Is self-reported height and weight accurate enough to be used in weight based FDG administration and SUV quantification in PET imaging?

Objectives: PET imaging with FDG has an important quantitative role in the assessment of and therapy in clinical oncology management and standardized uptake values (SUVs) are the tools used in quantifying FDG uptake. At our imaging site we use the lean body mass (LBM)-based SUV in our quantification analysis. Weight and height data is required in the formula to quantify SUV-LBM. At our PET Imaging Facility we use a weight-based formula for calculating the dose of F18-FDG to be administered for clinical PET imaging studies. Our goal was to retrospectively look at height and weight reported by patients and compared those to the actual measured weight and height obtained at our PET Clinic to determine if inaccuracies between the two could lead to incorrect dosing and/or quantification in PET imaging studies.

Methods: We collected height and weight data for clinical FDG-PET studies acquired at our center between May 2016 and September 2016. This data was both noted as reported by the patient at the time of scheduling and measured by the technologist in our clinic on the day the imaging was completed. Any data that could not be verified in both the above requirements was not used in this analysis.

Results: Of all patients imaged during this 4-month retrospective study, 234 reported their height and weight to the scheduler in the PET center and also had their height and weight measured by the technologist on the day of their PET scan. The percent error for each metric was calculated by subtracting the patient self-reported metric by the measured metric by the technologist, dividing the difference by the measured metric and expressed as a percentage. Thus, negative values represented cases where patients under-reported the metric and positive values were cases where the patient over-reported the metric. With respect to the height information collected in this dataset, no meaningful differences in percent errors were observed. However, with regards to the weight information collected from these 234 patients we did see a large variation in the range of errors reported. Analyzing the data set as a whole, we observed that, while the average reported discrepancy between self-reported weight vs. actual weight was only around 1%, the range of error was from -14% to +19%. When we introduced the patient’s sex into the analysis, we found that, among 94 male patients, the error in self-reported weight ranged from -8% to +9% (spread of 18%). When we looked just the 140 female patients, the range of errors in self-reported weight was -14% to +19% (spread of 34%). Of note, in the female population several cases of a more than 10% weight discrepancy were observed.

Conclusion: Although the average number of patients misreporting their weight was rather low, the results indicate concern for the effects of the large range in percent error of reported weight vs. actual weight. The significance of the large range in error can lead to a dosing misadministration in excess of what is allowed by most imaging protocols. These errors can affect the quality of a patient’s scan when under-reporting results in a lower than expected FDG administration, with a consequent poor target to background ratio in PET signal. In addition, the calculation of SUV-LBM is highly dependent on weight as a scaling factor. Therefore under-reporting and/or over-reporting this metric will negatively impact the quantitative value of PET imaging, as well as the consistency of those results in the therapeutic assessment between scans. Two sources of error to take into consideration for this analysis, the first being a transcription error by the scheduler when recording the patient’s reported values, i.e. inversion of numbers. The technologist’s technique when measuring actual patient values should be considered, as well.