Monte Carlo Based Assessment of Matrixx Array as a QA Tool for HDR
Mandar S Bhagwat, Desmond A O'Farrell, Robert A Cormack*, Dana Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MASU-E-T-76 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Verification of HDR dose delivered from surface applicators is challenging because there is a lack of backscatter and most brachytherapy TPS dose calculations do not accurately model such situations. IBA Matrixx™ provides a means to measure HDR dose, but brachytherapy sources are outside the manufacturers energy specification. This work investigates the use of an ion chamber array for HDR treatment verification.
A Harrison-Mick (HAM) applicator in conjunction with an Ir192 afterloader was used to deliver a superficial plan with 9 x 9 dwells on a grid separated by 1cm, normalized to 100cGy a depth of 3mm in skin. The Matrixx™ array was used to measure the dose. Backscatter contribution to the measured dose was changed by varying layers of solid water on the HAM applicator.
The brachytherapy module of EGSnrc was used to simulate this experimental setup. The microselectron V2 Ir192 source was carefully modeled following published literature. Rayleigh scattering, bound Compton scattering, photo-electric absorption were all simulated.
Taking the HAM applicators inherent backscatter of about 0.5cm as reference, the dosimetric contribution saturates around 6cm. An absence of backscatter material reduces the dose at 3mm into skin by about 3.5 - 4%. Dose from Monte Carlo simulation compares favorably with Matrixx™ measurements; beyond 5cm of backscatter the increase in measured dose is less than 0.5% and our results from Monte Carlo simulations reflect this.
A maximum difference of 0.5% between measured and simulated doses for different amounts of backscatter material indicates that the Matrixx™ ion chamber array, even when used in the kV energy domain of the HDR source, can be a satisfactory clinical QA device for checking planar dose distributions.