The impact of reclassifying cancers of unspecified histology on international differences in survival for small cell and non‐small cell lung cancer (<scp>ICBP SurvMark</scp>‐2 project)

Abstract Survival from lung cancer remains low, yet is the most common cancer diagnosed worldwide. With survival contrasting between the main histological groupings, small‐cell lung cancer (SCLC) and non‐small cell lung cancer (NSCLC), it is important to assess the extent that geographical differences could be from varying proportions of cancers with unspecified histology across countries. Lung cancer cases diagnosed 2010‐2014, followed until 31 December 2015 were provided by cancer registries from seven countries for the ICBP SURVMARK‐2 project. Multiple imputation was used to reassign cases with unspecified histology into SCLC, NSCLC and other. One‐year and three‐year age‐standardised net survival were estimated by histology, sex, age group and country. In all, 404 617 lung cancer cases were included, of which 47 533 (11.7%) and 262 040 (64.8%) were SCLC and NSCLC. The proportion of unspecified cases varied, from 11.2% (Denmark) to 29.0% (The United Kingdom). After imputation with unspecified histology, survival variations remained: 1‐year SCLC survival ranged from 28.0% (New Zealand) to 35.6% (Australia) NSCLC survival from 39.4% (The United Kingdom) to 49.5% (Australia). The largest survival change after imputation was for 1‐year NSCLC (4.9 percentage point decrease). Similar variations were observed for 3‐year survival. The oldest age group had lowest survival and largest decline after imputation. International variations in SCLC and NSCLC survival are only partially attributable to differences in the distribution of unspecified histology. While it is important that registries and clinicians aim to improve completeness in classifying cancers, it is likely that other factors play a larger role, including underlying risk factors, stage, comorbidity and care management which warrants investigation.