The 30-old-year Dutchwoman admitted she was shocked when she was told she was ill.
“It is obviously very scary when you hear this news,” Dekker said in a statement posted on the website of the Dutch Swimming Federation.
“For three years, I’ve been preoccupied with getting ready for the Rio Olympics, and then I heard this.
“Suddenly, my life became very quiet and I could see that everything was relative.
“I took a few days to get used to the news, but now I am looking with confidence towards the future.
“I am focusing on being positive and giving my all to recover well.
“I can only focus on the things I can influence, but I have certainly not put the Rio Olympics out of my head.”