# Co-Adjuvant Nanoparticles for Radiotherapy Treatments of Oncological Diseases
## 1 Introduction
_Nanoparticles_ (NPs) definition refers to those materials as large as 1- to 100-nm made from both inorganic and organic materials. NPs can be administered systemically (via injection inhalation, and oral intake) or locally (via intratumoral delivery and implantable devices such as implants loaded with NPs or anticancer agents) [34; 35; 36; 37]. Organic based NPs used in biomedical applications include liposomes [38], polymer constructs [39], and micelles [40]. Inorganic NPs comprise mostly metal, semiconductor, and insulating systems (see [41] and within references to have an overview of the available NPs). Within the past few decades, inorganic NPs have been proposed in the biomedical fields for the implementation of the cancer therapies (chemotherapy, drug delivery [42; 43], and hyperthermia [44; 45], as well as radiotherapy, photodynamic therapy and X-ray activated photodynamic therapy [17; 46]), or for the use as diagnostic agents for imaging techniques [12], as well as for the applications as theranostic tools for oncology and other fatal diseases [47]. The attractiveness of these NPs relies on the versatility and tunability of the physico-chemical qualities, which depend on their composition, morphology, size, and high surface-to-volume ratio. Specifically, NPs modifiable features, like shape, size, and surface charge [28], are key aspects for surface accommodation of functional groups or hydrophobic molecules. The functionalization of NPs surfaces results in enhanced solubility and biocompatibility, in improved stability and retention time in bodily fluids [48; 49], and in the ability of the nanosystems to cross physiological barriers for their use in vivo [50]. Moreover, the engineering of the NPs surfaces by targeting ligands enables the specificity of their diagnostic and therapeutic activity towards the malignancy and illness sites in the patients' body and the release of the antitumoral drugs in a stable and controlled manner [51; 52]. The adjustment of the physicochemical properties of NPs by facile doping strategies as well as by the engineering of their size, crystallinity, and surface chemistry would also affect their luminescence behavior. Fluorescent NPs, such as lanthanide doped inorganic nanoparticles [53; 54; 55], quantum dots [56], silica based nanosystems [57; 58], as well as metal clusters and nanoparticles [59; 60] have been proposed for in vitro and in vivo bioimaging, sensing, drug delivery, and light-triggered oncological therapies. The tuning of the luminescence is a crucial point to satisfy the performance requirements demanded to the nanomaterials for their exploitation in medicine [53; 61; 62]. Even though many NPs are composed by purely organic and inorganic materials, the development of organic/inorganic hybrid materials is eagerly hunted; hybrid NPs in form of core-shell structures, metal organic frameworks (MOFs), or of nanocomposites where hybrids NPs are dispersed into a polymeric matrix, enable to create a new class of functionalized theranostic NPs that possess unique advantages, exploiting synergistically the feature of their single components [63]. The world of nanomedicine and nanotheranostic is wide and covers many frontiers of medicine. The use of NPs of diverse composition can represent an alternative to the traditional oncology and, in parallel, a way to implement the established therapies and diagnostic tools. In this review, we focus our attention on the utilization of NPs in the X-rays based oncological treatments, with efforts in describing the multiplicity of the roles that the diverse NPs can play according to their compositions and surface functionalization from the radiosensitization in radiotherapy (RT), to the triggered activation of cancer killing agents in the photodynamic therapies (PDT) and in the X-ray activated photodynamic therapy (X-PDT). NPs suitable for X-rays can be divided in a simple way into passive and active nanoparticles.