A Philadelphia baby has died after a yearlong battle with a rare form of childhood cancer.

Relatives of Shane Metzgar's posted on social media and a fund-raising page on Tuesday that the 20-month old boy died Monday, a year and two days after he was diagnosed with alveolar rhabdomyosarcoma, known as alveolar RMS, a cancer of the muscles.

"He went peacefully," the Metzgars wrote. "This past weekend was a challenge as Shane took a turn for the worse. Time was of the essence. We were able to take final pictures and make various keepsakes with him. More importantly, we were able to say goodbye."

The Metzgars said the little boy's laughter and upbeat, smiling personality helped them through the months of medical appointments, chemotherapy and surgeries.

"Shane gives us the strength to fight," his mother, Michele Metzgar, said when Philly.com chronicled his case last year. "We have been through all the motions and emotions you can when your world gets flipped upside."

The family wrote on Tuesday that Shane "made us proud" and "fought until his last breath."

His tumors will be donated to the Keller Research Center in hopes of advancing research about the disease.

The little-understood cancer affects just about 350 people a year, one expert estimated last year. The survival rate for those, like Shane, at stage four, is 20 to 30 percent, and even lower for infants.

The family wrote last week on its fund-raising page, set up to help pay for treatment costs not covered by insurance, that Shane wasn't doing well.

His doctors concluded in early March that "the chemo wasn't working" and there were "no other treatment options left for Shane" at the Children's Hospital of Philadelphia, where he had been receiving care since he was diagnosed with alveolar RMS last March.

The family said last week he was under the care of CHOP's pediatric advanced care team and home hospice. Over the months, the boy's aggressive cancer had metastasized from his groin to his lymph nodes, pancreas, lungs, bone, bone marrow and ultimately his brain.

"The doctors told us from the beginning what Shane's chances were and we chose to ignore them," the Metzgars wrote. "We are glad we did."

The Metzgars have also set up a foundation, called Shane's Future Days, to raise awareness and money for rhabdomyosarcoma research, and to help families affected by the cancer.

The foundation was selected as the benefit charity for the Phillies' Phan Cave opening day tailgate. The family said it still plans to attend the event Monday at Citizens Bank Park.