Should I cancel?

I have arthritis in both knees and currently in physical therapy. My doctor said give it 6-8 weeks of pt and if doesn’t help I’ll probably need an mri. I’ve only had 3 appointments so far but my knee was killing me at work and felt like it was going to give out a couple times so I made an appointment for it. I’m thinking of just canceling it and see if another weeks of physical therapy help but I don’t want to possibly risk doing any damage to my knee. If nothing else maybe I’ll see about getting a stating I need to sit at work (I’m a cashier.)