Patients
A total of 31 IPF patients (23 men, 8 women) were included in the study. These patients were followed at the Erasme hospital until April 2018 and a [18F]-FDG PET/CT scan was performed between 2013 and 2016. The diagnosis of IPF was established in accordance with the ATS/ERS/JRS/ALAT recommendations (Raghu et al., 2011) through a multidisciplinary discussion involving pulmonologists, radiologists, internal medicine specialists (or rheumatologists) and pathologists experienced in the diagnosis of interstitial lung diseases (ILD). The patients were not treated by antifibrotic drugs at the time of the PET/CT, but most of them receive such a treatment during the follow-up period (pirfenidone, n = 15; nintedanib, n = 15; no treatment, n = 1). Patients under immunosuppressive therapy were excluded, with the exception of those receiving a low dose of corticosteroids (4 mg or less of methylprednisolone or equivalent). The protocol has been approved by the Erasme hospital Ethics Committee (ref. P2016/427; EudraCT/CCB: NA). Written informed consent for participation in the study was obtained from all the patients.
Clinical investigations
Information obtained included age, sex, tobacco exposure and treatments. For each patient, we analysed the pulmonary function tests (PFT) at the time of PET/CT and then every 3 months for at least 1 year of follow-up after the PET/CT evaluation. The following parameters were recorded: diffusing capacity of the lung for carbon monoxide (DLCO), forced vital capacity (FVC), vital capacity (VC) and total lung capacity (TLC). A 6-min-walk test (6MWT) was performed at the time of PET/CT and 1 year later. For each patient, the GAP index was calculated as described by Ley and al. (Ley et al., 2012). When available, differential cell counts from bronchoalveolar lavage (BAL) were recorded if BAL was performed 24 months or less before PET/CT (BAL performed during suspected infection or acute exacerbation were excluded).
To assess the clinical evolution of the patients at 1 year, the slope of the evolution of the VC, FVC, TLC, and DLCO were determined with a linear regression using least-squares method, in order to cope with the variability of PFT measurements performed routinely in clinical practice. The results of the 6MWT performed at the time of the PET/CT and at 1 year were also recorded. The difference between the pre- and post-exercise oxygen saturation (SaO2) at the time of PET/CT was calculated as well as the change in SaO2 post-exercise and the difference in walking distance at 1 year.
The disease activity for each patient was further characterized as “progressive” or “non-progressive”. A patient is categorized with a progressive disease if any of the following change occurred during the follow-up period of 1 year: ≥10% relative decrease from baseline in FVC, ≥ 15% relative decrease from baseline in DLCO, ≥ 5% decrease in the SaO2 after exercise at 6MWT, lung transplantation or death related to IPF.
PET-CT imaging
All imaging examinations were performed using a dedicated PET/CT scanner (Gemini GS16P, Philips Medical Systems, Cleveland, OH, USA). All patients were fasted for at least 6 h before the examination. Blood glucose level before [18F]-FDG injection had to be lower than 150 mg/dL. PET-CT images were obtained 60 min after injection of 300–370 MBq of [18F]-FDG. Low-dose CT was used for attenuation correction of the PET emission data. PET/CT was performed outside of an acute exacerbation. Acute exacerbation is defined by the sudden onset (within a month) of dyspnoea, associated with the appearance of new ground glass images on the chest CT, not fully explained by cardiac failure or fluid overload, and without alternative diagnosis such as pneumothorax, pleural effusion or pulmonary embolism (Collard et al., 2016).
Three-dimensional regions of interest were drawn on pulmonary parenchyma in the left and right lung using the Vivoquant software. In these regions, the SUVmean and SUVmax were obtained and corrected for the tissue fraction, according to a method already validated (Lambrou et al., 2011; Holman et al., 2015). Briefly, CT images were obtained for each patient to determine the lung density in Hounsfield unit (HULung). Then, a coefficient k corresponding to the tissue fraction was calculated as follow and as previously published (Lambrou et al., 2011):
$$ \mathrm{k}=\frac{\mathrm{H}{\mathrm{U}}_{\mathrm{Lung}}-\mathrm{H}{\mathrm{U}}_{\mathrm{Air}}}{\mathrm{H}{\mathrm{U}}_{\mathrm{Tissue}}-\mathrm{H}{\mathrm{U}}_{\mathrm{Air}}} $$
Where HULung, HUTissue and HUAir are the densities (in Hounsfield units) for the lung, soft tissues and air, respectively. HULung is determined on the CT, HUTissue is estimated equal to 50, and HUAir equals − 1000 HU. A matrix containing all these tissue fractions can be created, allowing the PET images to be corrected by dividing the SUV values in each voxel by the corresponding tissue fraction.
$$ \mathrm{SU}{\mathrm{V}}_{\mathrm{corr}}=\frac{\mathrm{SU}{\mathrm{V}}_{\mathrm{uncorr}}}{\mathrm{k}} $$
Statistical analysis
Correlations between imaging and clinical variables were analysed using Pearson correlation tests. Multiple comparisons were performed using ANOVA tests with Bonferroni post hoc analysis. According to the results of the D’Agostino & Pearson test used to assess the normality of the sample values, simple comparisons between two groups were tested by unpaired Student t tests or Mann-Whitney tests. Statistical analyses were performed using the GraphPad Prism 7.03 software. For all tests, a p-value of less than 0.05 was considered statistically significant.