Cancer, identified as the leading cause of death in South Korea, not only carries a high risk of fatality upon onset but also imposes significant economic burdens, including treatment expenses and costs for maintaining daily life. As a result, the number of individuals choosing cancer insurance to mitigate this risk is steadily rising. Since the introduction of cancer insurance in South Korea in 1980, numerous intense insurance disputes have emerged. In response, insurance companies have repeatedly revised their cancer insurance policy terms through court rulings and regulatory improvement orders.
 In 2011, insurance companies revised the cancer insurance policy terms by introducing the “Primary Site Criteria”. According to this provision, in cases where thyroid cancer (C73) and secondary lymph node cancer (C77) are diagnosed, the primary cancer - in this case, thyroid cancer (C73) - is considered the basis for insurance benefit payouts.
 In situations where decisions from lower courts and financial dispute resolution committees are unclear, insurance companies revised the primary site criteria once again in January 2021 and expanded the examples illustrating the application of this criterion within the explanation section. For example, if stomach cancer (C16) metastasizes to the brain and results in a diagnosis of brain cancer (C79.3), it is categorized as stomach cancer for insurance coverage purposes. Similarly, if breast cancer (C50) metastasizes to the lungs and is diagnosed as lung cancer (C78.0), it is classified as breast cancer. This serves as a basis to provide reduced insurance benefits compared to the final diagnosed cancer.
 Should insurers fail to convincingly demonstrate a rational basis for distinguishing cancer insurance benefits based on whether the cancer originated primarily or secondarily in the same location, such actions should be deemed an unjust limitation on the rights of insurance consumers.
 While insurance policy terms can be unilaterally drafted and implemented by insurers based on the principle of private autonomy, any clauses contradicting the principle of utmost good faith - which mandates that terms and conditions should be drafted fairly and in line with the reasonable expectations and legitimate interests of the contracting parties without compromising fairness - are inevitably subject to judicial oversight.
 This paper critically analyzes insurers' arguments regarding the “Primary Site Criteria” within cancer insurance policy terms and sheds light on the issues at hand. The research results are as follows: It is advisable to eliminate the current “Primary Site Criteria” in the existing cancer insurance policy terms and instead transition to a payment structure for cancer insurance benefits based on the final condition or stage of the patient (insured). Moreover, in cases where a diagnosis of advanced cancer (general cancer or high treatment cost cancer) follows an early-stage cancer diagnosis benefit, it is in line with the reasonable expectations of insurance consumers to introduce a provision that ensures payment of the difference between the respective diagnosis benefits.