Determination of the absorbed dose rate to water for the 18-mm helmet of a gamma knife.
OBJECTIVE
To measure the absorbed dose rate to water of (60)Co gamma rays of a Gamma Knife Model C using water-filled phantoms (WFP).


METHODS
Spherical WFP with an equivalent water depth of 5, 7, 8, and 9 cm were constructed. The dose rates at the center of an 18-mm helmet were measured in an 8-cm WFP (WFP-3) and two plastic phantoms. Two independent measurement systems were used: one was calibrated to an air kerma (Set I) and the other was calibrated to the absorbed dose to water (Set II). The dose rates of WFP-3 and the plastic phantoms were converted to dose rates for an 8-cm water depth using the attenuation coefficient and the equivalent water depths.


RESULTS
The dose rate measured at the center of WFP-3 using Set II was 2.2% and 1.0% higher than dose rates measured at the center of the two plastic phantoms. The measured effective attenuation coefficient of Gamma Knife photon beam in WFPs was 0.0621 cm(-1). After attenuation correction, the difference between the dose rate at an 8-cm water depth measured in WFP-3 and dose rates in the plastic phantoms was smaller than the uncertainty of the measurements.


CONCLUSIONS
Systematic errors related to the characteristics of the phantom materials in the dose rate measurement of a Gamma Knife need to be corrected for. Correction of the dose rate using an equivalent water depth and attenuation provided results that were more consistent.
Chung
Hyun-Tai
HT
Department of Neurosurgery, Seoul National University College of Medicine, Seoul, Korea.
Park
Youngho
Y
Hyun
Sangil
S
Choi
Yongsoo
Y
Kim
Gi Hong
GH
Kim
Dong Gyu
DG
Chun
Kook Jin
KJ
D000042:Absorption; D000388:Air; D000465:Algorithms; D002138:Calibration / Q000592:standards; D003037:Cobalt Radioisotopes / Q000627:therapeutic use; D004867:Equipment Design / Q000592:standards; D005720:Gamma Rays; D006260:Head Protective Devices / Q000592:standards; D008953:Models, Anatomic; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D010969:Plastics; D055618:Radiation Equipment and Supplies / Q000592:standards; D011874:Radiometry / Q000379:methods; D016634:Radiosurgery / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D012015:Reference Standards; D035501:Uncertainty; D014867:Water
D003037:Cobalt Radioisotopes; D010969:Plastics; D014867:Water