Six-year-old Dylan Williams's condition had baffled doctors.
They initially thought the swelling on the right side of his face was caused by either mumps, a tooth abscess or an allergy.
But the terrifying truth was that he had one of the world's rarest cancers and it had riddled his body. He was, in fact, just days from dying. Here are the signs and symptoms of the most common cancers that you should not ignore.
His mother, Siobhan O'Flynn, told Mirror.co.uk : “A couple more days and he would have died. Tumours were pushing out his eye, wrapping around his brain, spine, shoulder blades, ribs and kidneys.
“No one could believe that he’d survived - he was riddled with tumours, even his central nervous system was infiltrated. I am forever grateful to the NHS for catching it when they did. ”
Siobhan, 44, said that a swollen cheek had been the only physical sign that her 'healthy, robust little boy' could be so gravely ill.
“He had just started year one and I noticed a swollen cheek. My wonderful GP was abroad so I took him to the locum doctor several times and we were told that he might have mumps but I just knew that was not it.
"Then we took him to the dentist a couple of times thinking it might be a mouth abscess.
"One night he woke up in pain after his lips suddenly swelled up. We rushed him to A&E and they gave him steroids and anti-histamine and the swelling went down so they thought it was an allergy.
"But his cheek was still swollen and when his brilliant GP got back she immediately referred us to the dental clinic in hospital where he had an X-Ray and ultrasound.
"And the first X-Ray revealed this enormous mass in his head. The consultant kept us very calm and called a colleague to examine the scans.
"Then they took ultrasound images of Dylan's head and it was very quiet in the room as they looked at them.
"I knew something was wrong but I assumed it was just an abscess."
Then doctors asked to speak to Siobhan alone. They sat her down and told her they were 'greatly concerned'. Still not grasping the enormity of the situation, they asked her: "Siobhan, what's the worst thing you can imagine?"
Siobhan said: "Then I knew and I just collapsed. It was pure shock. But you go straight into doing things so you don't really have time to process it too much.
"He was riddled with cancer but there had been no indication. He had seemed like a healthy, robust little boy. He was eating, going to school. A swollen cheek was the only physical indication.
"He was admitted to hospital the same day and had biopsies, CT scans, MRIs, X-Rays and blood tests.
"In his first MRI one of his eyes looked bigger than the other because the tumour was pushing it out.
"I had to call his big brother Ruairi to break the news and all I could hear was sobbing on the other end of the phone as I tried to reassure him that we were doing all we could to help Dylan and he was in the best place possible to receive care.
"With childhood cancer there's no known reason or cause. Some children are born with it. No one really understands why."
Five days later, on Christmas Eve 2016, Siobhan and her partner Nick Williams, 48, were given an exact diagnosis.
Their son had stage 4 B-cell lymphoblastic lymphoma and he needed to start on intensive chemotherapy straight away.
Although the B-cell subtype of lymphoblastic lymphoma rarely manifests as solid tumours outside the lymph nodes, Dylan had many tumours all over his head and torso.
Dr Richard Simcock, a consultant advisor at the Macmillan Cancer Support charity, told Mirror.co.uk that Dylan's form of cancer was very rare.
He said that lymphoblastic lymphoma is itself a rare disease, occurring in about 11 cases per million population per year, usually presenting as enlarged lymph glands (lymphadenopathy).
But only approximately 25% of that small total present with extranodal involvement (tumours not in the lymph glands), making Dylan's presentation particularly unusual.
Mum-of-two Siobhan said: "Dylan was lying in the hospital bed with what looked like 100 people suddenly working on him. It was intense and I was told later on that if his chemo did not start then he probably would not be here now.
"He was given the chemo drug Methotrexate which is bright orange and so nasty the nurses wear face shields and gloves over their elbows when they administer it.
"You just don't know what's happening. It's an overwhelming feeling being helpless to help your child and you have to look to the professionals to do something. Your instinct as a mother is to pick him up and hold him but you can't.
"His cancer was so rare they had not seen it in the hospital for 17 years.
"His initial chemo lasted 8-9 months, it was really intense and the side effects were difficult. He was so weak he could barely walk. His long hair fell out and he struggled to write due to nerve damage.
"He had constant sickness, was incredibly fatigued and in constant pain.
"At one point he had so many mouth ulcers he couldn't even smile.
"He was housebound for weeks and couldn't go to school, the hospital or the shops after his immune system became so weak that he could not fight off anything. His friends could not even come to play with him. If he even caught a cold he would have to be hospitalised for 48 hours on an extreme course of antibiotics."
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So far, Dylan has received 28 general anaesthetics, 55 rounds of chemo and 20 lumbar punctures.
Every month, he has Methotrexate injected directly into his spine to flood his spinal fluid with the toxic but cancer-killing drug which then flows up to his brain to keep the aggressive tumours at bay.
But the difficult treatment plan managed to shrink Dylan's aggressive tumours and saved the little boy's life.
Siobhan said: “The conclusion was that the chemo started working just in time. Without the referral from my GP and the fast response of the consultant who saw him he quite simply wouldn’t have made it."
"Dylan's at school today which is good, he's getting stronger and I'm hoping he might manage the whole day."
Siobhan, from Cardiff, has two sons - her eldest Ruairi is 18 and Dylan is now 7.
She writes a blog called www.TheLittleBigC.com which helps her process the pain, fear and trauma of minding an acutely ill child while raising awareness about cancer.
"Being on this journey makes you realise how many kids are in a similar situation. Cancer changes your perspective and makes you re-evaluate what really matters in life.
"You suddenly have to find the resilience to help your child deal with whatever they will have to face.
"But the irony of going through a tough situation is that it brings peculiar positives I never would have thought of.
"It's brought us even closer as a family and it has made us stronger."
Siobhan said that Dylan is improving every day but the future is still uncertain.
"Now Dylan looks great. His hair is growing back and he is regaining his strength. He's started to ride his bike again and it just gives me the most immense pleasure to do regular things like bring him to the park and see him eat ice cream. Or see him able to play with his friends again. It's a cliche but it's true that the little things in life are the most important.
“He is now nearly one year into his maintenance therapy and he has just under two years left of treatment and we'll monitor him after that.
“He still struggles with some side effects and he still hasn’t managed a full week at school since he started back in September.
"We don't know what his prognosis is and he will probably need to be monitored all his life. He may never be able to have children, his heart will always have to be checked due to the affects of chemo, he has nerve damage and he may always have mobility issues.
"But the joy that the NHS has given by saving my son is incredible. I don't know how they do it, day in day out. They are absolutely fantastic.
"From the moment he had his first scan those medics came out, all guns blazing, saying 'we're going to sort this out.'
“Without the NHS my son’s diagnosis would have gone unnoticed and he wouldn’t be here today. The NHS has given him a second chance at life.”