Positron Emission Tomography (PET) is a proven imaging method in evaluating tumours by measuring their metabolic activity. A new concept in PET research is the noninvasive measuring of tumour heterogeneity (TH) using new recently published parameters. TH is a statistical calculation based on the voxel uptake values inside the volume of interest (VOI). The potential clinical value of TH is a theoretical correlation with tumour aggressiveness and response to therapy. To date, TH parameters predictive value is inconclusive and no standardized method of evaluating tumour heterogeneity exists.

The purpose of this study was to measure tumour heterogeneity using 33 of the new parameters, with the expectation of determining minimal voxel number needed for a valid measurement. Due to the spatial resolution of PET and the statistical calculation of the TH, a lower limit of tumour size for accurate measuring was to be explored.

A retrospective study was performed based on 85 selected human patients with solid pulmonary nodules. Segmentation was made by defining a boundary box around the pulmonary nodule with a threshold of 2.5 standard uptake value (SUV). The threshold was defined according to international guidelines. Built in calculations of the parameters were recorded and the data analyzed.

As expected, a minimal tumour size necessary to get a valid TH value was found. Based on the analysis, a value of the minimal tumour size was 120-150 voxels, corresponding to the range of 4 cm diameter in a spherical volume.

Unexpectedly, in 25 out of the 33 parameters a strong correlation was found between the tumour volume and the measured parameters, raising the question of the validity of those parameters in TH measuring.

In conclusion, this study suggests that in order to get a valid measurement of TH the tumour size needs to be at least 120-150 voxels and that only 8 out of the 33 measured new parameters should be used.