Head and neck cancer (HNC) encompasses a variety of subsites and ranks as the 7th most common cancer globally, with over 660,000 cases annually [1]. Smoking and alcohol consumption are well-established risk factors for HNC, though their impact varies by subsite. Human papillomavirus (HPV) infection is also a significant factor, particularly for oral and oropharyngeal cancers [1,2]. In South Korea, while HPV infection rates are rising, the incidence of HNC was expected to decline due to decreasing smoking rates, the strongest risk factor for HNC [1,3,4]. However, the incidence of HNC across all subsites has continued to rise since 1999, when statistics were first compiled [5]. This trend is partly explained by South Korea’s rapidly aging population [6].

Since the incidence of most chronic diseases, including cancer, increases with age, the overall cancer incidence in South Korea is naturally rising [6]. To evaluate cancer trends independent of population aging, age-adjusted incidence rates are essential [7]. Age-adjusted incidence is calculated by applying age-specific incidence rates from a population of interest to a standardized age distribution [7]. If the age-adjusted incidence of a cancer type increases or decreases over time, it is crucial to investigate the underlying causes. Recently, age-adjusted incidence has been increasingly used in studies of HNC in South Korea [8-10].

South Korea has a relatively large population of approximately 52 million, with all citizens enrolled in a single national health insurance system [11]. Cancer patients must register with the Korean Central Cancer Registry, managed by the National Health Insurance Service, to access various benefits. As a result, nearly all new cancer cases are recorded, enabling accurate HNC incidence data [8-10,12]. This robust system allows for precise calculation of age-adjusted incidence rates [7].

This study aimed to determine the population-based incidence and age-adjusted incidence of HNC in South Korea over the past 20 years. The findings will help estimate future medical demands and inform the development of new preventive measures.

This study was not subject to institutional review board approval, as it used publicly available data that did not contain any personally identifiable information, and informed consent was therefore not required.

The ICD codes, crude incidence rates, sex ratios, and age-adjusted incidence trends for all 12 subtypes of HNC are presented in Table 1. Although the crude incidence rates of nearly all HNCs have steadily increased from 1999 to 2020, the absolute values remained low in most cases, at less than 2 per 100,000. The male-to-female ratio was 1 or higher for all HNCs except thyroid cancer, and 10 or higher for laryngeal and hypopharyngeal cancers. The age-adjusted incidence trend showed a gradual increase for tongue cancer, tonsil cancer, major salivary gland cancer, and thyroid cancer, a decrease for laryngeal cancer, and no significant change for the remaining HNCs.

In this study, we observed that the crude incidence rates of nearly all HNCs have steadily increased from 1999 to 2020. However, when examining age-adjusted incidence trends, only tongue cancer, tonsil cancer, major salivary gland cancer, and thyroid cancer showed a gradual increase, while laryngeal cancer decreased. No significant changes were observed in the remaining HNCs.

Smoking is the strongest risk factor for HNC, particularly for laryngeal cancer [1,2]. The exact timing of laryngeal cancer development after smoking initiation is difficult to determine, as it depends on factors such as the amount of smoking, the age at which smoking began, and genetic predisposition. However, considering that Koreans typically start smoking between their late teens and late 20s, and laryngeal cancer most frequently occurs in individuals in their 60s, the estimated average lag time is approximately 40 years [5]. In the first comprehensive survey conducted in Korea in 1980, smoking rates for men and women over 20 were 79.3% and 12.6%, respectively. By 1985, these rates had decreased to 71.2% for men and 7.8% for women [13]. According to the National Health and Nutrition Examination Survey in 1998, smoking rates were 66.3% for men and 6.5% for women, and they have continued to decline, reaching 30.0% for men and 5.0% for women in 2022 (Fig. 5A) [14]. As smoking rates have dramatically decreased since the early 1980s, the age-adjusted incidence rate of laryngeal cancer dropped by 54% from 1999 to 2020. Given the approximately 40-year lag time between smoking initiation and laryngeal cancer development, and the ongoing decline in smoking rates, the age-adjusted incidence of laryngeal cancer is expected to continue decreasing in the future. However, the impact of smoking appears limited for other HNCs, as their age-adjusted incidence did not decrease significantly.

Alcohol consumption also significantly influences the occurrence of HNCs and is known to have a synergistic effect when combined with smoking [1,2]. Drinking rates in Korea remain high. According to the National Health and Nutrition Assessment, in 2005, 73.3% of men and 36.2% of women aged 20 or older reported drinking alcohol, with no significant changes observed by 2022 (Fig. 5B) [14]. Therefore, the decline in the age-adjusted incidence of laryngeal cancer is likely primarily due to reduced smoking rates.

HPV is known to cause various cancers, including tonsil and base-of-tongue cancers among HNCs [1,8,15]. HPV-associated tonsil cancer, which often occurs in young individuals who do not smoke or drink alcohol, responds well to chemotherapy and radiation therapy [8,15]. Although epidemiological studies on HPV infection rates in South Korea are limited, the infection rate is believed to be high and increasing (Fig. 5C) [3]. In this study, the age-adjusted incidence of tonsil cancer showed a significant increase. Given the decline in smoking rates and stable drinking rates, the rise in HPV infection is presumed to be a key factor driving the increase in tonsil cancer. Additionally, while most HNCs predominantly affect individuals in their 60s and 70s, tonsil cancer occurs most frequently in those in their 50s. This pattern suggests that HPV infection plays a significant role in tonsil cancer development. Nationwide education efforts to reduce HPV infection through vaccination and safe sex practices are needed to prevent tonsil cancer. Fortunately, the increasing trend of tonsil cancer cases among individuals aged 40–59 in Korea has slowed since 2008 [8].

The age-adjusted incidence of tongue cancer also increased significantly. While tongue cancer is strongly influenced by smoking, it is minimally affected by HPV, unlike oropharyngeal cancers such as tonsil cancer [1,2]. Logically, tongue cancer incidence was expected to decrease in the context of declining smoking rates and rising HPV infection rates. We propose three potential hypotheses to explain this phenomenon. First, cases of tongue base cancer may have been misclassified as oral tongue cancer, particularly in advanced stages where distinguishing the primary site is challenging. Second, HPV may directly influence the development of oral tongue cancer. Third, a global trend of increasing tongue cancer incidence among younger individuals may also be occurring in Korea [8]. Nevertheless, the underlying causes of this trend remain unclear.

Another notable finding is the rapid increase in major salivary gland cancer incidence from 1999 to 2020. Similar trends have been observed in Japan and the United States [16,17], although not in other countries [9]. Unlike other HNCs, major salivary gland cancer is not strongly associated with smoking or alcohol consumption [1,2]. Potential causes include occupational exposure to carcinogens, radiation, diet, family history, and viral infections [1,2]. However, the exact reason for this increase remains unknown [9]. Additionally, major salivary gland cancer occurs evenly across all age groups and has a similar sex ratio, which is unique compared to other HNCs that predominantly affect men over 60. This distinction may be due to the fact that most HNCs are squamous cell carcinomas, whereas major salivary gland cancer comprises 10 different types originating from various parts of the salivary gland [1,2,16,17].

The incidence of thyroid cancer in Korea has risen rapidly since the 2000s [10,18]. This increase may be attributed to high iodine intake, a high prevalence of the BRAF gene mutation, and a high frequency of chronic thyroiditis among Koreans. However, the primary driver is the increased diagnosis rate due to the widespread use of thyroid ultrasound examinations [18]. The incidence rate declined sharply around 2013 as public opposition to thyroid ultrasound screenings grew and treatment practices evolved. However, it has gradually increased again since 2015. According to a study by Kang et al. [10], which analyzed thyroid cancer staging in Koreans, advanced cases (T3 and T4) accounted for 47.1%, suggesting that this increase is not solely due to overdiagnosis.

The primary limitation of this study is the inability to precisely assess the individual impact of specific risk factors on HNC development. This limitation stems from the lack of detailed data on each patient’s exposure to known risk factors, such as smoking, alcohol consumption, and HPV infection. Therefore, the study could not determine the distinct contribution of each factor to HNC occurrence. Future research should address this gap by incorporating comprehensive exposure data to enable a more nuanced understanding of the relationship between these risk factors and HNC.

In conclusion, the incidence of all HNCs in Korea has steadily increased over the past 20 years. However, the age-adjusted incidence trend showed a gradual increase only for tongue cancer, tonsil cancer, major salivary gland cancer, and thyroid cancer. Laryngeal cancer incidence decreased, while no significant changes were observed in the remaining HNCs. A thorough analysis is needed to understand the factors driving the increase in cancers with rising age-adjusted incidence.