Focal FDG Uptake in Lung Parenchyma Without Structural Alterations on CT.

Transient visualization of a FDG avid abnormality in the lungs without any structural lesion is reported in a case of iatrogenic pulmonary microembolism.

Introduction

F-18 fluorodeoxy glucose (FDG) positron emission tomography/computed tomography (PET/CT) offers an effective diagnostic modality, especially in the field of oncology allowing metabolic and morphologic information to be acquired in a single examination session. For the correct interpretation of F-18 FDG PET/CT, it is crucial to have a clear understanding of the biodistribution of FDG, physiological variations in its distribution, and related medical and surgical histories of the patient.

Case Report

A 48-year-old female, under surveillance following surgery for gastric cancer was referred for control evaluation by the F-18 FDG PET/CT study. F-18 FDG PET/CT images demonstrated an area of increased FDG uptake in the right lung middle lobe parenchyma, without any structural abnormality on CT [Figure 1]. A repeat F-18 FDG PET/CT scan was performed after a brief interval of 4 days of the first study did not reveal any abnormality; the previously noted intense focus of FDG avidity having been completely resolved [Figure 2]; the latter suggesting the benign nature of the lung abnormality. On the basis of clinical presentation and imaging studies a final diagnosis of iatrogenic microembolism was made.

Figure 1

F-18 FDG PET/CT scan. Transaxial-, coronal-, and sagittal-section images of PET alone, CT alone and PET/CT fused images of the lungs show a discrete focus of increased uptake in the right lung middle lobe, without any structural abnormality on CT

Figure 2

A control F-18 FDG PET/CT scan performed after 4 days of the first PET/CT. Transaxial-, coronal-, and sagittal-section images of PET alone, CT alone and PET/CT fused images of the lungs revealed complete resolution of the previously noted FDG avid focus. No other abnormalities were noted

Discussion

Pulmonary infection or inflammation might predispose patients to localized F-18 FDG lung uptake mimicking pulmonary metastases, thereby limiting the specificity of whole-body scans performed in patients with cancer. Normally, the FDG avid lesions are invariably accompanied by structural changes as evident on the CT scan. However, F-18 FDG foci without pathological CT are a rare finding and have been associated with inflammation and iatrogenic microembolism caused during injection. In the medical literature, two main causes have been identified which may give rise to such findings. These include uptake by a pre-existing inflammatory vascular thrombus or an iatrogenic microembolism formed during injection[1] of the radiotracer.

A few publications are available in the literature.[1-8] In all instances the FDG avid abnormalities have shown complete resolution in the follow-up studies. In most instances the follow-up scans were performed after a period of a few weeks to a few months. It is interesting to note that in our particular case the follow-up F-18 FDG PET/CT study was performed only after a brief interval of just 4 days of the initial scan; and we were able to document complete resolution of the FDG avid focus. This is probably the only instance where an acute episode of iatrogenic microembolism and its prompt resolution within a few days have been documented by an F-18 FDG PET/CT study. It is interesting to note that the follow-up scan did not reveal any structural abnormality like infiltration, consolidation, or atelectasis, which is normally associated with such episodes of infection or microembolization.

Conclusions

Pulmonary iatrogenic microembolism or inflammation might predispose patients to localized F-18 FDG uptake in the lungs without any associated structural lesion. Such abnormalities should be treated with caution. A follow-up scan after a period of few days or weeks may help in arriving at a diagnosis.