Introduction
Idiopathic pulmonary fibrosis (IPF) is the most common type of idiopathic interstitial pneumonia. It has the worst prognosis with a median survival time of 3 years [
1]. The clinical course of IPF is variable [
2], and predicting its prognosis is difficult. Previous studies have reported that baseline and change in lung function over time and exercise capacity are associated with poor prognosis in IPF [
3‐
5]. Although high-resolution computed tomography (HRCT) is a pivotal modality for diagnosing IPF, it is unclear if it can be used for predicting the clinical course of IPF; previous studies on the association between HRCT patterns and survival, based on the 2011 IPF diagnostic criteria, showed inconsistent results [
6,
7]. Salisbury et al. reported that biopsy-proven IPF patients with a possible usual interstitial pneumonia (UIP) pattern on HRCT had significantly prolonged survival than those with a definite UIP pattern in multivariate Cox analysis adjusted by age, gender, smoking status, pulmonary function, and fibrosis extent [
6]. In contrast, Acardu et al. conducted a retrospective analysis of 350 patients with IPF and reported that a consistent UIP pattern was not associated with poorer survival than a possible or inconsistent pattern on HRCT when adjusted by age, sex, and lung function [
7]. These contradictory findings may be due to differences in baseline demographic features and disease severity between studies. Therefore, the usefulness of HRCT in predicting clinical course of IPF is still unknown.
In 2018, a new clinical guideline for IPF was released, which reclassified the HRCT patterns into four categories: UIP, probable UIP, indeterminate for UIP, and alternative diagnosis [
8]. The most important change from the previous guideline was that possible UIP pattern based on the 2011 guideline was subdivided into probable UIP and indeterminate for UIP [
9]. Therefore, it can be assumed that a more detailed HRCT classification may be more useful in predicting the clinical course of IPF. However, it is unclear whether these newly defined HRCT patterns can be used to predict the clinical course of IPF. Therefore, the aim of this study was to evaluate the impact of HRCT patterns on clinical course including lung function changes and survival in patients with IPF.
Discussion
In this study, IPF patients with an indeterminate for UIP pattern, which was a newly defined category in the revised IPF diagnostic criteria, exhibited more preserved lung function and a better clinical course (lesser decline in lung function and higher survival rate) than those with other HRCT patterns. Even when adjusted by clinical variables, including the baseline lung function and antifibrotic treatment, an indeterminate for UIP pattern independently predicted better survival. Conversely, IPF patients with probable UIP pattern had similar prognosis when compared with those with UIP pattern.
Chung and colleagues, in 201 patients with pulmonary fibrosis who had lung tissue samples taken, suggested that the possible UIP pattern could be subdivided into probable and indeterminate for UIP [
16]. Indeterminate for UIP is defined as having subtle reticulation or ground-glass opacity on HRCT that is not classifiable into the other categories [
8]. They demonstrated that histologic UIP was proven in 82.4% of patients with probable UIP on HRCT and 54.2% of patients with indeterminate UIP (
P = 0.01). In our study using biopsy-proven IPF patients, the indeterminate for UIP group had a better prognosis than the other groups. These findings implicated that the HRCT patterns may predict different clinical courses, as well as histologic correlation. Although it might be thought that a favorable prognosis is related to an early diagnosis, the findings of our study, in which the indeterminate for UIP group has been identified as an independent prognostic factor even when adjusted by age, lung function, exercise capacity, and antifibrotic treatment, suggest that the clinical course of the indeterminate group might be different from that of the other groups.
In our study, there were no significant differences in survival between IPF patients with the UIP pattern on HRCT and those with the probable UIP pattern. To date, inconsistent results regarding the association between survival and HRCT patterns were shown [
6,
7,
17]. Arcadu et al. in 350 patients with IPF (197 biopsy-proven cases), showed no significant difference in survival between patients with a UIP pattern and those with a non UIP pattern (possible and/or inconsistent UIP pattern; HR 1.29, 95% CI 0.90–1.83,
P = 0.15) on HRCT [
7]. Lee et al. in 606 patients with IIP whose HRCT pattern was UIP (n = 544) or possible UIP (n = 62), showed that a 3-year survival rate was different according to the HRCT patterns (44.6% in the UIP group vs. 56.8% in the possible UIP group,
P = 0.04). However, a propensity matching analysis of 122 patients (61 in the UIP group and 61 in the possible UIP group) showed that there was no survival difference between the two groups (48.7% vs. 61.1%,
P = 0.17) [
17]. In contrast, Salisbury et al. in 133 patients with biopsy-proven IPF, reported that patients with a definite UIP pattern on HRCT (n = 41) had a shorter survival period than those with a possible UIP pattern (median survival period: 2.27 vs. 6.95 years,
P = 0.002) [
6]. These conflicting results may be due to heterogeneous populations (various diagnoses and disease severity) in each study. Recently, Fukihara et al. in 311 IPF patients with HRCT pattern of UIP (n = 154) or probable UIP (n = 157), reported that the HRCT patterns were not significantly associated with survival in the multivariate analysis adjusted by age, sex, FVC, DL
CO, and use of antifibrotic agents (HR 0.883, 95% CI 0.640–1.218,
P = 0.447) [
18]. They categorized the HRCT patterns according to the revised diagnostic criteria, and showed consistent results with our findings. Therefore, insignificant survival differences between patients with UIP and probable UIP pattern on HRCT may advocate the current guidelines, in which surgical lung biopsy is conditionally recommended in the probable UIP pattern [
8,
19].
There were significant differences in lung function decline in IPF patients according to the HRCT patterns in our study. Although lung function changes were not significant between the UIP and the probable UIP group, lesser lung function changes were notable in the indeterminate for UIP group. Similarly, Raghu et al. in a post hoc subgroup analysis from the INPULSIS trial, reported that among the placebo group, the rate of lung function decline in patients who had a possible UIP pattern with traction bronchiectasis on HRCT was comparable to that in patients with a UIP pattern on a surgical lung biopsy or HRCT (− 221.0 vs. − 225.7 ml/year) [
20]. However, any other data have not existed on the trajectory of lung function according to each HRCT pattern. Instead, studies on the association between lung function changes and disease severity stratified into baseline FVC were reported [
21,
22]. Helen et al. in 416 patients with IPF, showed that the annual decline in FVC was not different (− 4.6% predicted/year vs. − 4.9% predicted/year,
P = 0.779) between patients with FVC ≥ 80% predicted and those with FVC < 80% predicted [
21]. Furthermore, Kolb et al. using the placebo group from the INPULSIS trial, showed that the rate of decline in FVC per year in IPF patients with baseline FVC ≥ 90% predicted was similar to that in those with FVC < 90% predicted (− 224.6 ml/year vs. − 223.6 ml/year) [
22]. Considering that patients with an indeterminate for UIP pattern showed better lung function (mean FVC ≥ 80% predicted) than the other groups, our results were inconsistent with previous findings. These contradictory findings may be due to differences in treatment status of the study population (no treatment [
22] or a small number of patients receiving antifibrotic agents [
21]). Recently, Cocconcelli et al. in 49 patients with IPF naïve of antifibrotic agents, showed that changes in alveolar score, defined as extent of ground-glass opacities, were associated with FVC decline (r = 0.66,
P = 0.002) [
23]. Although our study did not use a quantitative scoring system, our results suggest that along with quantitative measure and lung function, qualitative evaluation of HRCT at baseline could be a useful predicting prognosis.
In the current study, lung cancer tended to be more likely to be developed in patients with a UIP pattern on HRCT in addition to those with indeterminate for UIP pattern. However, previous study showed results inconsistent with our findings; Almeida et al. in 244 patients with ILD, reported that there were no significant differences in the lung cancer incidence according to the HRCT pattern (8.5% in UIP, 10.5% in probable UIP, and 16.7% in indeterminate UIP,
P = 0.551) [
24]. However, of the all subjects, only 38.4% had IPF and 29.1% was ever-smokers. In patients with IPF, an exceedingly high proportion of carcinomas, including squamous carcinomas, develop at the same sites where fibrosis and tissue remodelling are predominant (peripheral and/or basal lung areas), and are topographically associated with honeycomb lesions and epithelial metaplasia [
25,
26]. Calio et al. also demonstrated that the immunohistochemical characterization of lung cancer in IPF patients exhibited more varying bronchiole-related markers than that in non-IPF patients, [
27] suggesting that lung cancer in IPF arise from transformed small airways in honeycomb lung areas where abnormal bronchiolar proliferation takes place. These results support our finding showing higher incidences of lung cancer in IPF patients with an UIP pattern on HRCT.
This study has several limitations. First, this study was conducted in a single centre and had a retrospective design. However, the baseline characteristics of our patients were similar to those of previous studies [
6,
17,
20]. Second, our study only included IPF patients confirmed by surgical lung biopsy. As surgical lung biopsies cannot be done in patients that have a high risk of postoperative complications, the included subjects may have better-preserved lung function. Nevertheless, when lung function was adjusted in the multivariate Cox analysis, the HRCT pattern was still a significant predicting factor for survival in patients with IPF. Third, the number of patients with indeterminate for UIP pattern or alternative diagnosis, compared to that with UIP or probable UIP pattern on HRCT, was small. Previous studies reported that 35–60% of interstitial lung disease patients with a pathologic UIP pattern showed an inconsistent UIP pattern on HRCT [
16,
28]. However, the current study included only those diagnosed with IPF through a surgical lung biopsy, excluding nonspecific interstitial pneumonia or chronic hypersensitivity pneumonitis. Even considering a small number of subjects, our results suggest the usefulness of HRCT in predicting survival in patients with IPF. Finally, interobserver agreement for HRCT patterns was not high. However, interobserver agreement in our study is comparable to that of previous study (kappa value 0.66–0.69) [
29]. A recent study of 98 patients with ILD showed that the new software, IPFdatabase, can improve interobserver agreement among radiologists (kappa value of 0.18 to 0.64) [
30]. Further studies on enhancing the interobserver agreement are warranted.
In conclusion, an indeterminate for UIP pattern on HRCT was associated with more stable lung function changes and better survival in patients with IPF. These results suggest that HRCT may be useful in classifying subgroups with a better prognosis among patients with IPF. This supports the validity of the new classification guidelines regarding HRCT patterns.
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