It's unclear what type of cancer, but the 67-year-old said it's based in his pelvic area and the pain has kept him from working at City Hall for the past week.
Rosendahl said on his personal blog that the cancer is slow-moving and has not spread to any vital organs. His doctors are optimistic and he expects not only to return to work, but also seek a third term.
On Tuesday, Rosendahl's staff confirmed he was using medical marijuana to treat neuropathy. Rosendahl's spokesman, Tony Arranaga, did not say when the councilman might return to work.
City News Service contributed to this report.
