Descriptive epidemiology of cancer of unknown primary in South Korea, 1999–2017

1

Introduction

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 [ 1 ]. 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 [ 2 ]. 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 [ 34567 ]. 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 [ 8 ].

Recent studies estimate that the incidence of CUP is 3–10 % of all cancers diagnosed [ 9 ]. In Australia, CUP accounts for 2.7 % of all cancers diagnosed and 5.9 % of deaths resulting from cancer [ 10 ]. The declining incidence of CUP is likely related, at least in part, to advancements in diagnostic technology, such as radiology and histopathology [ 11 ], 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 [ 12 ].

The Korea Central Cancer Registry has operated a national cancer registration database since 1999 [ 13 ]. 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) [ 14 ].

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.