Biologic and physical fractionation effects of random geometric errors

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Delivery Systems

Background:

• Geometric errors and patient motion are critical factors influencing radiotherapy accuracy.
• Modeling these uncertainties is essential for predicting treatment outcomes and optimizing dose delivery.
• Understanding fractionation effects, including biological and physical components, is key to accurate dose modeling.

Purpose of the Study:

• To develop and validate a system for modeling the impact of random and systematic geometric errors in radiotherapy.
• To investigate the biological and physical fractionation effects of geometric errors and respiratory motion.
• To compare the resulting dose distributions with Gaussian blurring of the planned dose.

Main Methods:

• Simulated random geometric errors from a normal distribution, optionally combined with cranio-caudal respiratory motion.
• Conversion of physical dose to biologically effective dose using the linear-quadratic model, incorporating repopulation.
• Quantification of dose distribution differences by analyzing the distance between selected isodose levels.

Main Results:

• Fractionation with random errors resulted in a slight widening of the total dose distribution (0.4 mm for specific parameters).
• Respiratory motion introduced asymmetric deviations in dose distribution without additional biological widening.
• Negligible asymmetry was observed for respiration amplitudes ≤ 1 cm; larger amplitudes caused significant asymmetric isodose shifts.

Conclusions:

• Gaussian blurring is a valid approximation for random errors in fractionated radiotherapy.
• Significant respiratory motion (>1 cm amplitude) necessitates asymmetric margins due to asymmetric isodose line shifts.
• The study highlights the importance of accounting for motion asymmetry in radiotherapy planning and delivery.