An American surgeon found a tiny foot and other partially formed body parts inside a tumour he removed from an infant's brain.

Dr Paul Grabb, a paediatric neurosurgeon, said he operated on Sam Esquibel at Memorial Hospital for Children, Colorado Springs, after an MRI scan showed a microscopic tumour on the newborn's brain. Sam was three days old and otherwise healthy.

Grabb said that while removing the growth he discovered it contained a nearly perfect foot and the formation of another foot, a hand and a thigh.

"It looked like the breach delivery of a baby coming out of the brain," Grabb said. "To find a perfectly formed structure (like this) is extremely unique, unusual, borderline unheard of."

Grabb was not sure what caused the growth but said it may have been a type of congenital brain tumour. However, such tumours were usually less complex, he said.

The growth may also have been a case of "fetus in fetu" in which a fetal twin begins to form within another. But such cases very rarely occurred in the brain, Grabb said.

Sam's parents, Tiffnie and Manuel Esquibel, said their son was at home now but faced monthly blood tests to check for signs of cancer or regrowth, along with physical therapy to improve the use of his neck. They said he had mostly recovered from the surgery.

"You'd never know if he didn't have a scar there," his mother said.