Determination of dose distributions and dose rates in the gamma knife.
OBJECTIVE
To measure the dose parameters of the Leksell gamma knife unit and compare them with the treatment planning data.


METHODS
The dose distributions taken from treatment planning computer were confirmed with film dosimetry using a spherical uniform density polystyrene phantom. The dose rate measurements were made by using a 0.125 cc ion chamber and the spherical phantom for 4, 8, 14 and 18 mm helmets. The dose rate measurements were compared with the estimates given by the manufacturer.


RESULTS
The dose distributions obtained by film dosimetry were in agreement with those taken from the treatment planning computer only for the 70% and 50% isodose line. The results of dose rate measurement showed that the 0.125 cc ion chamber was suitable only for 18 and 14 mm helmets. Conversely, it was not suitable for 8 and 4 mm helmets.


CONCLUSIONS
This study showed that both computergenerated treatment plans and film densitometry estimates provided similar dose distributions without special beam blocking.
Isbakan
Z
Z
Department of Radiation Oncology, Marmara University, Istanbul, Turkey.
Bilge
H
H
Senkesen
O
O
Kucucuk
H
H
Sengoz
M
M