Women with some types of breast implant are up to 14 times more likely to develop a rare form of cancer than those with a similar product, Australian researchers have revealed, as two more cases are added to the official tally.
The Australian study has been watched by the international plastics community who are racing to understand what is causing breast implant associated anaplastic large cell lymphoma (BIA-ALCL).
The cancer has been linked with three deaths in Australia, one in New Zealand, and nine in the United States. But it is still considered rare - Australian health authorities estimate its prevalence among women who get implant to be between 1-in-1000 and 1-in-10,000 - and most cases can be cured by removing the implants.
Australia's medical devices regulator, the Therapeutic Goods Administration, revealed another two confirmed cases on Thursday, bringing the total to 53 women diagnosed since 2007.
A team lead by researchers at Macquarie University compared 55 Australian and New Zealand cases from 2007 to August 2016, with sales data from three leading breast implant manufacturers to better understand the risks patients face with different products and the possible cause.
"Higher surface area textured implants have been shown to significantly increase risk of ALCL in Australia and New Zealand," the researchers concluded in a paper published online in the American Plastic and Reconstructive Surgery Journal.
Of the three textured implants studied, Biocell salt loss implants had the highest risk for ALCL, with an estimated risk of one in 3800 breast implants being linked to the cancer.
That was 14 times that of the lowest risk Silitex textured implants which had a risk ratio of one in 60,000.
The risk of ALCL for Silimed polyurethane implants was one in about 8000, 10 times higher than the Siltex.
But the authors note the polyurethane implants have been available for a shorter time in Australia, which may downplay their risk profile.
All four recorded deaths in Australia and New Zealand were linked to the Biocell implants, manufactured by Allergan, the study stated.
Textured implants - as opposed to smooth - account for about 90 per cent of an estimated 40,000 implants used in Australia each year, said study author Anand Deva.
Professor Deva, an academic and plastic surgeon, believes the reason the Biocell and Silimed implants had a higher risk was because of their larger surface area, adding weight to the theory these cancers are being caused by bacteria contamination on the surface of the implant.
"The textured implants ... present bacteria with the most ideal forest or surface area to grow to huge numbers if left unchecked," he said.
"Now there's enough evidence to say if you fight bacteria and reduce the risk of contamination on these textured implants then you can reduce the risk of ALCL to close to zero."
A spokesperson for Allergan said all manufacturers of textured implants had confirmed cases of ALCL and it was important they worked with the medical profession and regulators to understand and address it.
"Plastic surgeons and patients have a variety of reasons for selecting a textured implant and patients are advised to have a thorough discussion with their plastic surgeon about the risks and benefits of each implant type in order to make an informed decision," the spokesperson said in a statement.
Allergan has been working to increase awareness of the disease, early diagnosis and techniques to limit bacterial contamination such as a 14-point plan advocated by Professor Deva, they said.
The TGA is not recommending that women without symptoms have breast implants removed or undergo regular screening.
Instead they are urged to stay alert to swelling or lumps and speak with their doctor.