Extracranial radiation doses in patients undergoing gamma knife radiosurgery

To determine extracranial doses in patients undergoing gamma knife radiosurgery and identify component sources of the extracranial doses using phantom measurements.

The lateral canthi, thyroid, sternum, and midpelvis region were monitored in 104 unselected patients during their gamma knife treatments using thermoluminescent dosimetry. Measured doses were normalized to integral dose, equivalent time (which is defined in relation to the activity of the cobalt-60 sources), and collimator size to correlate radiation doses with these parameters. A phantom was constructed from a polystyrene sphere as a model of the head adjacent to thoracic and pelvic body sections from a commercial humanoid phantom.

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 size was superior to other techniques. Phantom measurements supported the results from patient measurements and further refined estimates of component doses to extracranial sites.

Doses to extracranial sites ranged from 1.5% of the prescribed dose for the lateral canthi to 0.2% for the pelvis. Doses to the sternum and pelvis were proportional to the duration of irradiation. Scatter radiation contributed more than 50% of the dose to the canthi and thyroid. Leakage radiation typically 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.