Neurosurgeon Allan da Costa said the harpoon came within 1cm (0.4 inch) of killing Elisangela Borborema Rosa, 28.
She had emergency surgery to remove the harpoon and is expected to make a full recovery.
Officials said Ms Rosa's husband had accidentally fired his spear gun while cleaning it. Police said everything indicated it had been an accident.
The incident happened on Monday in the coastal town of Arraial do Cabo, 150km (93 miles) east of Rio de Janeiro.
According to her family, Ms Rosa was in the kitchen when she was hit by the harpoon, which her husband had been cleaning in the living room.
The harpoon, which her husband used for underwater fishing, entered through her mouth and was lodged against her spine.
She was taken to a local hospital, but immediately transferred to the regional hospital in nearby Araruama, where a team of specialised surgeons operated on her.
"This is the first case of this type I have seen," Dr Costa said.
"The peculiarity of this case is that the tip of the harpoon lodged itself between the anterior vertebral canal and the medullary canal," he said.
"One centimetre further in, and she would have been paralysed, one centimetre further out and it would have hit an artery which provides the brain [with blood] and she would have been dead."
Police said they would question her once she was recovered "to get a clearer picture of what happened".
