The impact of cancer survivors' extra risk of noncancer mortality on net survival estimation.

Relative survival (RS) with the general population as the reference is commonly used to estimate net survival (NS). However, cancer patients may face an increased risk of noncancer death compared to cancer-free individuals. We evaluate the impact of considering this relative risk (RR) on NS estimation. First, we compared selected NS values to generated RS under various theoretical scenarios, considering different RR, NS, age at diagnosis, time since diagnosis, and sex. Then, differences between NS and RS for 3 cancers were analyzed from cure model-based estimates using EUROCARE-6 data. We observed differences between RS and the true value of NS, larger for longer time since diagnosis, older patients, and higher NS. For head and neck cancer, the smallest differences were for young female patients at 5 years from diagnosis (4%) and the highest (32%) for older patients. For colorectal cancer, differences were <7% for all ages, both sexes, and times since diagnosis and for breast cancer, differences were <5% except for older patients after 5 years. If RR > 1, RS underestimates NS. Our findings aim to correctly interpret the differences between RS and NS, and contextualize the possible biases of assuming RS as a proxy for cancer-specific survival.