EXTRACRANIAL RADIATION-DOSES IN PATIENTS UNDERGOING GAMMA-KNIFE RADIOSURGERY

OBJECTIVE: To determine extracranial doses in patients undergoing gamm a knife radiosurgery and identify component sources of the extracrania l doses using phantom measurements. METHODS: The lateral canthi, thyro id, sternum, and midpelvis region were monitored in 104 unselected pat ients during their gamma knife treatments using thermoluminescent dosi metry. Measured doses were normalized to integral dose, equivalent tim e (which is defined in relation to the activity of the cobalt-60 sourc es), and collimator size to correlate radiation doses with these param eters. A phantom was constructed from a polystyrene sphere as a model of the head adjacent to thoracic and pelvic body sections from a comme rcial humanoid phantom. RESULTS: On average, 18 minutes of equivalent time and five isocenters were required to achieve the prescribed dose coverage. The median prescribed dose was 18 Gy. For the lateral canthi , thyroid, sternum, and pelvis, the median doses were 24, 20, 21, and 4 cGy, respectively. Normalization to equivalent time and collimator s ize was superior to other techniques. Phantom measurements supported t he results from patient measurements and further refined estimates of component doses to extracranial sites. CONCLUSION: Doses to extracrani al sites ranged from 1.5% of the prescribed dose for the lateral canth i to 0.2% for the pelvis. Doses to the sternum and pelvis were proport ional to the duration of irradiation. Scatter radiation contributed mo re than 50% of the dose to the canthi and thyroid. Leakage radiation t ypically contributed 80 to 90% of the dose to the sternum and pelvis. Radiation during patient couch transit contributed little to the doses at the measured extracranial sites.