Yorkshire terrier Casper joined the family after Emily, now 12, left hospital after five months of treatment for Burkitt's lymphoma.
But 10-year-old Casper has now been diagnosed with lymphoma and is even receiving the same chemotherapy drug, cyclophosphamide, that Emily had.
Mum Jackie, 42, of Dovedale Road, Seaburn Dene, spotted the lump on Casper's neck, and, after Emily's illness, knew something was wrong.
Jackie, who is married to Mark, 44, said: "Me and Emily just broke down when we heard.
"When I was told about Emily's lymphoma it was like having your insides ripped out and when I was told about Casper, briefly I got that feeling.
"Emily was heartbroken, we have to reassure her that he's going to get better.
"I said: 'You've done it, you've survived'. I think it reassures her, the fact that she can compare his with hers and she knows she survived."
Emily was diagnosed with lymphoma in February 2000 after her mum took her to the dentist because one of her adult teeth was loose.
Tests discovered a tumour on her jaw then more in her tummy and kidney and she was sent straight to Newcastle's Royal Victoria Infirmary to start her treatment.
But after nearly five months of intensive chemotherapy, Emily came home and her parents decided to get a dog to help her recover.
They heard about four-year-old Casper, who needed to be rehomed, through a kennel in Hartlepool and fell in love with him immediately.
Jackie says Casper is "heaven sent" and helped them through Emily's illness.
She added: "I always say he found us. He would snuggle up to Emily in front of the fire. Even for myself and my husband, he was a focus."
But Casper was diagnosed with lymphoma two weeks ago after Jackie noticed a lump on his neck.
She said: "I felt a lump and thought that something is not right because it was in his neck and Emily's was a swelling.
"It was the fact that it was in his neck, I just knew."
She mentioned it to the vet during Casper's annual visit to have his teeth scaled and, despite an initial test proving negative, further checks revealed that he had lymphoma.
They have been told it is a treatable form of cancer and he receives weekly doses of the chemotherapy drug, and takes tablets every other day.
Jackie said: "He's absolutely marvellous, he's great."
Jackie, a classroom support assistant, and Mark, a painter and decorator, both work for Tyne and Wear Autistic Society.
Jackie says Casper will get chemotherapy for two months and should then move on to maintenance treatment.


BURKITT'S LYMPHOMA

BURKITT'S lymphoma is a type of non-hodgkins lymphoma affecting lymphocytes (a type of white blood cell), found in the blood and lymph glands.
Burkitt's lymphoma is common in Africa but very rare in the UK
Lymphoma is the third most common cancer diagnosed in dogs.
Canine lymphoma (lymphosarcoma) is a cancer arising predominantly in the lymph glands of the body.
The cause of canine lymphoma is not known, however it is not a contagious disease and there is no chance of it spreading to other pets or to humans.