96 Chapter 7 Abstract The concept of tumor cell clusters or loose cells as a pattern of invasion in lung carcinoma has recently been proposed and is included in the 2015 WHO fascicle on the classification of lung tumors, so-called “spread through air spaces” or STAS. This inclusion is controversial, as there are significant data to support that this histologic finding represents an artifact of tissue handling and processing rather than a pattern of invasion. These data are summarized in this review and support the conclusion that the inclusion of STAS in the WHO classification for lung cancer as a pattern of invasion was premature and erroneous. In our opinion, these tumor cell clusters or loose cells appear to be simply an artifact, although one which may or may not pinpoint to a high-grade tumor with discohesive cells and adverse prognosis.
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