Dose homogeneity specification for reference dosimetry of nonstandard fields

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dose determination is critical in radiation therapy.
• Nonstandard radiation fields, such as those from Intensity-Modulated Radiation Therapy (IMRT), pose challenges for traditional dosimetry.
• Plan-class specific correction factors are used to improve dose accuracy in these complex fields.

Purpose of the Study:

• To investigate how dose distributions affect the plan-class specific correction factor in composite nonstandard field dosimetry.
• To evaluate the sensitivity of this correction factor to variations in dose delivery.

Main Methods:

• A PMMA phantom with a water-filled center was used to measure reference absorbed dose.
• Ten different TomoTherapy-based IMRT fields were created and their dose distributions analyzed.
• Plan-class specific correction factors were measured for Farmer-type chambers (Exradin A12, NE2571) and a smaller chamber (Exradin A1SL).

Main Results:

• Measurement uncertainties for the correction factor ranged from 0.3% to 0.8%.
• For target volume heterogeneity <5%, the correction factor was close to unity (within 0.7%).
• In more heterogeneous fields, Farmer-type chambers showed correction factors deviating up to 2.4% from unity, while the Exradin A1SL showed less deviation but higher variability due to positioning uncertainties.

Conclusions:

• A plan-class specific correction factor can be reliably determined based on plan evaluation parameters, particularly for Farmer-type chambers.
• This research offers a method for accurately selecting ionization chamber correction factors for nonstandard fields by quantifying dose distributions.