Although bone biopsy is considered the gold standard for quantifying bone turnover, it is limited to a single site at the iliac crest, is relatively invasive, complex and costly to perform, and is subject to significant measurement errors. In contrast, the measurement of bone turnover markers (BTMs) in serum and urine provides a convenient and practical alternative, but is limited because BTMs only provide information about global bone turnover.
The need for standardization of quantitative PET was recognized as early as 1998 by Schelbert et al and in 1999 by European Organization for Research and Treatment of Cancer take force. PET imaging can benefit substantially by using quantitative measures of uptake. Quantitation is not a substitute for thanking; however, it can be used to help determine the likelihood that a focus is malignant, its level of metabolic activity and whether its activity is decreasing with time. One of the most obvious ways to use a quantitation is to set a threshold for determining malignancy. Subsequently, several other studies reported the impact of various factors on PET quantification and provided recommendations for performing 18F-PET studies. Some of these studies focused mainly on the clinical use of or indications for 18F-FDG PET, on improving PET study quality and on providing guidelines for PET study interpretation or measurement of the response on therapy. Coleman et al. discussed various aspect and technical issues regarding the use of integrated imaging systems, that is, PET/CT. in the present review, the focus is on recommendation and standards given specifically for quantitative 18F PET oncology studies. The various factors affecting PET quantification and recommendations given in various reports are discussed.
Quantitation is not always necessary. If a patient with recurrent melanoma is found to have multiple sites of intense focal uptake throughout the body, the diagnosis is clear. However, FDG oncological studies are often not so easily assessed and quantitative measures of uptake become a welcome assistant.
The optimal method of quantitation for diagnosis, prognosis, and assessing response to therapy has not been defined and will undoubtedly change with time. It is dependent on the acquisition of a large body of empirical data. Currently, it is not clear that there is any advantage to the individual patient if more advanced quantitation methods are used. This is because we do not have data on FDG uptake for the multiple combinations of tumors, techniques, and clinical settings.
Several groups are addressing this problem. As such work proceeds, the investigators should avoid commitment to any given analytical method, but should collect additional data: height, weight, and blood activity, along with outcome data (pathological diagnosis, time to recurrence, duration of survival). This will enable determination of which method is the most effective way to address specific oncological questions.
Keys reported that PET images should be interpreted subjectively rather than quantitative interpretation of SUV. In contrast, Cook et al report that SUV has predictive value for evaluating bone metastases, whereas, qualitative ( visual ) interpretation tended to be less valuable.
SUV measurements from NAF-18 PET/CT have the potential to monitor treatment response. In patients with osteoporosis, SUV obtained from NAF18 PET/CT studies significantly decreased after treatment with bisphosphonates. This shows that simple SUV measurements may be sufficinent for monitoring disease response in metabolic bone diseases. Simplified SUV uptake measurements may be suitable subsitiutes for more complex kinetic modeling.
The simplest assessment is dichotomous visual assessment: either it is malignant or it is not. This approach is obviously subjective, involves numerous unstated assumptions, and requires considerable experience. It is adequate for simple questions, e.g., to rule out widespread metastatic disease, but is inadequate for addressing more subtle situations.
Graded visual assessment is slightly better. The abnormality is classified relative to normal structures on a four- or five-point scale. An example for lung cancer is: 0, not visible; 1, less than normal mediastinum; 2, similar to mediastinum; 3, brighter than mediastinum. This brings some objectivity to the assessment, and once well-defined criteria for positive and negative are defined, should result in reasonably consistent performance between observers.
Tumor to normal ratio:
Tumor to normal (TN) ratio is the next level of quantitation. It requires attenuation-corrected images, but no other calibration. It has the advantage that it can be applied to images long after original reconstruction without additional information. This approach is more objective but has significant limitations. Values derived depend on placement of regions of interest (ROIs), definition of normal tissue, reconstruction algorithm, image resolution, and whether maximum or average counts are used. The TN ratio method can be useful but should be implemented with explicit attention to these issues
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