160 Chapter 12 Figure 5. A, B, an example of lepidic adenocarcinoma with a pseudopapillary pattern (A, haematoxylin and eosin; B, elastin) with a focal papillary mimicker (blue arrows). C, Elastin staining shows the same pseudopapillae with focal elastin (yellow arrows). The stromal part of papillary carcinoma is mainly collagenous and focally desmoplastic. Note that the elastin in the interlobular septum is curly, as opposed to being stretched during maximal inhalation. The non-invasive nature of this papillary mimicker was recognized in 2005 by Ishikawa et al. as ‘Non-invasive papillary adenocarcinoma of the lung: a proposal of new entity’303. These pseudopapillary structures result from iatrogenic collapse but should not be considered as a distinct pathological entity. Instead, this entity represents a gap in the definition of the 1999 WHO classification12, in which specific architectural and stromal constituents, such as desmoplasia, were not incorporated11. Importantly, cross-sections of these papillary mimickers have a diameter of ⁓46 µm (range, 36–54 µm; Motoi and Thunnissen, unpublished data). The microscopic two-
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