Gamma knife output factor measurements using VIP polymer gel dosimetry

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Dosimetry

Background:

• Accurate output factor measurement is crucial for stereotactic radiosurgery.
• Small field dosimetry presents unique challenges due to detector volume effects.
• Gamma Knife radiosurgery utilizes small radiation fields requiring precise characterization.

Purpose of the Study:

• To measure the output factors of the smallest treatment fields (4 mm and 8 mm collimators) of a Gamma Knife Model C.
• To evaluate the performance of water equivalent polymer gel dosimeters for small field dosimetry.
• To compare measured output factors with vendor-recommended values.

Main Methods:

• Water equivalent polymer gel dosimeters and magnetic resonance imaging (MRI) were used.
• Three VIP normoxic gel samples were irradiated with 4 mm, 8 mm, and a reference 18 mm field.
• A refined data processing methodology was developed to analyze 3D dose distributions with reduced uncertainty.

Main Results:

• Polymer gel measurements for the 8 mm collimator showed excellent agreement with vendor values (0.955 ± 0.007 vs. 0.956).
• For the 4 mm collimator, polymer gel results indicated an output factor 3% lower than the vendor's recommendation (0.841 ± 0.009 vs. 0.870).

Conclusions:

• Polymer gel dosimetry provides accurate measurements for small fields in Gamma Knife radiosurgery.
• Discrepancies with vendor values for the 4 mm field highlight the need for precise small field characterization.
• The developed dosimetry method is comparable to other systems with fine spatial resolution and minimal radiation field perturbation.