Before starting any exercise regime is it best to discuss it with your treating doctor or surgeon as there are factors that will determine whether it is advisable for you to exercise or not.
Surgery, for example, and rehabilitation could affect your ability to exercise. Although mobilising earlier can improve recovery time, over exerting yourself may prevent healing.
Patients undergoing chemo and radiotherapy are usually advised to continue with gentle exercise as it can stimulate your appetite, and help manage fatigue and chemo brain. However, if you are neutropenic (low white cell count), anaemic (low red cell count), or thrombocytopenic (low platelet count), strenuous exercise, or exercising in a gym or outside may be ill advised.