Eintracht Frankfurt captain Marco Russ scored an own goal in their Bundesliga relegation play-off first leg with Nuremberg, one day after being diagnosed with a tumour.
The 30-year-old defender tested positive on Wednesday for high levels of growth hormone HCG, which was found to be caused by the illness.
He decided to play Nuremberg on Thursday but put Sebastian Kerk's free-kick into his own net after 42 minutes.
Mijat Gacinovic made it 1-1.
The second leg of the play-off, which decides who will play in the Bundesliga next season, is on Monday in Nuremberg.
Frankfurt were third from bottom in the Bundesliga, with their opponents finishing third in the second tier.