One definition of rare cancers is that with an incidence of less than six cases per 100,000 population each year . Identifying 198 cancers as rare leads to the estimate that they constitute 22% cancers . In the USA, the National Cancer Institute uses a definition of <15 per 100,000 which translates to rare cancers accounting for 25% of cancer-related deaths. The survival of patients with rare cancers is poorer than that associated with common cancers but they show some of the highest response rates to molecularly targeted therapies because there is less interpatient variability in the pattern of genetic mutations than with more common cancers .
The traditional path for clinical trials for conventional chemotherapy has been to commence with a dose-finding Phase I trial which escalates doses in cohorts of patients until a maximum tolerated dose (MTD) is reached and the dose-limiting toxicities identified. A dose is then identified to test for efficacy in a Phase II trial in a group of patients with a particular cancer.
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