Target and peripheral dose during patient repositioning with the Gamma Knife automatic positioning system (APS)...

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The GammaPlan treatment planning system does not account for leakage and scatter dose during Automatic Positioning System (APS) repositioning.
• Accurate dose delivery is critical in Gamma Knife radiosurgery, especially when treating targets near critical structures.
• Understanding dose contributions from system movements is essential for refining treatment planning.

Purpose of the Study:

• To measure the dose delivered to the target and its periphery during APS repositioning for the Leksell Gamma Knife model 4C.
• To quantify dose from defocus stage and intershot couch transit.
• To assess the impact of repositioning frequency and collimator size on delivered dose.

Main Methods:

• A Leksell 16 cm spherical phantom with a central ion chamber was used.
• CT images were acquired and registered in GammaPlan to define the target.
• Absorbed dose measurements were performed for 8, 14, and 18 mm collimator helmets, with couch pauses for defocus dose assessment.

Main Results:

• Measured dose increases with the frequency of APS repositioning and with larger collimator helmet sizes.
• During couch transit, the target receives more dose than peripheral regions; in the defocus position, the highest dose is superior to the target.
• Additional dose to the target site ranged from 3.87% to 5.71%, with peripheral dose varying by location.

Conclusions:

• The Automatic Positioning System (APS) in the Leksell Gamma Knife model 4C contributes additional, un-accounted dose during treatment.
• This additional dose, particularly in the defocus position, may be significant for treatments near critical brain structures.
• Further characterization and integration of these doses into treatment planning algorithms are needed to improve plan accuracy.