Metabolic pseudoprogression in a patient with metastatic KIT exon 11 GIST after 1 month of first-line imatinib: a case report

Abstract

Background: Positron emission tomography (PET) with 18-fluorodeoxyglucose (¹⁸FDG) has proven to be highly sensitive in the early assessment of tumor response in gastrointestinal stromal tumors (GIST), especially in cases where there is doubt or when the early prediction of the response could be clinically useful for patient management. As widely known, kinase mutations have an undoubtful predictive value for sensitivity to imatinib, and the inclusion of KIT and PDGFRa mutational analysis in the diagnostic workup of all GIST is now considered standard practice.

Case presentation: Herein, we described in detail a case of an exon 11 KIT mutated-metastatic GIST patient, who presented an unexpected metabolic progression at the early ¹⁸FDG-PET evaluation after 1 month of first-line imatinib, unconfirmed at the liver biopsy performed near after, which has conversely shown a complete pathological response.

Conclusions: This report aims to highlight the existence of this metabolic pseudoprogression in GIST at the beginning of imatinib therapy in order to avoid early treatment discontinuation. Therefore, an early metabolic progression during a molecular targeted therapy always deserves to be evaluated in the context of the disease molecular profiling, and in case of a discordant finding between functional imaging and molecular background, a short-term longitudinal control should be suggested.

Keywords: FDG-PET; GIST; functional imaging; gastrointestinal stromal tumors; imatinib.

Figure 1

Representative graphical output of the next-generation sequencing analysis, showing an exon 11 deletion of KIT (Gln556_Val559del). The different colors are referred to the single DNA basis.

Figure 2

Basal axial CT scan evaluation, showing a hypodense and partially colliquated liver lesion at segments II–III (arrow).

Figure 3

(A) Basal axial PET/CT fused images, showing a hypermetabolic lesion involving II–III liver segments (SUVmax 9.4). (B) Axial PET/CT fused images, restaging scan after a month of imatinib therapy: the hypermetabolic area appears increased in size and shows a greater ¹⁸F-FDG uptake: SUVmax 20.1 (vs. 9.4).

Figure 4

(A, B) H&E ×4: histologic features of the liver biopsy showing a vascular tissue with bland spindle cell fibroblast with collagenous stroma, admixed with a variable number of inflammatory elements (granulation tissue). (C, D) Immunohistochemical stain for DOG-1 (C) and CD117 (D) was all negative.

Figure 5

(A) Axial CT scan evaluation after 3 months of imatinib treatment (arrow), showing a partial response of liver lesion. (B) Axial PET/CT fused images scan, after 3 months of imatinib treatment, showing a marked decline in both lesion size and ¹⁸F-FDG uptake (SUVmax 7.9).