The US Anti-Doping Agency (USADA) produced a report on its decisions against Armstrong.
One of the key pieces of evidence against Armstrong was the testimonial of fellow cyclist Frankie Andreu about what he heard in an Indiana hospital room, while Armstrong was being treated for cancer. That testimony was initially challenged by Armstrong.
However, one of Armstrong’s doctors, Craig Nichols, confirmed that the
very next day, October 28, 1996, Armstrong was set to begin chemotherapy consisting of the intravenous administration of “an aggressive combination of cisplatin, etoposide, and infosfamide.”
So, a doctor testified in an affadavit that Armstrong was receiving chemotherapy, providing strong evidence that Armstrong was indeed sick with cancer as claimed.
(Side note: Armstrong later confessed that Andreu's testimony was accurate during the recent Oprah interview.)