264 Chapter 17 of the overlapping patterns and non-invasive growth on alveolar walls and markedly reduces the variation in assignment of invasion. As the elastin staining is also used as an aid in the assessment of pleural invasion, and CK7 in most laboratories part of the immunohistochemical armamentarium, the using these stainings to support the morphological diagnosis of pulmonary adenocarcinoma in situ should not have technical barriers and therefore be easily applicable in routine practice. Technically, for an optimal result, it is important to refresh the Lawson component of the staining at least every 3 weeks, while the von Gieson component should be refreshed every 4 weeks. A thin wall or relatively thin wall that has at least a clear fragment of elastin is based on morphological reasons part of a pre-existing alveolar wall. The elastin fibers are best recognized at high power microscope objective (40x). As mentioned above elastin is produced by mesenchymal cells and NOT by tumor cells. If the differential diagnosis on H&E stain is between papillary carcinoma and iatrogenic collapsed AIS. The presence of (discontinuous) elastin fibers in a tumor with fibrovascular cores lined by tumor cells denotes the fibrovascular core as a 2-dimensional cross cut of alveolar wall. Without other invasion characteristics the diagnosis is than (iatrogenic collapsed) AIS. Likewise, if a pathologist doubts about a collapsed AIS/ lepidic adenocarcinoma and acinar carcinoma the characteristic findings of continuous or fragmented elastin in the structures lined with tumor cells should suffice to diagnose that area as collapsed AIS/ lepidic adenocarcinoma. The papillary carcinoma is in the revised classification defined as a papillary tumor without elastin. This definition seems In larger tumors (> 3cm.) to match perfectly with those described in the original work of Silver and Askin11. Probably the in this way defined papillary carcinomas have a similar worse prognosis, as described by Silver and Askin. Table 1 shows the relationship between those 3-dimensional in vivo structures and the 2-dimensional microscopic view the pathologist uses. Awareness of the cross-sectional appearance of so called “papilla” is important, as this structure has the connotation of invasion according to the WHO classification of pulmonary adenocarcinomas. Realizing that if a so-called “papilla” would be part of a papillary tumor with several papillae, then there must be more oval cross sections than longitudinal appearances. If that is not the case, the likely interpretation is that the seemingly papillary structure is part of a pre-existing alveolar surface structure in the third dimension (see Table 1 and Figure 1). This is a clear example that 2-dimensional pattern recognition as applied in the WHO classification of pulmonary adenocarcinomas is dissociated from the 3-dimensional pulmonary morphology.
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