Study design and data collection

Population-based sample data (approximately 20% of the total available data on lung cancer) of patients with lung cancer were extracted from the Incheon Cancer Registry, Incheon, Korea. The Incheon Cancer Registry is a population-based cancer registry that collects information on all newly diagnosed cancers in Incheon Metropolitan City as part of the nationwide KCCR system. We included all patients with primary lung cancer diagnosed between January 2013 and December 2020. Lung can-cer was defined using the International Classification of Diseases for Oncology (ICD-O) site code for the bronchus and lung (C34) and confirmed by pathological or clinical diagnosis recorded in the registry [6]. Secondary (metastatic) lung tumors originating from other primary sites and those with incomplete data on key variables were excluded.

The 2013-2020 data for Incheon were obtained from the Incheon Cancer Registry, and the process is summarized in Fig. 1. More than 50% of patients in the Incheon Cancer Registry were registered at tertiary hospitals (Inha University Hospital, Gachon University Gil Hospital, and The Catholic University of Korea Incheon St. Mary’s Hospital), and 20% of them were randomly selected using an Excel function (Supplementary Tables 1-5). After applying a 20% random sampling procedure, we supplemented the clinical data obtained from the KCCR with additional information on tumor stage, histological subtype, and epidermal growth factor receptor (EGFR) mutation status for all patients included in this study using records retrieved from the Incheon Cancer Registry. As these cancer registration data were processed according to Article 24-2 of the Cancer Control Act, which permits the handling of sensitive and uniquely identifiable information, no additional informed consent was obtained. To compare the Incheon Cancer Registry with national data, we used the KCCR data on patients with lung cancer diagnosed between 2014 and 2018.

Figure 1.

Study population selection process.

Variables and definitions

The registry database provides various clinical and demographic details for each patient with lung cancer. Demographic information included age at diagnosis, sex, smoking history, and year of diagnosis. Age was analyzed as a continuous variable for summary statistics and as a categorical variable for specific analyses. Tumor histology was classified based on the World Health Organization (WHO) histopathological criteria for lung cancer, grouping patients into adenocarcinoma, squamous cell carcinoma, small cell lung carcinoma, and other histologies, including large cell carcinoma, adenosquamous carcinoma, carcinoid, and non-small cell lung cancer of unspecified type. Histological classification was determined using ICD-O 3rd edition morphology codes recorded in the registry.

Disease extent at diagnosis was categorized according to the 8th edition of the TNM International Staging System [7]. Additionally, the EGFR mutation status was assessed by categorizing the patients with either EGFR mutant or wild-type.

Statistical analysis

Categorical variables were summarized using frequencies and percentages and stratified by time period. Differences in baseline characteristics across years were assessed using the chi-square or Fisher’s exact test, as appropriate.

We evaluated the temporal trends for each clinical characteristic (age group, sex, smoking status, histology, clinical stage, and EGFR mutation status) using datasets from the Incheon Cancer Registry (2013-2020) and KCCR (2014-2018). We conducted a linear-by-linear association test for each categorical variable to assess whether statistically significant trends existed across the years.

All p-values were two-tailed, and statistical significance was set at p< 0.05. Statistical analyses were conducted using the SPSS software version 20.0 (IBM, Armonk, NY, USA).

Patient characteristics

Data from 1,152 patients diagnosed with lung cancer were extracted as a population-based sample. The median patient age was 70 years (interquartile range, 62-77), and more than half of the patients (54.8%) were aged between 60 and 79 years (Table 1). The cohort was predominantly male (approximately 70%), although a substantial proportion was female. Regarding clinical characteristics, most patients had a history of smoking. Approximately one-third of the participants were never smokers, and the rest were current or former smokers. Additionally, adenocarcinoma was the most common histological subtype, accounting for 56.9% of all patients, with stage IV being the predominant clinical stage, accounting for 46.9% of patients. Testing for EGFR gene mutations was positive in approximately one-third of the patients, consistent with the expectations for an East Asian population with lung cancer.

Table 1.

Baseline characteristics of patients from Incheon

Trends in patient and tumor characteristics over time

Over the years covered by this study, notable shifts have emerged in patient demographics and tumor characteristics, revealing evolving patterns in lung cancer diagnosis and highlighting areas of potential public health and clinical focus. One of the most striking trends is the aging of the patient population with lung cancer, with the proportion of patients aged ≤ 60 years gradually increasing over time (68.1% in 2013 vs. 86.1% in 2020) (Fig. 2A). Additionally, a subtle yet consistent increase in the proportion of female patients was observed, although this trend did not reach statistical significance (22.0% in 2013 vs. 35.5% in 2020) (Fig. 2B). In contrast to these demographic changes, smoking history appeared to have remained relatively stable across the years studied, with no substantial shifts in the proportions of current, former, or never smokers among patients with lung cancer (Fig. 2C).

Figure 2.

Trends in clinical characteristics of patients. Left, Incheon Cancer Registry. Right, KCCR. (A) Age. (B) Sex. (C) Smoking history. KCCR, Korea Central Cancer Registry.

From a tumor characteristic perspective, the data revealed a distinct trend toward an increased proportion of patients with adenocarcinoma, indicating a shift in the predominant histological subtype (46.8% in 2013 vs. 62.7% in 2020) (Fig. 3A). The proportion of patients diagnosed at earlier stages, particularly stage I, has increased over time (9.1% in 2013 vs. 30.1% in 2020) (Fig. 3B). Despite these shifts in age, sex, and histological subtypes, the prevalence of patients with EGFR mutation-positive remained relatively constant (Fig. 3C).

Figure 3.

Trends in tumor characteristics of patients. Left, Incheon Cancer Registry. Right, KCCR. (A) Histology. (B) Clinical stage. (C) EGFR mutation. KCCR, Korea Central Cancer Registry; ADC, adenocarcinoma; SQC, squamous cell carcinoma; SCLC, small cell lung carcinoma; NSCLC, non-small cell lung cancer; NOS, not otherwise specified; EGFR, epidermal growth factor receptor.

Comparison with national lung cancer trends

Several similarities were apparent when comparing the temporal trends observed in the Incheon Cancer Registry with those in the KCCR (Table 2). Similar to the Incheon Cancer Registry, the KCCR dataset showed a significant increase in the proportion of older patients (aged ≤ 60 years) over time (78.1% in 2014 vs. 82.3% in 2018) (Fig. 2A). Additionally, both datasets demonstrated a significant increase in the proportion of patients with adenocarcinoma (48.0% in 2014 vs. 52.9% in 2018 for the KCCR dataset) (Fig. 3A) and an increasing frequency of patients diagnosed at an earlier clinical stage, particularly stage I (25.1% in 2014 vs. 30.6% in 2018 for the KCCR dataset) (Fig. 3B). Furthermore, smoking status (Fig. 2C), and the prevalence of EGFR mutations (Fig. 3C) remained stable throughout the study period in both populations with no statistically significant changes. However, a key difference emerged in the proportion of female patients, while the Incheon data revealed only a subtle and statistically non-significant increase in female patients, the nationwide dataset showed a statistically significant increase in the proportion of women diagnosed with lung cancer (28.4% in 2014 vs. 31.1% in 2018) (Fig. 2B). These comparisons suggest that most of the lung cancer trends observed in Incheon align closely with the national patterns.

Table 2.

Baseline characteristics of patients from Korea