Table 1.
Techniques used for measuring the response of tumors to RT.
| Imaging-based methods |
|---|
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Method: Advanced imaging modalities can provide information on tumor size (X-ray computed tomography, ultrasound, magnetic resonance imaging) and give an estimation of hypoxia and proliferation heterogeneity within different areas of a tumor (positron emission tomography). Imaging can be conducted pre-, during and post-treatment. Advantages: Non-invasive. Real-time measurements of response can be obtained. Methods, protocols and criteria for visualizing and assessing changes in tumor size are already established within the clinic. Disadvantages: Changes in tumor size can be gradual and slow which may only be seen towards the end or after the treatment has finished; patients who fail to respond will initially go undetected. Patient safety concerns over repeat exposure to radiation and radioisotopes. |
| Cancer tissue-based biomarkers |
|
Method: Evaluating the expression levels of genes or proteins using biopsy samples. Advantages: Pre-treatment biopsies are already taken as part of standard of care treatment, meaning patients do not require an additional procedure. Protein assessment using IHC can identify the location of protein expression and provide information on functional status. Gene signatures may be prognostic and predictive of responses to RT. Disadvantages: Invasive. Unable to monitor response to treatment without further invasive procedures, which can be difficult to obtain and hard to justify. Some methods of analysis are cost prohibitive. Does not provide real-time measurements of response. |
| Blood-based biomarkers |
|
Method: Measuring the presence or expression levels of CTCs or proteins using liquid biopsy (blood) samples. Samples can be obtained readily pre-, during and post-treatment. Advantages: Non-invasive. Real-time measurements of response can be obtained. Disadvantages: Low sensitivity and specificity of tests developed to date. |
Advantages and disadvantages of each method are provided (IHC, immunohistochemistry; RT, radiotherapy; CTCs, circulating tumor cells).