Dosimetric impact and detectability of multi-leaf collimator positioning errors on Varian Halcyon

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Accurate radiation delivery is crucial for effective cancer treatment.
• Multi-leaf collimator (MLC) positioning errors can compromise treatment plan integrity.
• Varian Halcyon linear accelerators are increasingly used for advanced radiotherapy techniques like VMAT.

Purpose of the Study:

• To assess the dosimetric impact of MLC positioning errors (random and systematic) on Varian Halcyon VMAT plans.
• To evaluate the efficacy of portal dosimetry in identifying clinically significant errors.
• To determine the correlation between portal dosimetry results and dose-volume histogram (DVH) metrics.

Main Methods:

• Introduced systematic and random MLC errors into 11 head and neck VMAT plans (99 total plans).
• Delivered plans on a Varian Halcyon and captured fluence using portal dosimetry.
• Compared DVH metrics of erroneous plans against baseline plans.

Main Results:

• Systematic MLC errors caused significant dosimetric changes, while random errors had a lesser impact.
• The magnitude of dosimetric changes correlated with the size of MLC errors.
• Portal dosimetry detected all systematic errors but struggled with random errors ≤±5mm.
• Following AAPM TG-218 recommendations optimized error detection.
• A moderate correlation was observed between normal tissue DVH metrics and portal dosimetry pass rates.

Conclusions:

• Portal dosimetry on the Varian Halcyon is a reliable tool for detecting MLC positioning errors in VMAT plans.
• Systematic MLC errors are more likely to cause clinically significant changes than random errors.
• Portal dosimetry QA can help prevent the delivery of inaccurate radiotherapy treatments.