The population percentile allowance method for determining systematic spatial error tolerances for temporary...

Area of Science:

• Medical Physics
• Radiation Oncology
• Biomedical Engineering

Background:

• Temporary intensity modulated brachytherapy (IMBT) using partially shielded applicators is sensitive to spatial uncertainties in applicator position and shield orientation.
• Existing sensitivity analyses lack a generalized framework for rigorously determining spatial uncertainty tolerances for dose-volume parameters in IMBT.
• Conventional high-dose-rate brachytherapy (HDR-BT) dose distributions are primarily sensitive to applicator position, unlike emerging IMBT techniques.

Purpose of the Study:

• To derive and present the Population Percentile Allowance (PPA) method, a generalized mathematical and statistical framework.
• To evaluate the tolerance of temporary IMBT approaches to spatial uncertainties in applicator position and shield orientation.
• To establish a method for defining acceptable dose-volume errors within a patient population undergoing IMBT.

Main Methods:

• Developed a mathematical formalism for geometric applicator position and shield orientation shifts applicable to various brachytherapy techniques (RSBT, DMBT).
• Defined the PPA method by specifying the percentage of patients allowed to receive defined dose-volume errors.
• Applied the PPA method to multi-shield helical 169Yb-based RSBT for cervical cancer in 37 patients, assessing uncertainties against dose-volume requirements.

Main Results:

• For 90% of patients to have ≤10% dose-volume uncertainty (HR-CTV D90, organs at risk D2cc), tolerances were ~±1.0 mm for applicator position and ~±4.25° for shield orientation.
• With ±1.5 mm applicator position and ±5° shield orientation tolerances, 90% of patients experienced ≤12.8% maximum dose-volume uncertainty.
• The study quantified the relationship between systematic spatial uncertainties and resulting dose-volume errors in a patient cohort.

Conclusions:

• The PPA method provides a rigorous framework for determining spatial uncertainty tolerances in temporary IMBT.
• Tolerances were determined for multi-shield helical 169Yb-based RSBT for cervical cancer, applicable to treatment fraction design.
• The PPA method is versatile for various temporary IMBT approaches and can guide the design of delivery system components.