Clarence Dally (1865-1904), assistant to industrialist Thomas Alva Edison. The first recognized American X ray fatality.

Soon after the discovery of X rays in 1895, both men repeatedly exposed themselves to x rays in popular demonstrations and investigations of the exotic new imaging technology. Mr. Dally tested his X ray equipment on his own hands.

X ray "burns" on his hands turned cancerous. The cancer spread throughout his body and was ultimately fatal despite many efforts to cut out or otherwise treat the disease.

The discovery of x radiation blew everybody's mind; it was totally unexpected. Thomas Edison jumped right in and tried to exploit this unprecedented power to see inside solid bodies. He underestimated his ignorance, fatally. Of course, no one knew what effects the new rays could have on the human body. Fortunately for Edison, his blunder only caused him permanent persistent discomfort.
His main x-ray technician, Clarence Dally, who worked closely and consistently with the newly devised apparatii, was not so lucky. As we know now, you can absorb a large amount of X rays without any immediately visible signs. If the dose is spread out over a long enough period (say over days or weeks rather than minutes or seconds) you could conceivably take enough to kill you without feeling it. Eventually, like so many other unwitting "pioneers" who spent their lives on the new and magical X rays, Dally noticed what's euphemistically known as X ray "burn". By the time the burns showed up the damage was already potentially fatal in the years to come. Dally didn't stop even as his condition grew worse. He even kept working at the infernal machine when he knew his hands were cancerous. I won't go into the gruesome details, but he wouldn't stop and neither did the cancer.