Correlation between gamma passing rate and complexity of IMRT plan due to MLC position errors.

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Planning

Background:

• Intensity-modulated radiation therapy (IMRT) is a sophisticated radiotherapy technique.
• Multi-leaf collimator (MLC) position errors can compromise dose delivery accuracy in IMRT.
• Plan complexity is a factor that may influence the robustness of IMRT plans to delivery errors.

Purpose of the Study:

• To evaluate the correlation between IMRT plan complexity and susceptibility to MLC position errors.
• To determine if plan complexity quantitatively predicts the impact of MLC errors on the gamma passing rate.

Main Methods:

• Generated IMRT plans for 20 patients using sliding window and step-and-shoot techniques.
• Calculated Modulation Complexity Score (MCS) and introduced MLC errors (0.3-1 mm).
• Performed 3D gamma analysis (2%/2 mm criteria) to assess dose distribution accuracy and calculated gamma passing rate gradients.

Main Results:

• A strong linear correlation was found between plan complexity and the susceptibility to MLC leaf bank errors (R²=0.81-0.82).
• More complex IMRT plans showed a greater decrease in gamma passing rate with increasing MLC errors.
• Correlation of re-normalized gamma passing rate and complexity also showed a strong positive relationship (r > 0.75).

Conclusions:

• IMRT plan complexity is a significant predictor of susceptibility to MLC position errors.
• Dosimetric robustness varies with plan complexity when subjected to MLC errors.
• The observed susceptibility to MLC errors is independent of the IMRT delivery technique used (sliding window vs. step-and-shoot).