This is a complicated and difficult question and the best answer is maybe.
As an example, the both the Americans With Disabilities Act and Kentucky law make it unlawful for an employer to fire an employee because the employee is "regarded as" disabled or as suffering from a disability.
A breast cancer survivor won her suit under the Americans With Disabilities Act in Eshelman v. Agere Systems, Inc, No 05-4895 (January 30, 2009). Her claim was that her employer regarded her as disabled because of problems that she encountered as a result of chemotherapy.
The best answer is that the law may provide you some protection against being fired because you are diagnosed with cancer.
If you believe that you have been discriminated against by your employer because of your cancer diagnosis, please contact us.