P1.11-02 Pathologic Comparison of Prevalent vs. Incident CT Lung Screen Detected Cancer in NCCN High-Risk Subjects: Are They Different?

Abstract

CT lung screening (CTLS) detects two overlapping but potentially distinct groups of tumors. Prevalent tumors, found at baseline screening, are thought to be enriched with slow-growing, potentially indolent cancers while incident tumors, found on annual repeat scans, are thought to be more uniformly fast-growing and aggressive. Pathologically, squamous cell carcinomas, small cell carcinomas (SCC) and large cell neuroendocrine carcinomas (LCNEC) are uniformly fast-growing and aggressive while adenocarcinomas are more heterogenous in their growth-rates, behavior and histology. By comparing pathologic subtypes, I-ELCAP investigators reported a higher frequency of adenocarcinomas compared to squamous cell carcinomas in prevalent compared to incident tumors (ratio 8:1 vs. 3.4:1) and conversely a 4-fold lower frequency of SCC/LCNEC (7% vs. 30%, respectively). Based partly on these data, some worry about the risk of over diagnosis in CTLS subjects undergoing baseline screening in which the proportion of slow-growing potentially indolent adenocarcinomas may be enriched. Current guidelines recommend screening specific high-risk subjects for which pathologic comparisons of prevalent and incident cancers have not been thus far described. The pathology of 134 CTLS cancers detected at Lahey Hospital and Medical Center was reviewed, including 93 detected at baseline and 41 at annual repeat screening. All individuals met the NCCN Guidelines Lung Cancer Screening v1.2012 high-risk criteria, were asymptomatic, had no known metastatic disease and were free of lung cancer for at least 5-years. Detailed pathologic analysis was performed for 65 stage I resected adenocarcinomas whereas lineage alone was determined for the cytologically diagnosed tumors. The proportion of adenocarcinomas to squamous cell carcinomas was similar in prevalent compared to incident tumors (ratio 2.6:1 vs. 3.1:1). Small cell carcinomas were only half as frequent among prevalent vs. incident cancers (5% vs. 12%). Among stage I adenocarcinomas, a similar proportion of prevalent vs. incident cancers exhibited a ≥5% solid pattern (32% vs. 30%), ≥5% micropapillary pattern (23% vs. 22%), ≥10% cribriform pattern (30% vs. 26%), intermediate/high mitotic grade (57% vs. 57%), angiolymphatic invasion (43% vs. 48%), and STAS (36% vs. 39%). Visceral pleural invasion was actually higher in the prevalent cancers (27% vs. 17%) however no statistically significant differences were observed. Pathologic comparison of prevalent vs. incident CTLS cancers among NCCN defined high-risk subjects reveals less variability than previously described by I-ELCAP. In particular, histologically indolent adenocarcinomas are not enriched in prevalent compared to incident tumors and thus the risk of over diagnosis in baseline detected cancers may be less than once thought.