A type of thyroid tumour which has been classified as a cancer for decades is actually not a cancer at all, scientists have declared.

The finding means that thousands of people will be spared the aggressive radioactive iodine treatment and some of the surgery which is currently the standard treatment for thyroid cancer, the researchers say. It also could save millions of pounds in medical costs currently being spent on treatment which has now been revealed as unnecessary and challenges accepted approaches to diagnosing cancer, which could influence how other conditions including prostate, lung and breast cancer are treated and classified.

The finding has been heralded as a major breakthrough in how cancer is treated and diagnosed.

One of the experts who approved the reclassification, Dr Yuri Nikiforov, said of the finding: “To my knowledge, this is the first time in the modern era a type of cancer is being reclassified as a non-cancer.”

The announcement has been made in the scientific journal JAMA Oncology following a major study conducted by cancer specialists in which a total of 268 tumours were analysed by twenty-four working group pathologists.

The researchers found that some thyroid tumours currently classified as cancer do not actually meet diagnostic criteria, suggesting that they are often wrongly included due to their microscopic appearance meeting the criteria, despite the fact that they act like benign tumours.

Ronald Ghossein, Director of Head and Neck Pathology at the Memorial Sloan-Ketttering Cancer Cancer in New York and one of the researchers behind the study, told The Independent that the reclassification is expected to affect 45,000 thyroid cancer patients worldwide and also influence how patients are treated for other cancers including prostate, lung and breast cancer.

He said: “The big lesson here is that the nomenclature of a disease should reflect its clinical risks and behaviours and not only its appearance at the microscopic or molecular level. To put it simply, it is not because you dress like a criminal that you are one. The tumour should not be labelled cancer only on its appearance but on its clinical behaviour.

“This lesson could be applied to all diseases malignant or not. The cancers that can be affected are prostate, lung, breast and non-melanoma skin cancers among others. If reclassification of indolent lesions is performed in some of these common cancers, it will affect health care at so many levels from the psychological burden of cancer to therapy, screening and of course it will impact costs significantly.”

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Thyroid tumours are rare tumours which develop as a painless lump or swelling on the neck and account for less than one per cent of all cancer cases in the UK. According to the NHS, approximately 2,700 people in the UK are diagnosed with thyroid cancer every year.