On the interplay effect for moving targets treated with the CyberKnife static tracking system

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• The CyberKnife system uses static tracking for moving targets, but interplay effects between target motion and beam delivery can impact treatment accuracy.
• Understanding these interplay effects is crucial for optimizing treatment plans and ensuring effective dose delivery.

Purpose of the Study:

• To evaluate the interplay effect between different planned Monitor Unit (MU) distributions and respiratory patterns in CyberKnife treatments using static tracking.
• To assess how varying MU per beam and respiratory motion amplitude/frequency affect Gross Tumor Volume (GTV) coverage and dose conformity.

Main Methods:

• Simulated small (SRM) and large (LRM) respiratory motions in a dynamic phantom.
• Evaluated interplay effects for three MU plans (small, medium, large) and three beam-on phases for each plan.
• Measured dose distributions using radiochromic films in GTV axial and sagittal planes.

Main Results:

• Small-amplitude respiratory motion (SRM) plans generally yielded higher GTV coverage (95.2%-99.7%) and were less sensitive to beam-on phase variations compared to LRM plans (85.9%-99.8%).
• Maximum GTV coverage was achieved with large MU plans for SRM and small MU plans for LRM. Minimum coverage occurred with medium MU plans for both motion types.
• Increasing MU range decreased dose conformity for SRM but reduced inter-fraction variation for both SRM and LRM, particularly with large MU plans.

Conclusions:

• The interplay effect between target motion and beam irradiation time is confirmed in CyberKnife static tracking.
• Plans utilizing a large number of MU per beam improved GTV coverage for small motion amplitudes and reduced inter-fraction dose variations for large motion amplitudes.