AbstractID: 9135 Title: Dose perturbations due to contrast medium in Mammosite:
Dosimetric investigation of dose calculation accuracy
With the emergence of Mammosite high dose rate brachytherapy (HDR) as an alternative to
interstitial brachytherapy or external beam radiation therapy for early breast cancer, its
dosimetric accuracy needs to be verified. For imaging purposes, the Mammosite balloon is
filled with saline and radio-opaque contrast (max 25 % by volume). Due to the high-Z
medium and low energy of Ir-192 γ rays, the dosimetry on Mammosite may not be accounted
for accurately since inhomogeneity corrections are not available in most brachytherapy
planning programs. Of special interest is the dose perturbation, which is defined as dose with
radio-opaque medium in balloon to dose with saline only at the same point. Experimental
verification of the dosimetry in a Mammosite balloon was conducted with a small volume
parallel plate ion chamber placed perpendicular to the balloon surface and source axis.
Measurements were taken with various concentration of Hypaque (5-25%) for the same
balloon diameter (42 mm). Dose calculations were also performed using the Nucletron Plato
BPS system. Our results show that BPS predicts about 9% higher dose near the balloon
surface compared to measured values. Dose falloff with distance is a smooth exponential
function. The Hypaque concentration produces dose perturbation due to attenuation of the
radiation, up to 6%. Interface effect occurs at distances <1mm and hence the balloon wall
thickness safeguards this effect. Another issue that needs to be addressed is the dose
degradation along the source axis due to self-absorption.