162 Chapter 12 The application of the current WHO classification was discussed among the authors. A case with iatrogenic collapse with tumour cells in one or two layers along a thin wall led to agreement. However, when elastin was absent in similar structures, opinions ranged from a lepidic pattern to possible papillary outgrowth (and therefore invasion) (Figure 7). Figure 7. A flow chart with thought process for the differential diagnosis between collapsed AIS versus papillary/ other adenocarcinomas. *) In serial sections of collapsed AIS continuity of the alveolar walls is visible, with the exception of cross sections of alveolar duct endings. ”Of note, in desmoplastic stroma elastic fibers may also appear discontinuous due to degradation of elastin by elastase containing inflammatory cells (see text for location of elastin degradation). Moreover, the desmoplastic stroma is a morphologic clue towards invasion.” The difference in interpretation may result from arguments based on a retained background architecture (i.e., the collapsed underlying microscopic anatomy), whereas the criteria used in the WHO classification of lung cancer since 1999 are based on microscopically visual tumour cell patterns. Although it is realized that the iatrogenic collapse may have an effect on measurements of tumour size17, the WHO does not incorporate in the current lung cancer classification the effect of iatrogenic collapse on pulmonary adenocarcinomas120. It is clear that more data on this subject with sufficient follow-up are needed to resolve this uncertainty, in order to reach the level required for an evidence-based pathology classification. Desmoplastic stroma A mixture of tumour cells and dot-like or fragmented elastic fibers in combination with an increase in the number of loose collagen fibers (desmoplastic stroma304;
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