The club were informed of the positive test on Wednesday with Germany's anti-doping agency (NADA) suspecting that the high levels of growth hormone HCG found in the sample were due to illness rather than the use of performance-enhancing drugs.
Further tests by the club revealed they were indeed due to a tumour, Frankfurt said, without disclosing the location of the growth.
Russ is a Frankfurt youth product, having been at the club for almost his entire career apart from a two-year spell at VfL Wolfsburg between 2011-13.
"Despite this tragic diagnosis the player has said he is ready to play, something that has been confirmed by the doctors," Frankfurt said in a statement.
Frankfurt take on Nuremberg in their relegation/promotion playoff first leg later on Thursday after finishing in 16th place in the Bundesliga.
Image: Marco Russ
Photograph: Reuters
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