Dose attenuation effect of hip prostheses in a 9-MV photon beam: commercial treatment planning system versus Monte Carlo calculations.
OBJECTIVE
The purpose of this study was to investigate the dosimetric effect of various hip prostheses on pelvis lateral fields treated by a 9-MV photon beam using Monte Carlo (MC) and effective path-length (EPL) methods.


METHODS
The head of the Neptun 10 pc linac was simulated using the MCNP4C MC code. The accuracy of the MC model was evaluated using measured dosimetric features including depth dose values and dose profiles in a water phantom. The Alfard treatment planning system (TPS) was used for EPL calculations. A virtual water phantom with dimensions of 30 x 30 x 30 cm(3) and a cube with dimensions of 4 x 4 x 4 cm(3) made of various metals centered in 12 cm depth was used for MC and EPL calculations. Various materials including titanium, Co-Cr-Mo, and steel alloys were used as hip prostheses.


RESULTS
Our results showed significant attenuation in absorbed dose for points after and inside the prostheses. Attenuations of 32%, 54% and 55% were seen for titanium, Co-Cr-Mo, and steel alloys, respectively, at a distance of 5 cm from the prosthesis. Considerable dose increase (up to 18%) was found at the water-prosthesis interface due to back-scattered electrons using the MC method. The results of EPL calculations for the titanium implant were comparable to the MC calculations. This method, however, was not able to predict the interface effect or calculate accurately the absorbed dose in the presence of the Co-Cr-Mo and steel prostheses.


CONCLUSIONS
The dose perturbation effect of hip prostheses is significant and cannot be predicted accurately by the EPL method for Co-Cr-Mo or steel prostheses. The use of MC-based TPS is recommended for treatments requiring fields passing through hip prostheses.
Mesbahi
Asghar
A
Medical Physics Department, Medical School, Tabriz University of Medical Sciences, Tabriz, Iran. asgharmesbahi@yahoo.com
Nejad
Farshad Seyed
FS
D000465:Algorithms; D003198:Computer Simulation; D006622:Hip Prosthesis; D006801:Humans; D008670:Metals; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D017785:Photons; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011882:Radiotherapy, High-Energy
D008670:Metals