A Mathematical Framework for Determining the Effect of Rotational Errors on Single-Isocenter Multi-target...

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Single-isocenter multi-target (SIMT) radiosurgery improves efficiency but is sensitive to patient positioning errors, especially rotation.
• Rotational deviations can severely compromise target dose coverage, influenced by target size, rotation magnitude, and distance from the isocenter.

Purpose of the Study:

• To develop and apply mathematical models to quantify the impact of rotational errors on target coverage in SIMT radiosurgery.
• To elucidate the interplay between target characteristics and rotational errors for optimized planning target volume (PTV) margin selection.

Main Methods:

• Developed mathematical models simulating dosimetric consequences of rotational errors.
• Modeled targets and radiation fields as 3D spheres to calculate intersection volumes.
• Generated graphical representations of Gross Tumor Volume (GTV) coverage versus rotational error, target size, PTV margins, and distance from isocenter.

Main Results:

• Small targets far from the isocenter are most vulnerable to coverage loss from rotation, particularly without PTV margins.
• A 0.5 cm target showed <95% coverage with 0.5° rotation at 1.5 cm from the isocenter without margins.
• A 1.0 mm uniform margin generally ensured >95% GTV coverage for 1° rotations across evaluated parameters.
• Customized margins reduced treated volume and brain V12Gy compared to uniform margins in a clinical case.

Conclusions:

• Rotation's impact on target coverage depends critically on target size and location.
• Individualized PTV margin selection is essential for robust target coverage in SIMT.
• Mathematical modeling enables determination of optimal, customized margins to enhance treatment precision and minimize normal tissue irradiation.