance to tumor cell killing after exposure to ionizing radiation. This phenomenon, termed radioresistance, can be mediated by factors intrinsic to the cell or by the microenvironment.