Antibiotics prior to dental treatment this is a great topic and, in my humble opinion, misunderstood by many dentists and patients.

Antibiotics are prescribed to patients who are percieved to be at risk of developing bacterial endocarditis as a result of dental treatment. The basic regulations as to who needs them and how much has changed very little over the last 20 or so years. Here is a link to a site that explains the rationale and science of this situation; http://www.aafp.org/afp/980201ap/taubert.html

A long story short, the simple facts are that when you have your teeth cleaned bacteria that are already present in your mouth are introduced in large quantities into your bloodstream (the same thing happens when you brush your teeth.) Individuals who have prosthetic parts in their bodies especially in or around their hearts are at risk for the potential colonization of bacteria on these prosthetic parts which could lead to a potentially fatal result.
The American Heart Association; http://www.americanheart.org/presenter.jhtml?identifier=1200000
publishes guidlines for medical and dental practitioners to follow to best prevent this situation in patients who are at risk.
Radiation therapy and/or chemotherapy to the head and/or neck do not put a person at a higher or abnormal risk of developing bacterial endocarditis after dental treatment. There is no science or research that I am aware of that supports prescribing antibiotics to patients who are undergoing radiation and/or chemotherapy to the head or neck to reduce the risk of bacterial endocarditis.
So relax everyone and don't get anxious if your dentist doesn't prescribe an antibiotic before your filling or hygiene appointment. If you are at all concerned about this ask your dentist. I'm sure they would be more than happy to discuss it with you.

Cheers,
Mike