What are Rare Cancers? How do they differ from common cancers?

Neethu Sheri Kurian

Content Specialist
4baseCare

Cancers that involve a small percentage of the population in any age group are classified as rare cancers. These diseases can affect anyone of any age, gender, or organ system. Rare cancers account for nearly 22% of all cancers diagnosed globally, disproportionately impacting certain demographic groups, with a prevalence of less than 6 per 100,000 people each year. Anaplastic thyroid cancer, small bowel adenocarcinoma, fibrolamellar hepatocellular carcinoma, gynecological sarcoma, salivary gland cancer, thymoma, penile cancer, relapsed/metastatic anal cancer, and ocular melanoma are among the nine rare cancers in adults that the International Rare Cancers Initiative (IRCI) has outlined2, 3.

There is no universal agreement on what constitutes rare cancer. The National Cancer Institute (NCI) definition is fewer than 15 cases per 100,000 people per year3. As per this definition, rare cancers impact fewer than 40,000 people in the United States each year. They account for 27% of all cancers and 25% of cancer deaths.

More recently, a consortium from the European Union (RARECARE) classified rare cancers as those with fewer than 6 cases per 100,000 persons per year4. According to the Delhi Cancer Registry (DCR), 60.9 percent of men and 46.4 percent of women in India have rare cancers, according to the RARECARE criteria5.

How does it differ from common cancers?

• Rare cancer might be a subtype of a more common cancer; 
• Most cancers start in certain types of cells, such as skin cells and the cells lining the organs of the body. Rare cancer start in different types of cells than usual like bone cells;
• Cancer may be rare because it is in an unusual part of the body for that type of cancer. Melanoma (a form of skin cancer), for example, is a common type of cancer, but melanoma that begins in the eye is rare.
• Cancer is rare if it affects a child or young adult.

Any cancer diagnosis and management is difficult, however, rare cancers are especially challenging for patients, caregivers, and clinicians. Often, the cancers occurring most often get the most attention and the most research funding. In addition, the treatment options for rare cancers are often more limited and less effective than those for common cancers. This is partly because there is less clinical research and fewer clinical trials for rare cancers due to the small number of patients.

Furthermore, patients with rare cancers face unique challenges, such as delayed or incorrect diagnosis, difficulties in accessing clinical expertise and appropriate treatments, possible lack of interest in developing new therapeutic interventions, uncertainties in clinical decision-making, and a scarcity of available registries and tissue banks, as compared to patients with common cancers. As a result, the average outcome for patients with rare cancers is worse than those with more prevalent cancers.