Williams, a second-year animal science student at Massey University, saw a specialist annually in Hamilton and although the mole was growing slightly each year, was told not to worry.
"It never really affected my vision. It was like a mole you can get anywhere on your body. They said there was only a tiny chance it could turn into something bad."
After moving to Oamaru when she was 16, she started seeing specialists at Dunedin Hospital.
During her annual check-up in August last year, her doctor was concerned the mole was growing into the gutter of her eye, where the blood vessels were.
Specialist Dr Harry Bradshaw became involved, and a few days later told Williams and her mother the news they had been dreading: the mole had become cancerous.
As it was near the blood vessels there were concerns it could spread to the rest of her body. Either her eye would need to be removed, or she could seek specialist treatment at a clinic in England.
"I went into complete shock," Williams told the Weekend Herald.
"I went from thinking it was nothing bad, to thinking maybe a simple procedure, to, 'I could lose my eye'. I immediately wanted to look into the treatment rather than lose my eye."
Bradshaw had previously sent another patient for successful treatment at the UK Eye Proton Therapy Centre in the Clatterbridge Cancer Centre in Wirral, near Liverpool.
The treatment was an extremely precise form of low-energy radiotherapy, ideal for killing cancer cells without damaging critical tissue in the eye close to the tumour.
The Ministry of Health can fund treatment there out of a special high-cost treatment pool for one-off treatments not otherwise funded by the public health system.
There were 10 treatments funded from the pool in 2018, and generally around 10 to 15 cases were funded per year. Five New Zealanders have been funded for proton beam radiotherapy since 2008.
On December 1, Williams and her parents Christine and Bret flew over to England where, after some preparatory treatment, she received four sessions of proton beam therapy over a four-day period, each lasting 30 seconds.
Consultant proton physicist Dr Andrzej Kacperek said the treatment produced a precise proton radiation which was tailored to a particular patient lesion.
"Due to the sub-millimetric precision of the treatment, the patient sits on a special chair which can perform the necessary minute movements necessary.
"The patient has one visit for preparation which includes mask-making with a mouth bite, followed by planning to optimise the treatment for each individual patient – to kill the tumour with the least possible side-effects."
In Williams' case they only needed to penetrate the eye 5mm deep, which left the rest of the eye relatively healthy.
Williams said the treatment was painless.
"I was awake the entire time. I could not feel a thing."
Back home in Oamaru, her eye was now a little inflamed and sore, but she had been told it was normal.
Williams did not know if the treatment had been successful yet. She would be having check-ups every three months to see if it was growing still, or shrinking.
"It is a little weird not knowing if it has worked. We were told there was a 95 per cent success rate, so I am just trying not to focus on that slight chance it hasn't worked."
A cataract would likely form in six months and need to be removed. Other side effects included dry eyes. She could also need a stem cell transplant if the radiation had damaged the stem cells in the eye.
"Of course this is all so very minor compared to the alternative options of losing my eye or the cancer spreading."
Williams said she was incredibly grateful to all of those involved in her care and wanted to share her story to raise awareness about the treatment.
"We had no idea treatment like this existed, nor do some doctors.
"I have already spoken to people who had to have their eye removed because of similar things. If I had not been told about it by Dr Bradshaw I'd have had my eye removed."
Eye proton therapy
• The Clatterbridge Cancer Centre in Wirral, near Liverpool, was the world's first hospital based proton therapy service as previously this form of precise radiotherapy was limited to nuclear physics research institutes.
• Due to the sub-millimetric precision of the treatment it is possible to kill the tumour with the least possible side-effects.
• Since June 1989, 3450 ocular tumour patients have been treated. Patients have been referred from 23 countries and several continents.
• The Ministry of Health can fund such treatment for New Zealanders out of a special high-cost treatment pool for one-off treatments not otherwise funded by the public health system.