“Matching pairs” of s-5HIAA and CT/MRI assessments were defined for each patient to determine the biomarker’s ability to depict disease progression. In particular, biochemical changes in serum levels of 5HIAA were analyzed with respect to “matching changes” in tumor size, as defined by the RECIST1.1 for regression, stable disease, or progression [24]. The radiological assessment was made by a blinded radiologist with respect to the findings of the biochemical serum and/or urinary 5HIAA analysis. For urinary 5-HIAA levels, the reference value of 50μmol/L/24 h was used, whereas for serum 5HIAA we used the reference value of 123 nmol/L. A 25% change in serum concentration was set to distinguish between increased, unchanged, or decreased serum 5HIAA levels. The changes in serum 5HIAA levels were subsequently compared with the changes in tumor size on the corresponding CT/MRI (“matching pairs” of Δs5HIAA and 2 sequential CT/MRI scans). The same imaging technique was used in the individual patients to determine disease status. The urinary and serum 5HIAA samples were collected at varying intervals depending on differences in individual follow-up, as well as patient and/or responsible physician preferences.

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