to the absence of oxygen. The maximum of the OER curve lies at pO2 >10–15 mm Hg, above which the radiosensitivity does not increase much further. At a pO2 of about 2–5 mm Hg, the OER is reduced to about half the maximum level [19].
Secondly, tumor cells adopt a number of programs to survive a detrimental hypoxic microenvironment, which include hypoxia-inducible factor (HIF)-induced gene expression, the unfolded protein response (UPR) and – thereby among others – autophagy and mammalian targeting of rapamycin (mTOR) [20]. These programs are all directed to the survival of (tumor) cells under metabolic stress and lead to apoptosis resistance, increased metastasis, and increased genomic stability, etc. Additionally, the HIF-1 regulation of glycolysis and the pentose phosphate pathway lead to an aberrant cellular metabolism that increases the antioxidant capacity of tumors, thereby countering the oxidative stress caused by irradiation [21]. Finally, hypoxia will also lead to selection of particular tumor clones that are resistant to hypoxia-induced cell death. Combined with increased genomic imbalance, hypoxia will thus lead to the selection of more aggressive subclones, harboring p53 mutations, for example [22].
Causes of Hypoxia
The blood supply of a malignant tumor is often suboptimal as the vascular network is immature and chaotic. This chaotic vascular network results in tumor hypoxia [23]. Mainly 2 forms of hypoxia exist [14]: chronic, diffusion-limited and acute perfusion-limited hypoxia [17]. In chronic hypoxia, tumor cells at a certain distance from blood vessels (100–150 μm) are beyond the maximal diffusion distance for oxygen [14]. This distance is even shorter due to the increased oxygen consumption in rapidly dividing tumor cells, with chronic hypoxia depending on both supply and demand. (Chronically) hypoxic tumor cells have exceeded the oxygen capacity of the newly formed vascular network. This is due to the fact that the new microvasculature is often insufficient in providing normoxic circumstances in the distant tumor areas and will thereby contribute to diffusion-limited hypoxia [23]. Anemia and hypoxemia can also cause, and certainly contribute to, chronic hypoxia [17]. Imaging tracers are predominantly markers for chronic hypoxia, because hypoxia needs to persist for a period of time that is sufficiently long enough to allow targeting and binding [14].
Another form of hypoxia is acute, perfusion-limited hypoxia, which is caused by the transient opening and closing of blood vessels, producing fluctuations in perfusion of tumor regions, and changes in oxygen tension [14]. The structural and functional abnormalities in the newly formed vasculature cause malfunctioning of the blood supply. This in turn results in an unstable blood flow causing intermittent hypoxia close to poorly organized vessels. This form of hypoxia is characterized by rapidly changing oxygen concentrations [23]. Acute hypoxia cannot be reliably imaged using tracers. Its relevance for prognosis is unclear, whereas (acute) hypoxia during radiation treatment will surely attenuate treatment efficacy. The chaotic and complex vascularization of tumors results in a mixture of areas of predominant acute, chronic, or a mixture of these 2 states of hypoxia.
Counteracting Hypoxia
When imaging is found to indicate a more hypoxic tumor, or establishes which parts of a solid tumor are more hypoxic than other parts, treatment can be personalized by dose modification or “dose painting.” However, hypoxia might be an (additional) target for treatment itself, or may also be countered before or during radiotherapy (hypoxia modification). Hypoxic cells within tumors can be targeted through so-called hypoxia sensitization using bioreductive compounds [24, 25]. Modification imaging of hypoxia could be essential, as hypoxia modification for less hypoxic tumors is not only a futile effort, but might even potentially harm the patient as treatment-related toxicity could increase with hypoxia-targeted therapy [23]. Several different approaches have been used to resensitize hypoxic tumors, especially in anemic patients, such as hyperbaric oxygen treatment, erythropoietin, or red blood cell transfusions. These have been largely unsuccessful, or even counterproductive [26, 27]. A successful hypoxia modification strategy is the breathing of carbogen in combination with nicotinamide, which in combination with accelerated radiotherapy (ARCON) led to an improved regional control in laryngeal cancer patients with hypoxic tumors [28], and particularly in anemic patients [29]. Another successful approach is combining radiotherapy with the oxygen-mimetic nimorazole, which selectively sensitizes hypoxic cells to ionizing radiation by replacing oxygen in the chemical reactions that lead to the production of DNA damage [30]. Imaging patients for a priori selection of hypoxic tumors will be of the utmost importance of determining the effect of hypoxia-modification strategies.
PET-Based Hypoxia Imaging
There are 2 main tracer classes for imaging of hypoxia, the 18F-labeled nitroimidazole-like compounds and the Cu-labelled diacetyl-bis(N4-methylthiosemicarbazone) analogues (Cu-ATSM) [10]. Since 18F-FDG-PET visualizes both aerobic and anaerobic glucose consumption, this tracer is not suited for visualization of the oxygenation status of tissue [14, 31].
Mechanism of Nitroimidazole Uptake in Hypoxic Tissue
After injection into the bloodstream of the patient, the radiolabeled nitroimidazole derivatives spread throughout the whole blood volume. The lipophilic nature of the compound leads to passive diffusion into cells [32]. Within the cells nitroimidazoles combine with an electron, which has a high affinity for nitroimidazole, especially the -NO2 part. The electron and nitroimidazole form a radical anion: -NO2-. Although electrons have a high affinity for nitroimidazole, they have an even higher affinity for oxygen. In the presence of oxygen, the electron in the radical anion can be taken over by the oxygen and the nitroimidazole returns to its parent state and can exit the cell again [33]. In the case of hypoxia (pO2 <10 mm Hg), binding to an electron is not irreversible and the reduction continues. The reduction product remains trapped in the cell. Reduction of nitroimidazoles relies on the presence of active tissue reductases, which means that only viable hypoxic cells accumulate the reduction products and apoptotic or necrotic cells do not [23]. The accumulation of reduction products in hypoxic cells is inversely proportional to the local pO2[10]. When nitroimidazoles are radiolabeled, the radioactivity