"We demonstrated that this method is potentially quite valuable as well as non-invasive, feasible and safe," said study director professor Irving Waxman from University of Chicago.
"We had no complications related to portal vein blood acquisition," he added.
Primary tumours shed cancerous cells, known as circulating tumour cells (CTCs), into the blood. These have been widely studied as prognostic biomarkers for various cancers.
Because these cells are often larger, irregularly shaped and tend to cluster together, they get trapped in smaller vessels.
The authors hypothesised that most cells released from a gastrointestinal tumour would flow into the portal vein and then get trapped by the narrow vessels in the liver. Hence, they would not reach the peripheral venous system.
CTCs from gastrointestinal tumours are rarely identified in the peripheral blood until the cancer is widely metastatic.
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To test this theory, the researchers used an ultrasound-guided endoscope and a small needle to take blood from the portal vein during routine diagnostic endoscopies.
They found CTCs in 100 percent of 18 patients with suspected tumours in the pancreas and bile ducts. Tests using peripheral blood samples, the standard method, detected tumours cells in only four of the 18 patients.
The findings could offer doctors a method to diagnose pancreatic cancer earlier in patients.
"Access to circulating tumour cells may help us define the diagnosis and guide treatment," Waxman said.