Rachel Uchitel -- Major Brain Surgery Scheduled

Rachel Uchitel will undergo surgery so major, her entire brain will be lifted off the cavity.

We're told Rachel has been suffering from back pain for years, but all treatment failed. She recently went to a surgeon who told her she had a condition called Chiari, a birth defect that causes brain tissue to settle into the spinal canal.

Tiger Woods' former GF will go in for surgery in 2 weeks, where doctors will cut a hole in the back of her head and lift the brain off the spinal canal.

Rachel's terrified but doctors say they're fairly confident ... 50% become symptom-free, 10-30% improve significantly and the rest don't.