An estimate of the margin required when defining blocks around the prostate

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Quality Assurance

Background:

• Accurate patient positioning is critical in radiotherapy for effective cancer treatment.
• Variability in patient setup can lead to under- or over-dosing of the target volume.
• Quantifying uncertainties is essential for optimizing treatment margins.

Purpose of the Study:

• To evaluate systematic and random uncertainties in patient setup during radiotherapy for prostate cancer.
• To assess errors in radiation block production.
• To propose a method for combining uncertainties and determining optimal block margins.

Main Methods:

• Comparison of portal films with simulation films for 54 prostate cancer patients.
• Calculation of systematic and random uncertainties in the superior-inferior (UP), right-left (RT), and anterior-posterior (IN) directions.
• Analysis of field area discrepancies to determine block production errors.
• Development and application of a method to combine identified uncertainties.

Main Results:

• Systematic and random setup uncertainties were quantified (e.g., 1.6 mm UP, 0.3 mm RT, 1.3 mm IN).
• Block production error was found to be -0.7 mm (SD = 0.9 mm).
• Proposed method provides estimates for non-uniform block margins ranging from 13 mm to 21 mm.

Conclusions:

• Identified five sources of uncertainty in radiotherapy (three pre-treatment, two during treatment).
• The proposed method allows for the calculation of margins to ensure 95% target dose coverage in 95% of treatments.
• Non-uniform block margins are necessary to account for varying uncertainties in different directions.