Due to the fact that computed tomography and positron emission tomography scans measure different things, there can be a difference in the size of the lung nodule on the two scans. CT scans create anatomical images, whereas PET shows metabolic changes, as explained by the National Cancer Institute.
A CT scan only shows that a nodule is present, not whether this nodule consists of benign or malignant cells. A PET scan, however, does provide this information, as malignant cells are usually more metabolically active than noncancerous cells. Therefore, if some but not all cells in the nodule are malignant, PET would reveal a smaller tumor than CT.
In some cases, combined CT/PET scans are used to obtain a more complete image of a cancer's size and growth, explains the National Cancer Institute. These integrated CT/PET scans are better than the combination of separate CT and PET scans at determining the stage of the cancer and localizing malignant areas, according to a study published on National Center for Biotechnology Information. Furthermore, they can improve the targeting of biopsies and therapy. However, in the case of lung cancer, the accuracy of integrated CT/PET was only a little better than PET on its own, although the combined scans did increase the level of certainty in the findings.