It had been there for about eight years, but despite assurances from doctors that it was "benign", once it began to grow, the mother-of-two started to feel uneasy.

"I went to the doctor and all the imagery - X-rays, an ultrasound - all showed that it was benign," she said.

"I was referred to an orthopaedic surgeon and they did an MRI, and I was put on a one-year waiting list. But it continued to grow, and so did my concerns.

"I kept pushing for a review and they agreed to bring the surgery forward - but by that stage it had fully invaded my finger and a biopsy was done."

Ms Johnston, 33, was diagnosed with clear cell sarcoma - a very rare cancer of the soft tissue.

She had lived in Cairns for five years, but moved back to Newcastle to be closer to family, friends, and a specialist sarcoma centre in Sydney.

Ms Johnston had the lymph nodes in her right arm removed, and her right ring finger amputated.

Doctors hoped she was clear of the cancer, but during a routine PET scan, they found it had spread to other parts of her body.

"My sarcoma surgeon, who removed my finger, did my PET scan follow up report and he just kept saying, 'It's not good... It's not good'," she said. "I kept saying, 'What does not good mean?'"

At best, Ms Johnston may have "several years" left with her two young children.

At worst, 12 months.

"Because it's a rare cancer, there is limited information and limited data," she said.

"They consider it terminal, but they just don't know how my body will respond to certain treatments, or whether we'll get access to it, or how the cancer will respond.

"They don't know. They just... don't know."

Ms Johnston said she was the sole carer of her children - Jack, 4, and Maddison, 22 months.

"I live with my mum now - she helps me a lot with the kids. My dad drives me to radiation. I have my sisters, and family and friends," she said. "It is so scary. My son doesn't really understand that I have cancer. He just knows that I have sarcoma and that my legs aren't well.

"He asks a lot of questions. He is four. I have told him that I might get really sick.

"But it is really hard."

She said she wanted to fight for more time with her children - but some days, staying positive was tough.

The cancer had spread to her left calf, left hamstring, left "inguinal nodes" and right Achilles tendon.

Her legs were swollen. Walking had become difficult.

"Sometimes I feel like giving up to be honest, but I can't. Jack wants me around for every minute possible.

"I wish... I just wish I was never diagnosed.

"I just wanted to bring up my kids. Buy a house. I didn't ever want to live a spectacular, glamorous lifestyle. I just wanted to be a good mum and watch my kids grow old."

Ms Johnston has been granted compassionate access to a targeted therapy drug not funded by the PBS.

But if her cancer continues to grow, she will need to look at immunotherapies - which cost "thousands of dollars" a month.

Her current therapy costs $7250 a month.

Her oncologist, Dr Vivek Bhadri from Chris O'Brien Lifehouse in Sydney, said Ms Johnston had been "extremely unlucky".

"Her scans were completely clear, then three months later, it was all around her body - basically in the blink of an eye," he said.

"It is a very rare cancer of the soft tissue. We don't know why people get this.

"One of the problems with it being a rare diagnosis is when you look at treatments that work best, we only have a small number of patients to have reports about what has worked and what hasn't.

"What we do know from the past 20 years is that, unfortunately, not very many treatments work."

Normal chemotherapies were "not very" effective.

"The main problem with clear cell sarcoma is if it has spread - as it has done in Cassie's case - then you don't really have a lot of options," Dr Bhadri said. "This disease is very aggressive, it moves very quickly.

"We tried Cassie with one drug which really didn't work - her disease grew. We switched to another drug, and we are still waiting to see whether it's working, but at least it appears, clinically, that things have slowed down.

"The drugs we are talking about are many thousands of dollars per month."

A Go Fund Me - Help Cass Fight Cancer - has been set up to help Ms Johnston access these therapies.

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