THE INFLUENCE OF AIR CAVITIES ON THE OPTIC CHIASM DOSE DURING PITUITARY RADIOTHERAPY FOR ACROMEGALY

The treatment planning of acromegalic patients can be complicated by t he presence of abnormally large frontal air sinuses which allow increa sed anterior beam transmission causing increased dose to the optic ner ve and chiasm which, if excessive, can result in blindness. This poten tial problem has been investigated by computer treatment planning exer cises which have allowed for a variable thickness (1-3 cm) of air cavi ty beneath the frontal field and for different weightings of the later al opposed fields relative to the anterior field. The resultant overdo sage can be greater than 5% of the intended dose. The increased biolog ical effect from errors of this magnitude is sufficient to increase th e risk of optic nerve damage since neural tissue is very sensitive to small increments in fraction size. Even when the reduced attenuation i s allowed for in the computer plan, the thickness of the air sinuses v aries across the field so that irradiation through the frontal enlarge d frontal air sinuses causes inhomogeneous dose distributions in the t arget volume. Acromegalic patients should therefore be treated with a more superior beam which would avoid the air sinuses altogether.