Cytologic subtyping of lung adenocarcinoma by using the proposed International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society (IASLC/ATS/ERS) adenocarcinoma classification

Background: The significance of histologic subtyping of surgically resected lung adenocarcinoma (ADC) was recently proposed by the International Association for the Study of Lung Cancer/American Thoracic Society/European Respiratory Society (IASLC/ATS/ERS) classification. Approximately 70% of lung cancer patients present with advanced disease, and small biopsies or cytology specimens are frequently the only available diagnostic material. It is uncertain whether proposed morphologic subtyping of ADC can be applied to small specimens. The objective of this study was to assess the applicability of morphologic subtyping of ADC on cytologic specimens.

Methods: Consecutive, newly diagnosed primary lung ADC specimens from patients with matched surgical resection and cytology specimens (n = 66) were selected for the study. The dominant morphologic pattern was determined according to the IASLC/ATS/ERS classification. The number and percentage of malignant cells in cytology specimens were also evaluated.

Results: Concordant subtyping of ADC between the dominant pattern on resection and cytology specimens was observed in 26 cases (40%), and was discordant in 32 cases (48%). Concordance increased in specimens that had >200 cells and when correlating with the primary or secondary histologic pattern. The acinar pattern was the most common in concordant cases, whereas discordant cases had a predominantly solid pattern.

Conclusions: Application of the IASLC/ATS/ERS ADC classification to cytologic specimens is challenging and depends on the sufficient cellularity of cytologic preparations. The identification of solid and micropapillary patterns is prognostically important but may be unreliable and difficult on cytology specimens. Future studies are needed to establish reproducible cytologic criteria for the precise subtyping of lung ADC on small specimens.