Background Idiopathic pulmonary fibrosis (IPF) has a variable disease course and we lack biomarkers that accurately predict prognosis or treatment response. Positron Emission Tomography-Computed Tomography (PET-CT) provides structural and functional information about the lung. A study of 25 IPF patients reported increased 18[F]-FDG uptake in areas of normal lung compared to controls.¹ If confirmed, this raises the possibility that PET-CT can identify ‘microscopic fibrosis’ with prognostic implications. We assess 18[F]-FDG uptake in areas of lung with normal CT appearance in a second IPF cohort.

Methods PET-CT scans undertaken for cancer staging at an interstitial lung disease tertiary referral centre were reviewed. IPF patients and controls without lung disease were identified. 18[F]-FDG uptake was assessed using manual region of interest (ROI) placement in areas of lung with normal CT appearance in IPF patients and controls. ROI were placed away from the mediastinum and concomitant tumours. 18[F]-FDG uptake within ROI was expressed as maximum and mean standardised uptake values (SUV) normalised using body weight. Mean Hounsfield Units (HU) were evaluated to assess for subtle differences in radiodensity within ROI. Data are presented as mean ± SD. Unpaired, 2-tailed T-tests were used to compare between group differences with a P value < 0.05 considered significant.

Results Forty-five subjects were included in this study (15 IPF and 30 controls). Lung cancer was the most common concomitant malignancy in both groups.

There was no difference in mean HU within ROI between IPF and controls (−719 ± 79 HU in IPF and −723 ± 147 HU in controls. P = 0.92). Areas of normal lung in IPF patients exhibited increased 18[F]-FDG uptake compared to controls measured by maximum SUV (0.98 ± 0.32 in IPF and 0.70 ± 0.20 in controls, P <0.01) and mean SUV (0.80 ± 0.29 in IPF and 0.57 ± 0.18 in controls, P < 0.01).

Conclusions We confirm that in IPF, areas of normal appearing lung exhibit increased 18[F]-FDG uptake compared with corresponding areas in controls. A longitudinal study is required to establish the relationship between 18[F]-FDG uptake, disease progression and treatment response.

Reference

1. Win T, Thomas BA, Lambrou T, et al. Areas of normal pulmonary parenchyma on HRCT exhibit increased FDG PET signal in IPF patients. Eur J Nucl Med Mol Imaging 2013;41(2):337–42.