112P Retrospective analysis of the use of immunohistochemistry for the diagnosis of adenosquamous carcinoma of the lung in a Dutch national cohort

Adenosquamous carcinoma (ASC) is a subtype of NSCLC, which can be diagnosed if the tumor meets three criteria: (1) a combination of an adenocarcinoma and squamous cell carcinoma component, (2) the adenocarcinoma part must constitute at least 10% of the tumor, and (3) the squamous cell carcinoma component also has to cover at least 10%. The goal of this study is to determine to which extent immunohistochemistry (IHC) alone is used to diagnose ASC. A query was performed through the Dutch National Pathology Archive. All excerpts with a possible ASC were included. Subsequently the excerpts were categorized. Excerpts were included in the 'certain' group if ASC was the working diagnosis. Afterwards, this group was subdivided: (1) only IHC for the diagnosis of ASC and (2) all other methods to diagnose ASC. With this distinction the percentages of patients diagnosed with IHC only can be determined. The query resulted in 1468 excerpts. After categorization and removing duplicates 651 patients remained in the certain group, 267 patients were diagnosed with ASC based on IHC only. As seen in the table, the diagnosis of ASC slightly increases over time. This might be explained by the introduction of molecular diagnostics (MD). Over time, the guidelines for the performance of MD for systemic therapy of NSCLC changed from 'adenocarcinoma only' to all non-squamous carcinomas. Patients with inconclusive IHC were therefore amenable for MD, possibly leading to an increase in ASC diagnosis. Moreover, IHC-only diagnoses appear to have increased over this time period. This might show that IHC is a useful tool in the primary diagnosis of ASC, particularly with the advent of MD. Table 112PNumber of patients diagnosed with ASC each year with the method used for diagnosisYearTotal number ASCASC diagnosed with IHC only2004262 (7.7)2005316 (19.4)2006225 (22.7)2007318 (25.8)20083711 (29.7)20093512 (34.3)20103714 (37.8)20112814 (50.0)20123818 (47.4)20132713 (48.1)20143619 (52.8)20153316 (48.5)20163216 (50.0)20174620 (43.5)20184220 (47.6)20194622 (47.8)20203619 (52.8)20214215 (35.7)20222617 (65.4) Open table in a new tab IHC is increasingly being used for the initial diagnosis of ASC. Furthermore, the use of molecular diagnostics may have led to a pragmatic increase in the diagnosis of ASC.