- •Unknown primary cancer rates declined between 1999 and 2017 in Korea.
- •Incidence decreased for both men and women, though more so for men.
- •Patients aged 80 years and older had the highest incidence rate.
- •The survival rate increased from 14.2 % (1999–2002) to 27.3 % (2013–2017).
Cancers of unknown primary (CUPs) are tumors found after metastasizing from unidentified primary sites; these tumors generally have unknown treatment strategies, expected treatment results, and prognosis. We assessed the epidemiological characteristics of CUPs in Korea.
We extracted records for 1999 through 2017 from the Korea Central Cancer Registry using the International Statistical Classification of Diseases and Related Health Problems (10th revision) codes for CUP as defined by the International Agency for Research on Cancer. Age-standardized rates and relative survival rates were calculated.
The CUPs constituted 2.1 % of the total number of cancer registrations in 1999, declining to 0.7 % in 2017. The incidence rate decreased for both sexes (5.35 to 2.20 for men, 3.15 to 1.77 for women). Patients aged 80 years and older had the highest incidence rate at 40.2, and 86.3 % of CUPs occurred in those 50 years of age or older. The cases of retroperitoneum and peritoneum sites increased over time. Cases diagnosed by microscopic methods and death certification only were 62.3 % and 7.9 %, respectively. The malignant neoplasm of the retroperitoneum and peritoneum and unknown primary site had the highest and lowest survival rates, respectively. The 5-year relative survival rate increased over time from 14.2 % (1999–2002) to 27.3 % (2013–2017).
Our analysis of data from the Korea Central Cancer Registry found decreasing rates of CUP, although with consistent disparities by patient age and sex. Advancements in diagnostic technology may be decreasing the number of CUP diagnoses. Expanding the amount of information recorded in the registry may further improve diagnostic accuracy.
Cancer of unknown primary (CUP) is a disease in which malignant cells are found with an unidentified primary origin; it is mostly diagnosed following metastasis [ ]. Because the primary site is unknown, the development of disease-specific treatment and strategies is not possible, management of the disease is difficult, and expected treatment results are typically poor [ ]. However, over the past 10 years the development of diagnostic methods and therapeutic outcomes for CUP has been groundbreaking. Advanced diagnostic methods for CUP, such as immunohistochemistry and genetic testing, as well as treatment plans for CUP patients, can ensure better prognosis [ ]. Therefore, it is essential to determine, as far as possible, the primary origins of the cancer before diagnosing it as a CUP. This may be achieved by analyzing patient data to compare the incidence rates with those of known primary sites, tracking metastatic patterns of specific cancers, searching for causes of death by anatomical sites, and assessing survival rates by prognostic factors [ ].
Recent studies estimate that the incidence of CUP is 3–10 % of all cancers diagnosed [ ]. In Australia, CUP accounts for 2.7 % of all cancers diagnosed and 5.9 % of deaths resulting from cancer [ ]. The declining incidence of CUP is likely related, at least in part, to advancements in diagnostic technology, such as radiology and histopathology [ ], which are also reflected in improved completeness and validity of data in cancer registration systems. The International Agency for Research on Cancer (IARC) considers the rate of CUP in a cancer registry to be related to the quality of the diagnostic information that it contains [ ].
The Korea Central Cancer Registry has operated a national cancer registration database since 1999 [ ]. In 2017, newly diagnosed cancer cases and deaths from cancer numbered 232,255 (age-standardized rate (ASR) 264.4 per 100,000). Stomach, colorectal, and lung cancers were commonly diagnosed in both sexes. The quality indices of cancer incidence data were as follows: 98.2 % (completeness), 34.3 (mortality/incidence ratio), 91.2 % (microscopic verification), 0.9 % (death certificate only), and 0.0 % (age unknown) [ ].
Rigorous scrutiny of cancer diagnoses and treatment information—including registration reports, pathology reports, and death certificates—is conducted annually for newly diagnosed cancer cases, including CUPs. Cancer cases first identified from death certificates are tracked back to the certifying hospital as a source of information on new cancer cases. This study assessed the characteristics of CUP cases registered in the Korea Central Cancer Registry and presents the incidence rate, trends, and survival of CUP patients in Korea.