Quantitative assessment of CT lung abnormalities in cystic pulmonary Langerhans cell histiocytosis using parametric response mapping

<bold>Introduction:</bold> In pulmonary Langerhans cell histiocytosis (PLCH), distal bronchioles are destroyed by infiltrating LCH cells, leading to lung cysts on computed tomography (CT) sometimes replaced by emphysema-like lesions. <bold>Objective:</bold> To evaluate the extent of small airways disease (SAD) and lung parenchyma lesions by parametric response mapping (PRM) quantitative method and their contribution to pulmonary function tests (PFTs). <bold>Methods:</bold> Nineteen PLCH patients were evaluated by inspiratory and expiratory chest CT and concomitant PFTs. PRM analysis was performed to differentiate normal density (PRM^Norm), persistent low-density (PRM^Pers) areas and functional SAD (PRM^fSAD). <bold>Results:</bold> We excluded one patient due to insufficient expiration. Eighteen patients (11 females, mean age of 45±11 years, 12 current smokers, pack-years (PA) 28 ±16) were analysed. Inspiratory and expiratory PRM volumes were strongly correlated with TLC and RV (R=0. 89 and 0.80, p<0.001). Emphysema-like lesions and cysts were classified as PRM^Pers. The median [IQR] values of PRM^Norm, PRM^fSAD, PRM^Pers were 60% [51.3; 88.8], 20% [7.3; 28.8] and 12% [3.0; 19.5]. PRM^Pers and PRM^fSAD were correlated with % predicted (pred) TLC, RV, RV/TLC, FEV₁ and FEV₁/FVC (p<0.05). PRM^Pers was correlated with pred D_LCO (p=0.04). PRM^Pers was associated with higher pred RV (β=3.8, 95%CI [0.8; 6.9], p=0.02), lower pred FEV₁ (β=-1.4, 95%CI [−2.7; −0.1], p=0.04), and lower FEV₁/FVC (β =-1.1, 95%CI [−1.9; −0.4], p=0.006), independently of PRM^fSAD and smoking PA. <bold>Conclusion:</bold> These results suggest that PFTs alterations in PLCH are more related to the extent of cysts and emphysema-like lesions on chest CT than to SAD.