Figure 5. Three-dimensional appearance of a PLCH lesion. Artist’s rendering, based on the reconstructions by Kambouchner et al (40), illustrates the elongated morphology and variable cellular and fibrotic composition of PLCH with correlative histologic sections. As a PLCH lesion evolves, the nodule of densely packed cells (bottom, a) is centripetally replaced by fibrous tissue and ultimately becomes a stellate scar (top, c). This continuum of change may be evident within a single lesion. PLCH lesions are bronchiolocentric and propagate both proximally and distally along the small airways. The involved bronchiolar lumen may become either dilated or obliterated. The histologic sections correspond to the early, middle, and late phases of PLCH. In the early phase (a), there is a densely cellular nodule with delicate stellate extensions along the adjacent alveolar walls (original magnification, x12; H-E stain). As the disease progresses (b), cellularity diminishes as fibroblasts replace the lesion (original magnification, x19.2; H-E stain). Note that the stellate extensions have become more prominent, the central bronchiole (*) is dilated, and adjacent alveolar spaces have coalesced because of focal destruction of alveolar walls (paracicatricial air-space enlargement). In the final phase (c), the characteristic LCH cells are absent and only a fibrous, stellate scar remains (original magnification, x24; H-E stain). This phase is often accompanied by paracicatricial air-space enlargement (**).