A new study has found that in order to prevent thyroid cancer, one should avoid excess weight or obesity.

The research has been published in the International Journal of Cancer. The world-first study led by Dr Maarit Laaksonen from UNSW's School of Mathematics and Statistics found that one in five future thyroid cancers in Australia is attributable to current levels of overweight and obesity.

Dr Laaksonen, a senior lecturer in Data Science, said that this is concerning as the prevalence of obesity has doubled during the last two decades, with 75 per cent of Australian men and 60 per cent of Australian women being overweight or obese. "This finding translates to close to 10,000 thyroid cancers in the next 10 years," Dr Laaksonen said.

The main authority for cancer research, the International Agency for Research on Cancer, has concluded that body fatness is a causal risk factor of thyroid cancer. But Dr Laaksonen said this is the first time a study has evaluated the thyroid cancer burden attributable to current levels of overweight and obesity and compared this burden by sex.

Thyroid cancer is one of the few cancers that are 2-3 times as common in women as in men, but Dr Laaksonen said the study found that the future thyroid cancer burden attributable to overweight/obese is higher for men compared with women. Being overweight or obese explained two in five thyroid cancers in men and one in 10 thyroid cancers in women.

"It is still not well understood what causes the sex difference in overweight/obesity-related thyroid cancer risk," Dr Laaksonen said.
"But our findings add evidence to the urgent need to halt and reverse the current global trend in weight gain, especially obesity and especially in men," Dr Laaksonen added.

"Other lifestyle factors do not appear to be implicated but ionising radiation exposure, iodine deficiency and some genetic factors and family history are known to increase the thyroid cancer risk. Thyroid cancer is a bit like prostate cancer that its risk factors are not yet very well understood," Dr Laaksonen said.

The study findings were based on seven Australian cohort studies involving 370,000 participants, which enabled the evaluation of less common cancers such as thyroid cancer. "We linked the data from these seven studies, which all ascertained the participants' Body Mass Index [BMI] at study baseline, with national cancer and death databases, which allowed us to estimate the strength of BMI-cancer and BMI-death associations during the follow-up," Dr Laaksonen explained.

"We estimated up-to-date prevalence of overweight and obesity in the Australian population from the latest National Health Survey from 2017-2018, and then combined the strength of association and exposure prevalence estimates to estimate population attributable fractions [PAF].

The PAFs described what fraction of future cancers at the population level is explained by current exposure. We did this by applying advanced PAF methods which I developed," Dr Laaksonen continued. The advanced PAF methods allowed for unbiased disease burden estimates, as well as for the comparison of preventable disease burden by population subgroups in the study.

"These methods require access to large cohort data," Dr Laaksonen said. As the prevalence of overweight/obesity is higher in men than women it adds to the sex difference in thyroid cancer burden. "Finally, we multiplied the PAF estimates by the projected numbers of thyroid cancers in the next ten years (2021-2030) to get the absolute numbers of thyroid cancers that are expected to occur due to body fatness," Dr Laaksonen said.

The aim of the project, which was jointly funded by the National Health and Medical Research Council and the Cancer Institute NSW, was to evaluate for the first time the preventable future burden of cancer in Australia and its determinants. The team involved in this study included collaborators and representatives of the seven cohort studies on which the study was based.

They were from various Australian universities, institutes and Cancer Councils. Dr Laaksonen had previously published findings on preventable lung, colorectal, breast, endometrial, ovarian, pancreatic, kidney, bladder and head and neck cancers.

Her next steps include a study on the preventable burden of stomach and oesophageal cancers and a summary paper on the preventable cancer burden in Australia across all cancers.