The cancer risk of thyroid nodules classified as "indeterminate" under an international standard (which means they are considered neither particularly worrisome nor safe) is higher than previously thought, a local study has found.
It was thought that they turn out to be malignant 5 per cent to 15 per cent of the time. Instead, researchers here found that the risk is much higher, with more than one in four patients getting cancer, a finding which corroborates other international studies.
What this means for patients is that doctors may ask them to have surgery rather than wait for a biopsy, said the authors of the study.
Thyroid lumps affect about 4 per cent to 7 per cent of the population, with women four times more likely than men to be affected.
While most lumps are benign and do not affect everyday life, some lumps grow into a visible bulge in the neck and can cause shortness of breath or difficulty swallowing, and can turn cancerous.
Thyroid nodules are lumps found in the thyroid glands in the neck. To determine if one is malignant or benign, a thyroid surgeon uses a fine needle - thinner than a blood test needle - to extract cells for testing.
>1 in 4
Ratio of patients with thyroid nodules classified as "indeterminate" getting cancer.
At the National University Hospital (NUH), the nodules are then classified by a pathologist into six categories using an internationally approved standard - the Bethesda System for Reporting Thyroid Cytopathology. Category three, or what doctors call the "in between" group, is the one that causes difficulty.
One of the study's authors, Assistant Professor Ngiam Kee Yuan, a consultant at NUH's division of general surgery (thyroid & endocrine surgery), said: "The challenge is how do you offer the right treatment for the patient. Surgery or a follow-up?"
Current guidelines recommend that the patient undergoes a follow-up fine-needle aspiration after an appropriate interval, typically three months later.
But, with the recently published local study showing that the cancer risk is much higher, Prof Ngiam said surgery to remove one side of the thyroid gland might sometimes be a better option.
Between 1,000 and 1,500 fine-needle aspirations are performed on thyroid nodules at NUH each year, and about 6 per cent fall into this "indeterminate" category.
The findings came from a study of 309 patients who sought treatment at NUH between 2008 and 2014.
It is the first such local study to be published and one of the largest of its kind internationally, in terms of sample size. It was published in Cancer Cytopathology, a peer-reviewed journal of the American Cancer Society.
Associate Professor Nga Min En, a senior consultant at NUH's department of pathology who was also involved in the study, said the team also tried to examine whether the "difficult category" could be further refined.
"We wanted to see if we could further refine it into patients who are more likely to require surgery versus those who might actually benefit from a repeat fine-needle aspiration," said Prof Nga.
The researchers found that the risk of cancer differed for two types of thyroid nodules - those with nuclear atypia, where the cells have an abnormal nucleus, and architectural atypia, where the cell patterns are abnormal.
The risk of cancer for thyroid nodules with nuclear atypia was found to be 36.8 per cent, compared with 14.7 per cent for those with architectural atypia.
This suggests that patients who have thyroid nodules classified in category three but with nuclear atypia might benefit more from surgery, said Prof Nga.
Prof Ngiam hopes that it will lead to better outcomes for patients.
"From a clinician-patient perspective, you want to be able to tell patients something you know is validated in Singapore and not in America or Europe," he said.