oxygen species (ROS). b ROS mediates DNA damage. c Under normoxia, O2 mediates the fixation of this damage, leading to detrimental double strand DNA breaks, whereas (d) under hypoxia, the ROS is reduced.
In general, it should be kept in mind that the PET voxel size does not match with the microscopic level on which the biological processes occur that we are interested in [16]. This complicates the identification of small-scale heterogeneity in uptake. Nevertheless, considering this and being aware of what is actually measured and the robustness of the measurement, there are PET-based imaging biomarkers that are worth further investigation.
Imageable Biomarkers
Imaging of Hypoxia
One of the most interesting imageable biomarkers related to radioresistance is hypoxia, i.e., the lack of oxygen often encountered in solid tumors. Hypoxia is generally referred to as the status when the oxygen level in tissues is below physiological levels. Radiobiological hypoxia usually refers to the very low levels that are required to make cells maximally radioresistant, which is at levels less than 5–10 mm Hg [17]. Hypoxic cells are up to 3 times more radioresistant than normoxic cells, and hypoxia is associated with (radio)therapy resistance and poor prognosis [18]. Therefore, imaging of hypoxia in tumors can be useful in establishing a prognosis, might predict response to (radiation) treatment, and can be used to personalize cancer treatment.
Hypoxia and Radioresistance
Several mechanisms can explain the lack of radiosensitivity in hypoxic tumors. Firstly, the effect of DNA damage induced by radiotherapy depends on the presence of oxygen. While DNA damage induced by free radicals can be restored in hypoxic areas, in the presence of oxygen the damage becomes permanent and irreparable. In other words, oxygen fixates the damage, and therefore this mechanism is called the oxygen fixation hypothesis (Fig. 3). The oxygen fixation hypothesis contributes to the so-called oxygen enhancement ratio (OER), which indicates that cells are 2–3 times more radiosensitive in the presence of oxygen relative 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
