Researchers at Lund University have cracked a frustrating paradox in breast cancer care: by the time imaging or symptoms reveal recurrence, the disease has often been spreading for months or even years. Now a blood test called Pathlight is changing that equation, detecting the earliest whispers of cancer's return up to 13.8 months before any imaging can spot it—and in some cases, nearly four years earlier than conventional methods.
The breakthrough matters because the vast majority of breast cancer deaths come from metastatic disease, when cancer spreads beyond the breast. Today, doctors rely on imaging scans and patient symptoms to catch recurrence, but by then a patient's disease is already advancing. Pathlight takes a different approach: it hunts for tiny fragments of tumor DNA floating in the bloodstream, using each patient's tumor as a genetic blueprint to spot even minuscule traces of cancer's return.
"Every tumor has a unique genetic fingerprint," explains Lao Saal, the senior researcher leading the study published in EMBO Molecular Medicine. "By measuring these alterations in the patient's blood, we can detect extremely small amounts of residual tumor DNA with high precision, even when a recurrence is not yet visible with today's imaging methods or has begun causing symptoms."
The study followed 136 patients treated with chemotherapy and surgery for various types of breast cancer, collecting blood samples from diagnosis through six years of follow-up. The results were striking. Nearly 90 percent of patients had detectable tumor DNA in their blood before treatment even began. After chemotherapy given before surgery, tumor DNA persisted in about 21 percent of patients—a signal that worried oncologists. When tumor DNA levels failed to decline clearly during treatment, it correlated strongly with high recurrence risk.
Perhaps most compelling: Pathlight proved more accurate at predicting treatment response than pathological complete response (pCR), the standard method oncologists currently use. Among patients who later developed metastatic disease, the blood test signaled danger a median of 13.8 months before clinical visibility, giving doctors precious months to intervene. The presence of tumor DNA after surgery was strongly associated with future relapse.
What makes this method practical is its balance of simplicity and accuracy. The blood test provides less detailed information than exhaustive genetic sequencing but remains fast and cost-effective while maintaining the precision needed to measure treatment response and identify high-risk patients. For patients and doctors alike, that efficiency matters—it means the test could realistically integrate into standard care.
The implications ripple in two directions. For high-risk patients, Pathlight could enable earlier intervention, potentially improving survival. Equally important: the test could identify low-risk patients who might otherwise undergo unnecessarily aggressive therapy, sparing them harsh side effects. As Niklas Loman, a senior oncologist at Skåne University Hospital involved in the research, notes, "With further research, the technology may provide opportunities to improve treatment for patients at high risk of recurrence" while helping others "avoid unnecessarily intensive therapy."
The findings represent the kind of incremental yet profound shift that moves medicine forward—not a cure, but a clearer window into what's actually happening inside a patient's body, months or years before the old tools could see it.