Danish nationwide cohort study of cancer rarity and mortality

Patients with rare cancers have consistently been reported to have poorer survival than patients with common cancers. A nationwide cohort study in Denmark investigated whether this excess mortality could be attributable to differerences in tumor histology and stage at diagnosis. The study included adults diagnosed with a first primary solid malignancy in Denmark between 2004 and 2023. Cancers were classified according to Tier 2 of the RARECAREnet classification, with rare cancers defined as incidence < 6/100.000. A total of 20.6% of cancer patients were diagnosed with a rare cancer. Patients with rare cancers had 74% higher mortality than those with common cancer (HR 1.74; 95% CI 1.72-1.76), but mortality did not generally increase with increasing rarity. After adjustment for tumor histology and stage at diagnosis, the HR was close to unity (HR 0.96; 95% CI 0.95-0.98). The authors conclude that tumor histology and stage at diagnosis largely explain the survival disadvantage in rare cancers.