Although each individual rare cancer affects a small percentage of patients, the collective group of 181 rare cancers is responsible for significant disease burden in the United States.
Although each individual rare cancer affects a small percentage of patients, the collective group of 181 rare cancers accounts for significant disease burden in the United States.
According to a recent study published in CA: A Cancer Journal for Clinicians, 20% of all cancer patients diagnosed in the United States between 2009 and 2013 had one of the 181 cancers considered rare. Rare cancers are defined in the study as those occurring in fewer than 6 patients per 100,000 people per year.
Using prevalence and survival data from several cancer registries, the researchers set out to quantify the burden of rare cancer in the United States. They found that the prevalence of these rare cancers varies by patient characteristics. Male cancer patients were slightly more likely to have a rare cancer than women (21% vs 19%), and Hispanics and Asians/Pacific Islanders had higher rates of rare cancers than blacks and whites. The occurence of rare cancers reduced with increasing age, as they made up 71% of diagnoses in patients younger than 20 years but just 17% to 20% of cancer diagnoses in those aged 40 and older.
Prevalence of rare cancers also varied by cancer site. Just 5% of cancers occurring in the breast or the male genital system were rare, while cancers affecting the hematopoietic system or digestive system had the highest chances of being rare, at 38% and 36%, respectively.
Having a rare cancer was associated with lower survival rates than with a common cancer, as 57% of patients with a rare cancer and 74.5% of those with a common cancer had an overall relative survival time of 5 years. Differences in cancer stage when diagnosed could contribute to the survival disparities; 59% of rare and 45% of common cancers were diagnosed at regional or distant stages.
According to the study authors, “delays in diagnosis are nearly inevitable, because, by definition, clinicians will not see rare cancers often and will initially seek to rule out more common causes of symptoms.”
However, gaps in the likelihood of survival between rare and common cancers were reduced for younger patients, although those with rare cancers still had worse chances of survival. For instance, the 5-year relative survival rate was 82% for rare cancers and 92% for common cancers in patients aged 0 to 19 years, but for those older than age 80, the survival rates were 35% for rare cancers and 60% for common cancers.
The researchers noted that younger patients with rare cancers might have survival rates comparable with those with common cancers because of the growing number of specialized children’s cancer centers that collaborate on research. However, barriers to improving rare cancer prognosis persist among all age groups due to the lack of treatment guidelines and screening regimens. Furthermore, the rarity of these cancers results in a limited number of patients eligible for clinical trials of potential new treatments.
Efforts like the International Rare Cancers Initiative have been implemented to address these difficulties, as institutes around the world begin to cooperate on clinical trials for rare cancers. Still, the study authors write that “continued efforts are needed to develop interventions for prevention, early detection, and treatment to reduce the burden of rare cancers.”
They added that these efforts will benefit all cancer patients, not just the 20% diagnosed with rare cancers, as “the study of rare cancers has led to the identification of numerous cancer genes and has increased understanding of cancer biology.”