I called to CNS to inform that I wouldn't be able ...

I called to CNS to inform that I wouldn't be able to attend the medical control on the specified date because I would be doing chemotherapy all day, and one person who I talked to told me: "You have to choose between the treatment and the appointment for medical control. You should decide what is more important for you", without even tell me that I have the option to make a new appointment or send a certificate from my oncologist justifying my absence.