Development of a PMMA phantom as a practical alternative for quality control of gamma knife® dosimetry.

BACKGROUND: To measure the absorbed dose rate to water and penumbra of a Gamma Knife® (GK) using a polymethyl metacrylate (PMMA) phantom. METHODS: A multi-purpose PMMA phantom was developed to measure the absorbed dose rate to water and the dose distribution of a GK. The phantom consists of a hemispherical outer phantom, one exchangeable cylindrical chamber-hosting inner phantom, and two film-hosting inner phantoms. The radius of the phantom was determined considering the electron density of the PMMA such that it corresponds to 8 g/cm2 water depth, which is the reference depth of the absorbed dose measurement of GK. The absorbed dose rate to water was measured with a PTW TN31010 chamber, and the dose distributions were measured with radiochromic films at the calibration center of a patient positioning system of a GK Perfexion. A spherical water-filled phantom with the same water equivalent depth was constructed as a reference phantom. The dose rate to water and dose distributions at the center of a circular field delimited by a 16-mm collimator were measured with the PMMA phantom at six GK Perfexion sites. RESULTS: The radius of the PMMA phantom was determined to be 6.93 cm, corresponding to equivalent water depth of 8 g/cm2. The absorbed dose rate to water was measured with the PMMA phantom, the spherical water-filled phantom and a commercial solid water phantom. The measured dose rate with the PMMA phantom was 1.2% and 1.8% higher than those measured with the spherical water-filled phantom and the solid water phantom, respectively. These differences can be explained by the scattered photon contribution of PMMA off incoming 60Co gamma-rays to the dose rate. The average full width half maximum and penumbra values measured with the PMMA phantom showed reasonable agreement with two calculated values, one at the center of the PMMA phantom (LGP6.93) and other at the center of a water sphere with a radius of 8 cm (LGP8.0) given by Leksell Gamma Plan using the TMR10 algorithm. CONCLUSIONS: A PMMA phantom constructed in this study to measure the absorbed dose rates to water and dose distributions of a GK represents an acceptable and practical alternative for GK dosimetry considering its cost-effectiveness and ease of handling.

Influence of phantom thickness and material on the backscatter factors for diagnostic x-ray beam dosimetry.

Most of the existing backscatter factors for the dosimetry of clinical diagnostic x-ray beams have been calculated for 15 cm thick phantoms; these data are used for skin dose determinations which in general ignore the influence of phantom material and thickness. The former should strictly be required whenever dosimetry measurements are made on phantom materials different from those used for the backscatter factor calculations. The phantom or patient thickness is of special importance when skin dose determinations are made for infants or paediatric patients. In this work, the recently published formalism for reference dosimetry and comprehensive database of backscatter factors for clinical beams and water phantoms have been extended using two correction factors which account for phantom material and thickness. These were determined with simulations using the PENELOPE Monte Carlo system, for PMMA to analyse the influence of the phantom material relative to water, and for a broad range of thicknesses of water and PMMA to investigate the role of this parameter in patient dose estimates. The material correction factor was found to be in the range 3-10%, depending on the field size and the HVL. The thickness correction factor was in the range 2-12% for a 5 cm thick phantom and square field sizes between 5 and 35 cm, reaching a plateau of about ±1% for thicknesses beyond 13 cm. Expressions in the form of surface fits over the calculated data are provided which streamline the determination of backscatter factors for arbitrary thicknesses and phantom materials, as well as field sizes. Results demonstrate the inadequacy of using conventional backscatter factors (calculated for 15 cm thick phantoms) without correction factors that take into account the phantom material and its thickness.