There are several issues with so-called "non-compliant" patients. In the elderly, simple forgetfullness or confusion may be the problem.
It's a fact that many medications have unpleasant side effects. Some people are, for one reason or another more susceptible to these effects. Often, the reason a patient skips a medication is because of a side effect. It is important to routinely question patients about side effects, and if they are significant to report the incident promptly to the FDA.
It may or may not be appropriate for you to provide such advice, depending upon what physician or hospital you work for. I'm not saying that it is inappropriate for you to provide input. I'm saying that the situation is often not that clear-cut. You have to play it by ear.
Because cancer involved mortality, and the outcome often means death, advice on risk and benefits and treatments is a very touchy subject, particuliarly if you are involved with an interdisciplinary team, everyone with a different ricebowl.
There is also great disagreement about informing a patient of the seriousness of his illness. This, of course, has a great deal of impact on whether or not the patient chooses to follow a medical protocol. For many year, as an example, some physicians would not inform a patient they had a malignancy.
The other side of the coin is that there is disagreement on cancer treatment protocols, and it is not always unreasonable for a patient to refuse radiation or a specific treatment modality. Oftena physician is bound by the "standard of care" doctrine and is unable to suggest an alternative. It may not be appropriate for you to "convince" a given patient to "accept the radiation".
Sadly, some physicians have an interest in convincing patients to follow a certain protocol, particuliarly in a drug study, and it remains to be seen whether you should exert influence one way or the other.
You ask a good question, and I by no means intend to disrespect you, but life working in a medical care facility can become complicated.