Exercise Ball Workout Week 1

I have a tumor that started in my back and protrudes forward into my abdomen. I have already completed 12 weeks of chemotherapy a year ago. I started using an exercise ball to strengthen my core. I start with my feet on the ball and hold a plank position for up to 30 seconds. At this point, I am dealing with back pain: sharp, stabbing sensations at times and a dull ache. I also have lost much of my aerobic endurance so I carefully breathe and clear my head as best that I can. I do sit ups while seating on the ball. I use a slow controlled motion. I burn very quickly but I patiently work though it.