Talk to the company let them know whats about to happen, and see how they can help. Most companys don't want to be seen as a bully when dealing with someone fighting cancer. But they have a bottom line to look after too. It depends on each case. My company is too small to pay someone for not being here, but I know of some that have paid their people just like they were there working. Sometimes you can shift schedules, work afternoons, weekends or when you are able to instead of 5 days a week rigid schedule. As a business owner, I would be open to someone working when they could, when they felt better. I also have heard of companys that held tight with the rules, and if you couldn't work and used up your alloted days, they got let go. that is not as common though.
I worked through three different diagnoses, had to take off a day or so each week, when I was working for someone else, they were good enough to let me have off whenever I needed it, and still paid me.
Last time, working for myself, radiation made me droopy, I took off after 2 each day after about 3 weeks of treatments.