I have a friend who has recovered from CFS, although she does not have the stamina I know she wants. So I think of her when I see "fatigue." That is, I know there is a substantive difference between being shagged out after a long squalk and suffering from exhaustion and fatigue. What these differences are, tho, dunno.
For the majority of patients, it is only a temporary condition that resolves over time when treatment is completed; but for others it may be chronic (meaning it doesn't go away).
The cause of fatigue is poorly understood but contributing factors may include anemia, sleep disturbances, depression, the disease itself or the treatment(s).
Here we go! Anemia: pretty straightforward -- you're anemic or not. Can you get iron shots? Supplements? or not?
Sleep disturbances. This is not about car alarms go off at 4 am nor about a wretched night with two hours -- which happens to everyone. What it is precisely, I'm not sure.
Depression. OK. This will eventually be another topic. But for now: depression the side effect is different from garden variety depression, chemical imbalance, situational depression and exhaustion. If my experience with the birth control pill for abdominal pain / ovarian cysts leading to clinical depression is any indicator, depression brought on by radiation therapy is the real depression, difficult to explain while you're suffering from it that no you're not actually depressed like "hey, life sucks and then you die, oops, I have cancer, spiral, sucking noise," but "40 ton monty python weight falls on head after being zapped by a huge machine" depression.
more cut n' paste I will digest later:
Patients may feel weak and dizzy with a desire for rest and sleep. Other symptoms include pain in the legs, or difficulty climbing stairs, walking short distances, or being short of breath after only light activity, like cooking a meal or taking a shower. Some patients report difficulty thinking, forgetfulness, and an inability to concentrate.
There are no medical tests to measure fatigue. No one knows the exact cause of cancer-related fatigue, but causes are usually multiple. Any physical or emotional change can deplete energy. Varying levels of fatigue may be experienced, depending on the individual patient situation and cancer treatment(s).