SU-E-T-113: Volume Effect Correction Factor KV for Small-Field Photon Dosimetry with Ionization Chambers

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiotherapy Physics

Background:

• Ionization chambers exhibit a volume effect, causing spatial averaging in dose measurements.
• This averaging can be modeled as a convolution of the true dose profile with the detector's Gaussian response function.

Purpose of the Study:

• To derive a volume effect correction factor (kV) for ionization chambers.
• To improve the accuracy of output factor measurements, particularly for narrow photon beams.

Main Methods:

• A sixth-degree polynomial was fitted to the true dose profile (D(x)).
• The measured dose profile (M(x)) was calculated via convolution with a Gaussian function.
• The correction factor kV was derived as a function of polynomial coefficients and Gaussian standard deviation (s).

Main Results:

• The correction factor kV was calculated for lateral (kV,lat) and longitudinal (kV,long) directions for common ionization chambers.
• kV,lat fell below 1.01 for field widths > 2 cm.
• kV,long fell below 1.01 for field widths > 4 cm.

Conclusions:

• A method to correct for ionization chamber volume effects at dose maxima was developed.
• This correction is crucial for accurate output factor measurements in narrow photon beam calibration.