Joan Rivers Personal Doc During Fatal Procedure ENT to the Stars

9/17/2014 11:25 AM PDT

Joan Rivers' personal doctor who was working on Joan when she stopped breathing is Dr. Gwen Korovin, who treats the biggest celebs in the world ... multiple sources tell TMZ.

We're told Dr. Korovin was not authorized to practice medicine at Yorkville Endoscopy center. CNN has reported Joan's personal physician took a selfie while performing a biopsy on the comedienne.

Dr. Korovin's patient list reads like a celeb who's who. Her patients include Celine Dion, Lady Gaga, Hugh Jackman, John Mayer, and Brooke Shields.

Dr. Korovin's Facebook page has a pic of the doc in her office with pics of her celeb patients lining the walls.

The doctor has done interviews in which she says she brings a black bag to the dressing rooms of A-list stars and administers "on-the-spot treatment."

We reached out to Doctor Korovin's office and were told, "We do not comment on matters related to patients."