While there have been reports in medical literature of patients who developed tumors after stem cell injections, few patients have been as open about their quest for a cure, the money they paid and the tragic consequences as Mr. Gass, a 66-year-old former chief legal counsel for Sylvania who lives in San Diego, has been.

Image An M.R.I. image of Mr. Gass’s spine. The cavity to the right of his lower spine is filled with a tumor. Credit... New England Journal of Medicine, 2016

After the stroke, Mr. Gass lived independently although his left arm was useless and his left leg weak. Now, however, with the foreign body in his spine, he is paralyzed from the neck down, except for his right arm. He needs a hoist to move from his bed to a wheelchair, and his doctors do not know how to stop the tumor from growing.

The doctors wrote about his case in a letter published on Wednesday in The New England Journal of Medicine.

Experts who have written articles in medical and scientific journals bemoaning “stem cell tourism” said this case could make a difference.

“We scientists and clinicians say, ‘Don’t do this,’” said Dr. Jaime Imitola, a neurologist and stem cell researcher at Ohio State University Wexner Medical Center who has written about the dangers of stem cell tourism. But, he said, their warnings often do not resonate.

Dr. Imitola said that Mr. Gass “puts a human face to a tragedy.” Patients might see what happened to Mr. Gass “and say, ‘Oh my God, that could be me.’”

“This is a really cautionary tale,” said Timothy Caulfield, research director of the Health Law Institute at the University of Alberta, who wrote a recent commentary on stem cell clinics.