Clinical experience with EPID dosimetry for prostate IMRT pre-treatment dose verification

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Traditional pre-treatment dosimetry relied on film and ionization chambers, which can be time-consuming and less efficient.
• The integration of advanced imaging devices like amorphous silicon electronic portal imaging devices (a-Si EPID) offers potential for improved accuracy and efficiency in radiotherapy quality assurance.

Purpose of the Study:

• To demonstrate the replacement of film and ionization chamber measurements with a-Si EPID dosimetry for routine pre-treatment quality assurance in clinical practice.
• To illustrate the application of EPID dosimetry in identifying and resolving clinical issues related to Intensity-Modulated Radiation Therapy (IMRT) plan delivery.

Main Methods:

• IMRT prostate plans were delivered to a phantom, with transit images acquired using an a-Si EPID for each segment.
• An in-house back-projection algorithm reconstructed dose distributions, and segment dose images were summed for field comparisons using gamma analysis (3%/3 mm criteria).
• EPID measurements were compared against film and ionization chamber measurements for both field-by-field and isocenter dose verification.

Main Results:

• EPID and film measurements showed excellent agreement for 50 fields (gamma avg = 0.16, gamma max = 1.00, P gamma < 1 = 100%), leading to the discontinuation of film use.
• EPID and ionization chamber isocenter dose measurements agreed well (average ratio = 1.00), with both systematically underestimating the planned dose (average ratio = 0.99).
• Comparison of EPID mid-plane dose images with 3D dose grids revealed under-dosage in 7 plans (5-16%), attributed to segment overlap or junctions, which was resolved by adjusting treatment planning system parameters.

Conclusions:

• a-Si EPID dosimetry is a reliable and accurate replacement for film and ionization chambers in pre-treatment verification of IMRT plans.
• EPID dosimetry efficiently detects systematic errors, enabling adjustments to treatment planning system parameters and clinical plans, thereby enhancing treatment accuracy.
• The efficiency and accuracy of EPID dosimetry make it a valuable tool for routine IMRT quality assurance, requiring only one set of measurements per plan.