Trust in your doctors is probably just an important thing as anything else. 2500 calories is just an average for everyone. It really goes by your weight, height, BMI, activity level, special requirements like fighting disease, after surgery, etc. With cancer treatment It could be as high or higher than 3500. Most CCC have a nutrionist on board that can help you or go to one of those calorie calculator sites, and there are formulas used by the professionals, which is not difficult, but don't recall it off hand, and use one from their nutritional society. Some go by a half a gram of protein for every pound, some say a gram of protein for every pound you weigh, then for carbs, fats too is writhed. The same goes with water. It's usually half your body weight, more with added elevation, activity, etc, in liquid ounces, and with certain chemo's you want to flush the toxins out of your kidneys. The other think is cancer feeds on nutrients, so it's not all you can eat Buffett, but I don't know anyone who does during treatments.

Good luck.