SU-E-T-15: Small and Nonstandard Photon Field Dosimetry Characterization Using Monte Carlo Methods

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Accurate dosimetry is critical for effective radiotherapy.
• Small photon fields used in modern techniques like IMRT present unique calibration challenges.
• Existing calibration protocols may not fully address the complexities of small field dosimetry.

Purpose of the Study:

• To quantify uncertainty reduction in small photon field dosimetry.
• To characterize ionization chambers and calibration conditions using Monte Carlo methods.
• To benchmark simulations against NIST-traceable measurements.

Main Methods:

• Detailed EGSnrc Monte Carlo models were used to simulate phase space profiles.
• Simulations covered various ionization chambers (Farmer-type, micro, scanning) in static and dynamic fields.
• Calibration conditions included different field sizes, beam qualities (6 MV, Co-60), and phantom materials (water, acrylic).

Main Results:

• Simulated small field response values generally agreed with measured values within uncertainty.
• A database was created comparing proposed small field calibration conditions to Co-60 reference conditions.
• The database includes beam quality correction factors, tolerances, and dose calibration uncertainties for small fields.

Conclusions:

• Characterizing calibration conditions enhances understanding of applying Co-60 absorbed dose to water calibration coefficients to small fields.
• The developed methodology and database aid in reducing dose uncertainty for small and nonstandard field calibrations.
• This work supports future recommendations for small field calibration protocols in radiotherapy.