Histologic Analysis of Idiopathic Pulmonary Fibrosis by Morphometric and Fractal Analysis

Abstract

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive fibrotic lung disorder, ultimately leading to respiratory failure and death. Despite great research advances in understanding the mechanisms underlying the disease, its diagnosis, and its treatment, IPF still remains idiopathic without known biological or histological markers able to predict disease progression or response to treatment. The histologic hallmark of IPF is usual interstitial pneumonia (UIP), with its intricate architectural distortion and temporal inhomogeneity. We hypothesize that normal lung alveolar architecture can be compared to fractals, such as the Pythagoras tree with its fractal dimension (D_f), and every pathological insult, distorting the normal lung structure, could result in D_f variations. In this study, we aimed to assess the UIP histologic fractal dimension in relationship to other morphometric parameters in newly diagnosed IPF patients and its possible role in the prognostic stratification of the disease. Clinical data and lung tissue specimens were obtained from twelve patients with IPF, twelve patients with non-specific interstitial pneumonia (NSIP), and age-matched "healthy" control lung tissue from patients undergoing lung surgery for other causes. Histology and histomorphometry were performed to evaluate D_f and lacunarity measures, using the box counting method on the FracLac ImageJ plugin. The results showed that D_f was significantly higher in IPF patients compared to controls and fibrotic NSIP patients, indicating greater architectural distortion in IPF. Additionally, high D_f values were associated with higher fibroblastic foci density and worse prognostic outcomes in IPF, suggesting that D_f may serve as a potential novel prognostic marker for IPF. The scalability of D_f measurements was demonstrated through repeated measurements on smaller portions from the same surgical biopsies, which were selected to mimic a cryobiopsy. Our study provides further evidence to support the use of fractal morphometry as a tool for quantifying and determining lung tissue remodeling in IPF, and we demonstrated a significant correlation between histological and radiological D_f in UIP pattern, as well as a significant association between D_f and FF density. Furthermore, our study demonstrates the scalability and self-similarity of D_f measurements across different biopsy types, including surgical and smaller specimens.

Keywords: IPF; UIP; fractals; histomorphometry.

Figure 1

Correlation table and histologic depiction of UIP pattern: (a) This correlation table shows a summary of the main result in shadows of blue (positive correlation: 1 = perfect positive correlation) and red (negative correlation: −1 = perfect inverse correlation); (b) Histologic UIP pattern at low power, characterized by honeycombing, architectural distortion, and temporal inhomogeneity (1× original magnification, Hematoxylin and Eosin stain).

Figure 2

Fractal dimension analysis results: (a) D_f in the UIP certain subgroup was associated with a shorter life expectancy; (b) D_f was consistently higher in the UIP certain subgroup; (c) higher architectural distortion and complexity is confirmed by D_f measurements compared to NSIP and control patients (ns = p > 0.05; * = p ≤ 0.05; ** = p ≤ 0.01; **** = p ≤ 0.0001).

Figure 3

Role of radiologic fractal dimension: (a) a high degree of correlation was obtained between histologic specimens and radiologic images; (b) the _f in CT scans of the UIP certain histologic pattern was significantly different from the UIP indeterminate patients (ns = p > 0.05; * = p ≤ 0.05).

Figure 4

Survival proportion according to D_f and FF density: (a) D_f could actually identify a larger number of patients with unfavorable prognosis compared to (b) FF density by itself.

Figure 5

Scalability between the whole section image (~400 mm²) and the ROI, mimicking a cryobiopsy area (10–36 mm²): (a) an almost perfect correlation between the values was achieved (+0.97, r² = 0.94); (b) consistent results among each specimen were high, despite a different UIP histologic pattern.