is attenuated by accelerated repopulation. In order to increase the treatment response by limiting the effect of accelerated repopulation, different approaches, such as accelerated radiotherapy or radiotherapy combined with another therapy, e.g., cetuximab, have been suggested. Since these are all at the cost of increased side effects [16], careful patient selection for this treatment modification is crucial. Imaging of proliferation, or accelerated repopulation, has the potency to select those patients or patient groups that require an intensified treatment, and could thereby contribute to an improved treatment outcome.
PET-Based Proliferation Imaging
One way to measure proliferation, or the effect of proliferation, is to measure the anatomical size of a tumor. Besides the delay in the visibility of the size-effect, even after weeks, size does not provide information on the tissue viability and is therefore an impaired biomarker. This makes 18F-fluorothymidine (18F-FLT) PET imaging, which visualizes cell proliferation, an attractive alternative. As it is not (yet) routinely used in the clinical practice of radiotherapy, it is an often-used imaging biomarker in research. Dose escalation based on 18F-FLT PET has been suggested and was shown to be technically feasible [49].
Mechanism of 18F-FLT Uptake and Accumulation
18F-FLT is a radiolabeled variant of the nucleoside thymidine. After intravenous administration, 18F-FLT gets transported into the cell by nucleoside transporters. The enzyme thymidine kinase 1 (TK1) leads to monophosphorylation of 18F-FLT and thereby to trapping of 18F-FLT in the cell [9, 31]. During the S-phase of the cell cycle (DNA synthesis) TK1 is increased about 10-fold [49], which makes 18F-FLT visualize cell proliferation. This is confirmed by comparison of the 18F-FLT uptake with the cellular proliferation biomarker Ki-67. Ki-67 is exclusively present in the active phases of the cell cycle, and therefore strictly associated with cell proliferation [50]. 18F-FLT uptake and immunohistological analysis of Ki-67 expression showed a positive correlation [3], with the advantage of 18F-FLT being noninvasive and providing information of the entire imaged volume (whole body is possible), at the cost of a much lower resolution.
Imaging of 18F-FLT
18F-FLT PET provides reproducible quantitative imaging results [3, 14]. A decrease in uptake of 20–25% is greater than the reproducibility error [31] and can therefore be attributed to a decrease in proliferation. Imaging can be performed about 1 h after administration of the 18F-FLT, which is similar to the incubation period for imaging of 18F-FDG, the frequently used glucose analogue for PET imaging. Beyond 1 h of incubation, proliferation might be underestimated by leakage of the metabolite back into the blood [51].
Compared to FDG, FLT has a lower tumor uptake. For example, for contouring of the functional tumor volume, for FLT a threshold of 1.4 of the SUV results in a volume that is similar to the volume calculated with FDG with a 2.5 threshold [52]. When using imaging of proliferation for early response prediction, it should be considered that, although proliferation is a characteristic of the tumor, it is not exclusively a tumor cell characteristic. Not only do proliferating tumor cells accumulate 18F-FLT, but also, for example, proliferating B-lymphocytes in inflammatory lymph nodes [31] and in bone marrow [53]. Radiotherapy could increase uptake by inducing the proliferation of inflammatory cells in certain areas [31]. A relatively high background uptake in the liver and genitourinary system complicates the imaging of proliferation with 18F-FLT in these areas. This limits the specificity of this biomarker and makes 18F-FLT imaging better suited for the monitoring of treatment response than for staging [9, 14, 53].
For early response assessment, in head and neck cancer it was shown that already in the second week of treatment the decrease in 18F-FLT uptake is indicative of the treatment outcome [54]. This allows the treatment protocol to be changed, for example accelerated. Since the 18F-FLT uptake decreases at least during the first 4 weeks of therapy [31], the timing influences the outcome of the quantification.
The preparation of 18F-FLT is different from 18F-FDG. It requires, for example, a purification because of toxic side products. In contrast to the widely used FDG, in most institutes FLT is not easily available, which limits its use.
Perspective
Although 18F-FLT-PET proved to assess radiotherapy treatment response earlier than 18F-FDG-PET and CT tumor response [55], currently 18F-FLT-PET is only applied within clinical studies. Unfortunately, these studies are mostly single-center studies with a small sample size. Similar to studies applying 18F-FDG-PET, standardization of 18F-FLT-PET is required to enable the comparison and combination of clinical study results. Guidelines like the EANM guideline for 18F-FDG-PET should also become available for other tracers, such as 18F-FLT. This would stimulate harmonization and therefore increase the chance of success for these biomarkers.
The Future of Biomarker Imaging for Radiotherapy
Both hypoxia and the proliferation of imaging are illustrative for a more general challenge we face in the development of biomarker imaging for radiotherapy. Despite the many initiatives, based on many different biological characteristics, currently only a few biomarkers have the potency to enter clinical routine. These have to first overcome the challenge of nonuniformity in study designs, imaging, and quantification, which slows down further development by hampering the combination and comparison of results.
The value of the use of biomarkers lies in patient selection. The challenge of biomarker studies is to first select those patients that can demonstrate the working of the biomarker. By both biological and technical harmonization, biomarkers will further demonstrate their usefulness for radiotherapy. Biomarkers have the potency to be useful for both research and individual patients.
References
1West CM, Huddart RA: Biomarkers and imaging for precision radiotherapy. Clin Oncol 2015;27:545–546.
2Strimbu K, Tavel JA: What are biomarkers? Current Opin HIV AIDS 2010;5:463–466.
