Cancer-specific survival of patients with multiple cancers: an application to patients with multiple breast cancers

In the analysis of cause-specific survival, the causes of death must be known. For single-cancer patients with a known cause of death, the estimation of the cause-specific survival rate is straightforward. For multiple-cancer patients with two primary cancers, however, the analysis of cause-specific survival rates is more complex, particularly if the cancers are of the same primary site. In these situations, a concept of cancer-specific survival may also be distinguished from cause-specific survival. Cancer-specific survival rates are studied here by introducing two models, the primary one where the death from cancer is attributed to one of the cancers, and an alternative where such an attribution is not necessary. The models are illustrated using data on patients with multiple breast cancers. The model-based survival rates are compared with each other and with the corresponding relative survival rates based on analogous modelling of relative survival. The results show that for the subsequent breast cancer, the cancer-specific survival rates based on the alternative, where the distinction between the cancers as a cause of death was not necessary, tended to be higher than those based on that distinction. It is thus possible that the subsequent cancer was too often coded as a cause of death, particularly when being localized at diagnosis.