74 Chapter 5 Clamping Edema During lobectomies and pneumonectomies, lymph fluid in the lung drains into local and regional lymph nodes. However, during video-assisted thoracoscopic procedures the lymph vessels are not emptied before clamping. The staple gun compresses the lung tissue before adding the 2 rows of staples. This rapid compression squeezes the lymph in the lymph vessel columns both centrally and peripherally. The centrally directed lymph remains in vivo, but the peripherally pressed lymph dilates lymph vessels, leading to interstitial overflow. This is usually most prominent in segmental interstitium, as shown in Figure 4, A and B. This clamping artifact is only seen in video-assisted thoracoscopic specimens. One should not mistake this artifact for either septal edema or lymphatic dilatation89. Figure 4. Images supporting the hypothesis of the clamping artifact. Spaces with asterisks (*) denote dilation by lymph fluid, which is most prominent in the septum and only minorly present in the pleura, emphasizing that the lymph vessels running toward the hilum are compressed during clamping (hematoxylin-eosin, original magnifications x1.25 [A] and x5 [B]). Spreading Through a Knife Surface An extraneous tissue contaminant on a slide is called a floater. This potential source of diagnostic confusion occurs in approximately 1.2% of prepared slides102. Floaters occur at the slide level in 73% and at the paraffin-block level in 16% of cases103. Approximately half of these contaminations are derived from the same patient sample103. When the origin of the tissue floater is uncertain, DNA analysis can be helpful104,105. Although contamination may occur outside the pathology laboratory, it mainly happens inside the laboratory during gross handling, cutting, or slide processing102,106. The location of the contamination may be from the specimen surface or from a cut surface102. During lung resection specimen prosection, tumor cells may be displaced by the knife along the plane of sectioning: spreading through a knife surface (STAKS). The contaminants may either be displaced within the specimen or left on the surface of the tissue. Depending on the size of the loose tissue or cellular fragment, the floater
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