Background
Neuroendocrine tumour (NET) is a relatively rare tumor of neuroendocrine system origin, which poses a number of clinical challenges due to its late clinical presentation, few treatment options and the limitations of current imaging and biomarkers. Therefore, there is an urgent need for more accurate biomarkers that can directly evaluate the biological behavior of tumor cells and provide real-time feedback to clinicians. Based on the need for novel NET biomarkers, an expert panel of 18 multidisciplinary NET clinicians met in the United States in October 2014 to discuss eight major categories: background on consensus development, diagnostic markers, bronchial and pulmonary neuroendocrine tumor markers, circulating markers, imaging and circulating markers, pathological markers, circulating tumor cells, and novel biomarkers. The consensus is summarized below. This article translates the consensus opinion as follows.
Discussion on the clinical application of NET biomarkers
The expert group believes that there is an urgent need for a sensitive and specific biomarker for NET with a sensitivity of at least 80% and specificity of at least 90%, and a positive and negative predictive value of >80%. In terms of obtaining specimens, venous blood collection is the easiest and safest way to obtain biomarker specimens.
Any biomarker that can be used for accurate diagnosis and prediction of outcome is of high clinical value. In addition, markers that can quantify tumor load are important in patients with small tumor volumes. Ideally, NET biomarkers should also be multidimensional, i.e., provide information on the ability of tumors to proliferate and metastasize. However, the currently clinically used biomarkers including chromogranin A, B, and C, pancreatic inhibition, and neurokinin A are unable to meet this multidimensionality. In addition, the panel believes that any circumstances leading to false negative and false positive results including the collection of blood specimens and the detection of markers need to be clarified.
Current biomarkers used for diagnosis
Currently applied biomarkers including chromogranin A, neuron-specific enolase, pancreatic polypeptide, pregastrin, serotonin, and urinary and plasma 5-HIAA can be used to assist in the diagnosis, but are not sufficient to establish the diagnosis or to help determine the site of tumor origin. The panel believes that the ideal circulating biomarker must be specific for the diagnosis of NET and be able to distinguish between functional and non-functional tumors by detecting circulating biomarkers, especially in patients presenting with non-characteristic symptoms, as they may be caused by NET, for example, flushing may be caused by carcinoid syndrome. The relationship between current circulating biomarkers and tumor load, grading and degree of benignity and malignancy is also uncertain. Therefore, the panel believes that most of the currently applied single-component biomarkers for NET do not meet the needs of clinical diagnosis and treatment.
Bronchial and pulmonary neuroendocrine tumor markers
Current biomarkers for gastroenteropancreatic NETs are not suitable for bronchial and pulmonary NETs, and there are no specific circulating biomarkers for bronchial and pulmonary NETs.
Clinical applications of circulating markers
The applications of circulating markers in NET include tumor diagnosis, follow-up, and prediction of tumor response to drug therapy. In addition, they are also used to clarify the outcome of surgery and the aggressiveness of residual tumor tissue. In addition, the functions of circulating biomarkers should include aiding in the detection of microscopic tumors and definitive surgical cure, as well as serving as predictors of tumor recurrence and prognosis.
Imaging and Circulating Markers
Most experts believe that CT and MRI combined with growth inhibitor receptor imaging can be used as a routine test for NET, and PET-CT with 68Ga-labeled growth inhibitor analogs or 18F-DOPA as the imaging agent is the best imaging method for NET in centers where available. In addition, the RECIST criteria do not apply to all types of NET, and the group believes that circulating biomarkers can provide useful reference information for imaging, ideally in combination with radiographic imaging, to obtain valuable diagnostic information. However, there are no imaging studies that correlate with circulating biomarkers more than 80%.
Pathological markers
Tissue biomarkers such as chromogranin A are more useful in the diagnosis of NET, while neuron-specific enolase (NSE) and tryptophan are of little value. In terms of quantifying tumor proliferation capacity, mitotic spindle markers (e.g. phosphorylated histone H3) are superior to mitotic counts, but the former have not been extensively evaluated in NET. As for Ki-67 index, the accuracy of its evaluation of tumor proliferation rate can be affected by bias between different laboratories, between different observers or even by the same observer, and it is imprecise to estimate Ki-67 index with the naked eye. The Ki-67 index may not be homogeneous within the same tumor, and the final grading of a tumor is determined by the proliferation hotspot with the highest Ki-67 index. The panel believes that diagnostic punctures should not be performed more than four times when assessing the proliferative activity of metastatic lesions. It is worth noting that the Ki-67 index is subject to change at different clinical stages of tumor development. Furthermore, the Ki-67 index does not predict micro-metastases. The panel agreed that despite its multiple limitations, the Ki-67 index is currently the best indicator for pathologic assessment of NET biological behavior.
Circulating tumor cells
Currently, the only FDA-approved method for circulating tumor cell detection is the CellSearch system, which has been shown to have potential application in metastatic NET, but further validation is needed. Current methods of circulating tumor cell analysis do not reliably detect all NETs, and circulating tumor cells are not sensitive and specific enough as diagnostic markers for NETs, and their relationship with tumor load, grade and prognosis is unclear. The group believes that more research is needed before accepting circulating tumor cells as an effective biomarker for NET.
Novel biomarkers
Several novel NET biomarkers are gaining further clinical investigation, including a whole blood RNA multigene marker (NETest) for gastroenteropancreatic NET based on multiplex cluster analysis. Although new single-component markers for NET still need to be further investigated, the panel concluded that single-component biomarkers are less effective than multi-component markers. The panel agreed that the results obtained by applying genomic or metabolomic techniques should be used to screen for novel markers of NET. The panel also considered circulating DNA and miRNA as novel biomarkers with potential applications.
Conclusion
Current clinically applied NET biomarkers have many limitations, and circulating biomarkers that accurately reflect disease activity and therapeutic efficacy are urgently needed. Cluster analysis of multicomponent genomic indicators is a promising direction for future research. NETest has a greater potential for clinical application, while circulating tumor cells and NET-specific miRNAs need further study. The panel also recommended blood biomarker testing of patients every 3 months to monitor tumor stability or progression.