Quantification of intrafraction prostate motion and its dosimetric effect on VMAT

Area of Science:

• Radiation Oncology
• Medical Physics
• Image-Guided Therapy

Background:

• Intrafraction prostate motion during radiation therapy (RT) delivery reduces treatment accuracy.
• Existing metrics for quantifying prostate motion may not accurately reflect the dosimetric impact on patients.
• Volumetric Modulated Arc Therapy (VMAT) is a common RT technique for prostate cancer.

Purpose of the Study:

• To quantify intrafraction prostate motion using novel and existing metrics during VMAT.
• To evaluate the dosimetric impact of prostate motion on target coverage and organs at risk (OARs).
• To identify motion metrics that best correlate with dosimetric deviations and establish an action level threshold.

Main Methods:

• Quantified prostate motion in 18 patients (294 fractions) using various metrics during VMAT.
• Evaluated dosimetric impact via dose reconstructions and correlated metrics with PTV D95%, CTV D99%, and OARs (rectum, bladder).
• Used coefficient of determination (R ²) for correlation analysis and iterative regression to find action level thresholds.

Main Results:

• Average mean prostate motion was 1.5 mm (range 0.3-9.9 mm).
• Several motion metrics strongly correlated with PTV D95% (R ² range 0.43-0.81).
• The mean of the highest 50% of motion metric showed strong correlation with PTV D95% and an action level threshold of 3.0 mm was identified, indicating a 6.2% average PTV D95% reduction when exceeded.

Conclusions:

• Several motion metrics effectively predict the dosimetric impact of intrafraction prostate motion on PTV coverage during VMAT.
• The identified action level threshold of 3.0 mm for the mean of the highest 50% of motion can guide treatment course adaptation.
• These findings support the use of specific motion metrics for real-time monitoring and adaptive radiotherapy in prostate cancer treatment.