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

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiosurgery

Background:

• Single-isocenter multi-target (SIMT) radiosurgery improves efficiency but is sensitive to patient positioning errors, especially rotations.
• Rotational deviations can significantly reduce target dose coverage, influenced by target size, rotation magnitude, and distance from the isocenter.
• A lack of robust mathematical frameworks hinders precise prediction of these dosimetric impacts.

Purpose of the Study:

• To develop and apply mathematical models to quantify rotational error effects on target coverage in SIMT radiosurgery.
• To elucidate the interplay between target size, isocenter distance, and rotational error magnitude.
• To inform individualized planning target volume (PTV) margin selection for optimized coverage and reduced normal tissue irradiation.

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 diameter, PTV margins, and isocenter distance.
• Applied methodology to clinical cases for deriving customized margins.

Main Results:

• Small targets farther from the isocenter are most vulnerable to coverage loss from rotational errors, particularly with small or no PTV margins.
• A 0.5 cm target experienced <95% coverage with a 0.5° rotation at 1.5 cm from the isocenter without a margin.
• A 1.0 mm uniform PTV margin generally ensured >95% GTV coverage for 1° rotations across evaluated parameters.
• Differential margins derived from the model reduced treated volume and brain V12Gy in a clinical case compared to uniform margins.

Conclusions:

• Rotational errors significantly impact target coverage based on target size and location.
• Individualized PTV margin selection is critical for robust target coverage in SIMT radiosurgery.
• The developed analysis enables clinicians to determine optimal, customized margins, enhancing treatment precision and minimizing healthy tissue exposure.