Australian track cyclist Matthew Glaetzer has withdrawn from Wednesday's individual sprint and been replaced by Matthew Richardson, Olympic organisers said on Wednesday.
No details were given other than to say the 28-year-old could not race for medical reasons.
Glaetzer is a former world champion in the sprint and finished in fourth place at the Rio Olympics.
Glaetzer was diagnosed with thyroid cancer in October and returned to action a few months later, after surgery, before continuing to receive radiotherapy treatment into 2020.
He rode in the men's team sprint on Tuesday when the Australians finished fourth.
(Reporting by Martyn Herman; editing by Clare Fallon)
