183 Pattern recognition in pulmonary AdC; a modified classification The pattern of iatrogenic collapsed AIS adjacent to the scar is more or less similar to AIS without scar, except for the border zone between scar and AIS. In the border zone, a reduction of glandular structures may be observed (Figure 6A and B). The sometimesangulated compressed alveoli may mimic an acinar pattern317, but the elastin containing structures support non-invasive nature (Figure 6C). In the CK7 this change in glandular size also shows a regular pattern (Figure 6D), in line with the WHO classification19. Figure 6. Case of adenocarcinoma in situ showing at the periphery a regular pattern of a monolayer of tumour cells with a central “scar” (A, H&E, 100x). At the border area of the scar a reduction of alveolar structures is seen, but no desmoplastic stroma (B, H&E, 200x). In the elastin staining there are elastin fibers in the peripheral preexisting alveolar septa, while in the “scar” area remnants of alveoli with increased elastin are visible (C, Elastin, 100x). In cytokeratin 7 staining at the periphery regular pattern is seen, with condensation of alveolar or “gland-like” structures towards the edge of the “scar” (D, CK7, 200x). Note that the “scar” area fits in the description of biological collapse with smaller alveoli with increased elastin. In the proposed modified classification, the elastin staining is used for the differential diagnosis between true papillary carcinoma and collapsed AIS: the presence of elastin demonstrates that these structures represent pre-existing collapsed alveolar walls308 and should, without other signs of invasion, be interpreted as part of the pre-existing peripheral pulmonary framework, supporting the diagnosis of AIS. When invasion is present in other areas, the growth pattern that occurs along the alveolar surfaces of adenocarcinoma is referred to as “lepidic”. We distinguish several other forms of intra-alveolar growth, besides a monolayer as in collapsed AIS or papillary carcinoma: 14
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