Dosimetric Characteristics of the Penumbra Regions for Conventional and MLC Fields using a SLIC-EPID

The characteristics of radiation fields set up using conventional collimators and multileaf collimators (MLCs) for a range of rectangular and rounded fields were investigated. The transmitted dose maps measured using a scanning liquid filled ionization chamber electronic portal imaging device (SLIC-EPID) was used to evaluate the size of radiation field, and the characteristics of penumbra region, including penumbra width (80 – 20 % isodose curves) as well as the undulation of isodose curves. Results showed that the radiation field sizes set up using MLCs are generally larger than those set up using conventional collimators. A significant difference was also observed between the penumbra width for radiation fields set up using conventional collimators and MLCs. Due to the beam divergence the penumbra width varies with the increase of radiation field size. A low dose deposition (approximately 1.5% of central dose) was also detected in the outside of the Region Of Interest (ROI) for radiation fields created using MLCs. Due to the “staircase” shape of semi-circular fields, compared to the rectangular fields, a significant increase of penumbra (approximately 1 cm) was observed from the minimum to the maximum width of penumbra. SLIC-EPID was found to be a sensitive device to evaluate the characteristics of the radiation fields created with MLCs. It can be used to detect the variation of radiation field sizes as well as to evaluate the characteristics of penumbra region, including the width of penumbra and the undulation of isodose curves.

Dosimetric Characteristics of the Penumbra Regions for Conventional and MLC Fields using a SLIC-EPID Keywords:

SLIC-EPID , Multileaf Collimator , transmitted Dosimetry