Nearly half of childhood tumors treated with common types of chemotherapy showed detectable DNA changes linked to treatment within 18 months, according to a new international study led by The Hospital for Sick Children (SickKids). Researchers say these changes could eventually help clinicians spot treatment resistance earlier, before cancer returns or spreads, opening the door to more precise use of chemotherapy.

For their study published in Nature, researchers analyzed more than 600 tumors from 544 patients in Canada, Australia and the United States. By combining whole-genome sequencing with detailed medical records and advanced computational methods developed at SickKids, the team found distinct genomic signatures of DNA changes left behind by different chemotherapies. Some of these patterns appeared as early as 91 days after treatment began.

“There’s a long-standing belief that pediatric cancers are genetically quiet because they haven’t had much time to mutate,” says lead author Dr. Adam Shlien, senior scientist, Genetics & Genome Biology, and a lab director in Genome Diagnostics at SickKids. “Instead, it was mind-blowing to see how many mutations found in tumors that had relapsed or spread were linked to the chemotherapy used to treat the cancer in the first place.”