69 Artifacts and histologic pitfalls in the lung However, in 3 dimensions, these ‘‘papillae’’ are connected with other fragments above or beneath the plane of section, indicating that the stromal components of these ‘‘pseudopapillae’’ are actually the stromal components of preexisting alveolar walls. The elastin stain is helpful in this instance, since the identification of elastin in these thin alveolar walls points to the underlying architecture of peripheral lung tissue90. Conceptually, in statistical terms the probability of finding a straight papilla in a section of peripheral lung is low, probably as close as the probability of finding a completely straight hair in a histologic section of the skin. The probability of finding 15 papillae in 1 microscopic field of view is much smaller, providing an additional argument that the proliferation represents cross-sections of lepidic pattern as opposed to true papillae. In our opinion in case of thin ‘‘papillae’’ the presence of (frequently discontinuous) elastin fibers is diagnostic of lepidic pattern and supports in the distinction of a papillary carcinoma90, 6. A second pitfall may occur if preexisting fibrosis is present adjacent to lepidic patterns of adenocarcinoma. This fibrosis may be collagenous or of the infarction type with abundant fibroelastosis. Owing to the partial collapsed irregular alveolar walls, structures looking like irregular acini may mimic acinar adenocarcinoma, also seen in an earlier reproducibility study among 28 pathologists14 (Figure 2, A through F; images not published with this previous study). This may introduce the possibility of an erroneous diagnosis of invasive adenocarcinoma adjacent to a fibrotic area14. Moreover, in lung cancer resection specimens with smoking-related lung disease, the stromal changes in remodeled lung should not be a priori attributed to lung cancer, leading one to consider any epithelial atypia as neoplastic. Preexisting scarring should be taken into account as well. Usually, this fibrosis consists of dense collagen fibers with scarce fibroblasts. The third pitfall occurs in partial collapse giving an appearance of ‘‘tufting,’’ that is, the piling of the cells in airspaces without fibrovascular core formation. Sometimes this may give the impression of loose cells floating freely in alveolar spaces in close association with a lepidic growth pattern. As tufting is the morphologic hallmark of micro- papillary carcinoma, the occasional appearance of this pattern should not be diagnosed as micropapillary carcinoma. Although the minimal amount of micropapillae required for a diagnosis of papillary carcinoma is not established, practical considerations suggest that it should be extensive13. 5
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