The prevailing view that the evolution of cells in a tumor is driven by Darwinian selection has never been rigorously tested. Because selection greatly affects the level of intratumor genetic diversity, it is important to assess whether intratumor evolution follows the Darwinian or the non-Darwinian mode of evolution. To provide the statistical power, many regions in a single tumor need to be sampled and analyzed much more extensively than has been attempted in previous intratumor studies. Here, from a hepatocellular carcinoma (HCC) tumor, we evaluated multiregional samples from the tumor, using either whole-exome sequencing (WES) (n = 23 samples) or genotyping (n = 286) under both the infinite-site and infinite-allele models of population genetics. In addition to the many single-nucleotide variations (SNVs) present in all samples, there were 35 “polymorphic” SNVs among samples. High genetic diversity was evident as the 23 WES samples defined 20 unique cell clones. With all 286 samples genotyped, clonal diversity agreed well with the non-Darwinian model with no evidence of positive Darwinian selection. Under the non-Darwinian model, MALL (the number of coding region mutations in the entire tumor) was estimated to be greater than 100 million in this tumor. DNA sequences reveal local diversities in small patches of cells and validate the estimation. In contrast, the genetic diversity under a Darwinian model would generally be orders of magnitude smaller. Because the level of genetic diversity will have implications on therapeutic resistance, non-Darwinian evolution should be heeded in cancer treatments even for microscopic tumors.

As the population of cancer survivors has increased and continues to age, the occurrence of second cancers has risen dramatically-from 9% of all cancer diagnoses in 1975-1979 to 19% in 2005-2009. The Childhood Cancer Survivor Study, a cohort of more than 14,000 childhood cancer survivors with detailed exposure data and long-term follow-up, has substantially contributed to our understanding of the roles of radiotherapy and chemotherapy in second cancer occurrence. In particular, dose-related risks have been demonstrated for second cancers of the breast, thyroid, central nervous system, gastrointestinal tract, and sarcomas following radiation. Cytotoxic chemotherapy-which has long been known to be leukemogenic-also appears to contribute to risk for a range of other second cancer types. Individuals who develop a second cancer are at particularly high risk for developing additional second cancers. A genome-wide association study of survivors of Hodgkin lymphoma who received radiotherapy identified a locus on chromosome 6q21 as being associated with second cancer risk, demonstrating that recent advances in genomics are likely to prove invaluable for elucidating the contribution of genetic susceptibility to second cancer etiology. Among adults, risk of second cancers varies substantially by type of first and second cancer, patient age, and prevalence of second cancer risk factors, including primary cancer treatments, environmental and lifestyle exposures, and genetic susceptibility. Further research is needed to quantify second cancer risks associated with specific etiologic factors and to identify the patients at highest risk of developing a second cancer to target prevention and screening efforts.