'This wasn't on the scale of my worst fears.' The mother and baby fighting brain cancer.

In March this year, Melbourne mother-of-two Sara Chivers was told that, after eight years, her brain cancer had returned.

The 34-year-old had developed three new tumours – all were inoperable, all were incurable.

But four weeks ago, Sara and her husband Leigh were given another devastating diagnosis: their 18-month-old son, Alfie, also had brain cancer. It’s something the family’s neurosurgeon describes as “lightning striking the same place twice”.

While Alfie’s cancer is a completely different type to his mum’s, it’s also aggressive, malignant and terminal.

"You just don't expect the worst scenario to keep happening over and over again," Leigh told ABC's 7:30 this week.

"But it has for us. A couple of times."

"This wasn't even on the scale of my worst fears," Sara - who was undergoing treatment for her own cancer when she noticed Alfie's symptoms - said of the family's current situation.

The heartbreaking story of Sara Chivers and her son Alfie who both have terminal brain #cancer. More tonight on #abc730. @Milliganreports pic.twitter.com/EihQLR9CdR

— abc730 (@abc730) November 1, 2017

What was initially thought to be symptoms from teething or an infection was actually a mass the size of a pear in little Alfie's brain.

When Leigh called Sara - who was staying with her mother at the time - to share the news, she "started screaming".

While Sara had an operation in late October to shrink the largest of her tumours, and has been receiving treatment for her cancer, the mum has penned a heartbreaking letter to her two sons - Alfie, and her eldest son, three-year-old Hugo - to help guide them after she's gone.

"I won't be around to see you grow up. It's a hard thing to say and even harder to face," she wrote for The New Daily.

"There are some things I want you to hear from me... Love hard. As they say, it is better to have loved and lost, than never to have loved at all. That's how I feel about you both.

"Heartbroken doesn't come close to describing the pain I feel at not being in your lives in the future, but I would never change or forego the time we have spent together and the immense joy you have brought me. You are without a doubt my proudest accomplishments."

Sara also offers words of wisdom to her sons about school, travel, friendship, manners and putting family first.

She also spoke about her husband, telling her boys to "be kind" and "never let him doubt himself" as he faced bringing the boys up on his own.

"Your Dad is the most admirable, courageous man I have ever known. He is my companion, my rock, my everything," she wrote.

"I will be forever grateful for the time we spent together, the memories we created, the love we shared. It was always him. Always will be."

Sara is using the time she has left to help raise awareness of brain cancer, and help her young son with his fight.

"He hasn't been able to do all of the things that I've been able to do, you know, in my 34 years to date," Sara told 7:30.

"The mortality rates for children aren't great. And I won't let him be a statistic."

Sara encourages donations to the Cure Brain Cancer Foundation. Her sister, Stephanie, has also set up a GoFundMe to help the family with their ongoing expenses.

LISTEN: Carrie Bickmore on juggling motherhood and a career, and the inspiration behind Beanies 4 Brain Cancer.