A test that monitors blood levels of DNA fragments released by dying tumour cells may serve as an accurate early indicator of treatment success in people in late stages of one of the most aggressive forms of skin cancer, a new study finds.

The study looked at adults with undetectable levels of freely circulating tumour DNA (ctDNA) four weeks into drug treatment for metastatic melanoma tumours that cannot be removed surgically (unresectable). It showed that these patients, all of whom had common genetic changes (BRAFV600 mutations) linked to cancer, were living nearly twice as long without cancer growth as those who continued to have detectable levels.

Normally, the study authors say, physicians would have to wait three months before an X-ray, CT scan, or other measures could reveal whether a tumour is growing or shrinking in response to treatment, the researchers, including David Polsky from NYU Langone Health in the US, said. "Our findings suggest that levels of ctDNA may serve as a fast and reliable tool to gauge whether an anticancer medication is working. The blood test results could help support continuing the current treatment strategy or else encourage patients and physicians to consider other options," Polsky said.

For the study, published in the journal The Lancet Oncology, the research team analyzed blood samples from two pivotal clinical trials involving 383 men and women. All were receiving targeted treatment with drugs dabrafenib and trametinib for unresectable metastatic melanoma tumours that had mutations in the BRAF gene, which is found in about half of all patients with the disease, investigators say. The investigators measured levels of ctDNA from patients whose tumours had this mutation before treatment and one month into therapy. As part of the clinical trial, patients received periodic checkups using CT scans until treatment ended.

Among the study's other findings were that patients with 64 or fewer copies of ctDNA per millilitre of blood before treatment were likely to respond well to therapy, surviving nearly three years on average. By contrast, levels above that threshold were linked to a significantly poorer chance of survival, with patients living just over a year.