A-level student Chloe Jones was having routine tests after complaining of headaches and dizziness.
Optometrist Brian Borland found a swelling pressing on an optic nerve which resulted in the youngster being rushed to hospital and undergoing brain scans followed by urgent surgery.
Chloe told her local newspaper in Merthyr Tydfil that she "couldn't believe it" when medics told her they had removed a growth "the size of an orange" from her head.
Recalling her shock at the sequence of events, she said: "I couldn't help feeling a bit nervous as I knew I was literally going to have my forehead peeled back during surgery.
"But thankfully the whole operation was a success which was a huge relief for me and my family. It's so weird to think there was something that big in my brain that I didn't even know about."
Chloe's wound was closed with 40 staples in her scalp following the six-hour operation, which has left a long scar across the top of her head.
Her optician Brian Borland said it was extremely lucky that Chloe came in for her eye test when she did: "It's quite common to see swelling during eye tests but in seven years as an optometrist I have only seen one other case where this has been caused by a brain tumour."
Very lucky indeed!