Microionization chamber for reference dosimetry in IMRT verification: clinical implications on OAR dosimetric errors.

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• Intensity modulated radiotherapy (IMRT) uses small beamlets, necessitating small ionization chambers (ICs) for dose verification.
• Verification measurements often occur under non-reference conditions, differing from standard clinical beam calibration.
• Accurate dose measurement is critical for organs at risk (OARs) in IMRT.

Purpose of the Study:

• To estimate the dose error when using a micro ionization chamber (microIC) for absolute OAR dose measurement in IMRT.
• To evaluate the impact of non-reference conditions on microIC dosimetry for IMRT.
• To determine the clinical significance of potential dose errors in OARs during IMRT.

Main Methods:

• Utilized Monte Carlo (MC) simulations for detailed geometrical modeling of the microIC and dose verification setup.
• Calculated the dose ratio D(water)/D(air) as an MC estimator for the correction factor under non-reference conditions.
• Evaluated dose errors in two clinical IMRT cases, focusing on microIC placement within OARs.

Main Results:

• The D(water)/D(air) dose ratio was calculated for microICs in OARs for both clinical cases.
• The study quantified the dose error arising from assuming dosimetric parameters are the same under reference and non-reference conditions.
• Monte Carlo simulations provided a precise estimation of absorbed dose to water.

Conclusions:

• The dose error associated with microIC measurements in OARs for IMRT was estimated.
• The clinical impact of the calculated dose error was found to be negligible for the studied IMRT treatments.
• This study validates the use of microICs for OAR dose verification in IMRT, confirming minimal impact on treatment accuracy.