Doctors then sat her and her husband, Nadim, down and told them there was little they could do.
Even if they had found the tumour earlier, it would not have changed the prognosis.
"I had a lot of bad days after that. It was the worst thing in the world to hear," Mrs Truter said.
They also had to work out what to tell their bubbly son.
"We decided that we wanted to let him enjoy his life for as long as possible and let him be a child ... We're not sure how to have that conversation with him yet."
The parents settled on telling Arian he has a "naughty lump at the back of his head" and that they're hoping to make it smaller with radiation. This will improve his quality of life and may prolong it a few months.
Even if this treatment is successful, the tumour will likely grow back within a few months and re-radiation is not possible because the brainstem needs two years to heal.
But the family are not giving up.
Both parents now care for Arian full-time and Mrs Truter said they've been researching other treatment options overseas, including a $70,000 procedure in Germany. She believes this is her son's best shot.
She has set up a Givealittle page in her son's name to help get him the operation. By last night it had raised nearly $19,000.
Mr Truter said they were so grateful for everything people had already generously donated to his son's cause.
"It's a shame it's a situation like this that you see that coming through. It's so great to have that support.
"It's the one great thing about this, finding out how kind and generous people are."
Getting Arian treatment overseas is reliant on his getting into clinical trials and being fit to travel.
But if it transpires that it can't happen, the parents want to improve the 5-year-old's quality of life by using the money to give him the experiences he's always wanted.
"He loves space so we want to build him a rocket in the backyard. He loves rockets and he's always asking me how old you have to be to go to space," Mr Truter said.
"We also want to take him to see the snow and maybe to Disneyland - he'd love that."
They're also considering celebrating his 6th birthday early, depending on how his health holds out as he draws closer to the end.
"We just want him to be as happy as possible."
Mr Truter said they would donate any remaining money to research into DIPG to help other children like Arian.
To donate go to: givealittle.co.nz/cause/teamarian