http://www.canceradvocacy.org/resour...mployment.aspx
The page above provides a good overview of our rights as cancer survivors in reference to employment. You can request a copy of the publication that this webpage references.
It is reassuring and suggests that although you do not have to reveal your illness to the employer, it may be in your best interest to do so in order to request what is called a "reasonable accomodation" for you to get your treatment. This reasonable accomodation may simply be a flexible work schedule once every three weeks in order to provide you with the opportunity to recieve treatment.
Specifically, "Federal law and most state laws require an employer to provide you a reasonable
accommodation. An
accommodation is a change, such as in work hours or duties, to help you do your job during or after cancer treatment. For example, if you need to take time off for treatment, your employer may accommodate you by letting you work flexible hours until you finish treatment. An employer does not have to make changes that would be unduly costly or disruptive."
There are also many other tips.
Good Luck