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 A liquid biopsy examining blood or urine can help gauge the effectiveness of therapy for colorectal cancer that has just begun to spread beyond the original tumour, a new study suggests.

Such a biopsy can detect lingering disease and could serve as a guide for deciding whether a patient should undergo further treatments due to some tumour cells evading an initial attempt to eradicate cancer, indicates the paper published in the Journal of Clinical Oncology Precision Oncology.

Patients in this study had what is known as oligometastatic colorectal cancer, meaning each patient's cancers had spread beyond his or her original tumour but only to a small number of sites. Such patients undergo chemotherapy to shrink the tumours before having surgery to remove whatever remains of the primary tumour.

There is a debate in the field about whether, after initial therapy, oligometastatic cancer should be treated like metastatic cancer, with more chemotherapy -- or like localised cancer, with more surgery plus radiation at those limited sites. Contributing to the problem is that doctors have a limited ability to predict how patients will respond to early chemotherapy, especially since most patients don't have access to cancer genome sequencing to identify the DNA mutations in their original tumours.

"Being able to measure the response to early chemotherapy without prior knowledge of the tumour's mutations is a novel idea and important for being able to determine whether the patient responded well to the therapy," said researcher Aadel A. Chaudhuri from the Washington University.

Liquid biopsies for colorectal cancer detect tumour DNA that has broken free of cancer and is circulating in the blood and, to a lesser extent, has collected in the urine. First, most such biopsies have been developed to track metastatic cancers or to verify that local cancers have not started to spread. Second, most liquid biopsies for cancer rely on knowledge of the original tumour's mutations, to see if those mutations are still present in the blood after therapy.

But many patients don't get the opportunity to have their original tumours sequenced. Instead, the new biopsies rely on detecting DNA mutations in the blood or urine and comparing them with DNA mutations measured in the treated primary tumour, after it's surgically removed.

And finally, the urine biopsy is unique for colorectal cancer as most urine biopsies have been limited to use in cancers of the genitourinary system, especially bladder cancer. The study showed that lower circulating tumour DNA levels correlated with better responses to early chemotherapy. Indeed, most patients who had undetectable levels of tumour DNA in blood samples also had no measurable cancer in their surgical specimens.