A CT image that conflicts with improvements in symptoms and clinical picture^1,2

This is the first of 2 examples referred to as “indeterminate response,” since at least 1 of the CT images does not definitively classify the outcome. This example describes a GIST outcome where a CT image conflicts with observed improvements in patient symptomatology and the clinical picture. In this situation, the patient may report feeling better, but the larger tumor size seen in the CT image could be interpreted as disease progression.

FDG-PET is sometimes useful when evaluating GIST outcomes. While CT imaging allows for an assessment of change in tumor size and density, FDG-PET provides a measurement of the tumor’s metabolic activity. FDG-PET is not a substitute for CT—which is the standard imaging modality in GIST—but FDG-PET should be performed when the CT findings are inconclusive or inconsistent with the clinical presentation.

Here, FDG-PET is recommended as a complementary diagnostic modality to CT. FDG-PET is used to measure the metabolic activity of tumors, whereas CT provides information on tumor size and density. Importantly, in order for FDG-PET to be useful, a baseline FDG-PET must be obtained.

A spurious new lesion¹

This example demonstrates that on CT scans, it may be challenging to visualize a lesion if it enhances to the same degree as the surrounding tissue. Spurious new lesions are not actually new; in this case, the lesion was always present but became apparent in the CT image only when the density decreased in response to treatment. It should not be interpreted as a new lesion or as progression; the response is characterized as “indeterminate.”

In all cases of GIST, contrast-enhanced, 3-phase CT should be used to ensure identification of these lesions at baseline. In this example, the lesion became evident only in the post-therapy scan, an enhanced portal-venous–phase CT. A PET scan may also aid in this case if the lesion is FDG avid at baseline.