The effect of a metal hip prosthesis on the radiation dose in therapeutic photon beam irradiations.

Prostate and cervical cancer patients are often treated with external X-ray beams of bi-lateral incidence. Such treatment may incur some dose effect that cannot be predicted precisely in commercial treatment planning systems (TPS) for patients having undergone total hip replacement. This study performs a Monte Carlo (MC) simulation and an analytical calculation (convolution superposition algorithm which is implemented in ADAC TPS) of a 6 MV, 5 x 5 cm2 X-ray beam incident into water with the existence of hip prosthesis, e.g. Ti6A14V and CoCrMo alloy. The results indicate that ADAC TPS cannot precisely account for the scatter and backscatter radiation that a metal hip prosthesis causes. For percent depth dose (PDD) curves, the maximum underdosage of ADAC TPS up to 5mm above the interface between dense material and water is 5%, 20% and 27% for PDD(Bone), PDD(Ti) and PDD(Co), respectively. The dose re-buildup, which occurs behind the hip region, becomes more and more obvious for denser medium existed in water. Increasing inhomogeneity also enhances the underdosage of ADAC for greater depth (> 10cm), as the figures of nearly 2% in PDD(Bone), PDD(Ti) and 4-5% in PDD(Co) reveal. Overestimating the attenuated power of high-density non-water material in ADAC TPS causes this underdosage. For dose profiles, no significant differences were found in Profile(Bone) at any depth. Profile(Ti) reveals that MC slightly exceeds ADAC at off-axis position 1.0-2.0 cm. Profile(Co) reveals this more obviously. This finding means that scatter radiation from these denser materials is significant and cannot be predicted precisely in ADAC.