The Danish actress' youngest son, Raoul Jr., was first diagnosed with a benign tumour when he was nine and overcame his health issues after a lengthy stint in hospital.
But the 16 year old was recently involved in a scooter accident, leading doctors to discover the tumour after a series of tests.
Nielsen tells British TV show Lorraine, "When my son was nine he was diagnosed with a benign brain tumour and you feel so guilty and confused and angry... How can you be lucky to have an accident? When he was nine he was almost two years in the hospital, now he is 16 and I thought he was going strong, he overcame it.
"But four days ago they called and told us the tumour is back. Because he had a scooter accident, and so he had all the tests done and they saw there was bleeding and they found it was back in his brain.
"So it's a tough moment, but it just encourages me even more... I always thought, 'It can't happen to me, it will happen to some other family.' But it could happen to anyone."
The Rocky IV star has revealed the teen will now undergo further surgery - but he's determined to remain positive.
She adds, "He is going to be going through some major operations, and brain operations are never good. So of course it's tough, he is 16 so he likes to be with girls and look good - it's a long process and it's a tough process... But I spoke to him yesterday and he was like, 'Come on mum, I am going to be just fine!' What's amazing is that they are just so much stronger than we are."