"We're just so in love everyday, we're so thankful," their mother, Alissa Dunn, tells PEOPLE

Credit: Courtesy Alissa Dunn

Eight years ago, Michigan parents Alissa and Michael Dunn got a double dose of terrible news when they learned their identical twins Madeline and Isabella were both born with a rare cancer – and that the cancer appeared to have been passed from one daughter to the other in the womb.

Now cancer-free, the twins celebrated their eighth birthday over the Fourth of July weekend.

“We’re just so in love every day, we’re so thankful,” mom Alissa tells PEOPLE.

But the future wasn’t always so certain.

While the Dunn girls looked healthy at birth, just two months later their parents noticed Madeline’s stomach was starting to swell. They rushed to the pediatrician for their two-month check-up.

Her enlarged stomach turned out to be a softball-sized tumor on her adrenal gland. Doctors examined Isabella as well, finding both twins had stage 4S Neuroblastoma, a very rare cancer found in children, typically under the age of 1.

It appeared to grow in Madeline while she was a fetus, and it was then passed on to her sister, Dr. Jeffrey Taub, MD, FAAP of the Children’s Hospital of Michigan at the Detroit Medical Center tells PEOPLE.

“The scenario is extremely, extremely rare,” he says. “It starts in the adrenal gland, which sits on top of your kidney and it spreads to other parts of the body. Because they’re identical twins, their blood vessels are connected to each other so blood cells can pass from one fetus to the other.

“Since they’re identical, her immune system wouldn’t recognize them as being foreign.”

The twins began chemotherapy immediately, completing four rounds before they were finally pronounced free of cancer in December 2007.

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Now, with only small scars to show the pain of their first year, the “spunky” twins celebrated their birthday at the lake, surrounded by family, over the Fourth of July.

Alissa says Isabella is very caring and loving and Madeline is full of energy. The girls are enrolled in gymnastics and will start second grade in the fall.

“We celebrate every day that they wake up and they’re healthy,” Alissa says.

Taub, who attended their first birthday bash and is excited to watch them as they grow older, says he sees them once a year now for a check-up – and a photo of them in his office reminds him everyday how special they are.

“Being an oncologist is a really stressful, challenging type of business, but when you have patients like Isabella and Madeline, that sort of reminds me why I went into this and the outcomes we can achieve,” he says. “That’s what we’re always aiming or striving for.”