Management of hip prosthesis in radiotherapy
For patients with metallic hip prosthesis, treatment planning for tumours in the pelvic region (prostate, bladder, rectum, and anus) has to consider the prosthesis material as well as its position and form, if photon beams are administered through the prosthesis. However, knowledge about the prosthesis material may be missing, or the actual geometry may deviate from the assumed one, so that serious under- or over-dosage of the tumour, or an increase in dose in organs at risk, may occur. In order to avoid this risk, it is recommended to design a treatment technique using fields which are not administered through the prosthesis. If such a solution is not possible, then a good estimate has to be made of the influence of the prosthesis on the resulting dose distribution. When the prosthesis is bilateral, treatment planning is further complicated because only a limited number of beam angles can be used to avoid the prostheses. IMRT using either coplanar or non-coplanar beams may produce favorable results in the treatment of patients with bilateral prostheses.
Treatment planning systems currently available are not always able to predict accurately the dose distribution around metallic prosthesis. Firstly, when implanted objects of high atomic number (Z) material are present, severe image artefacts are generated in conventional CT, strongly hindering the ability to delineate some organs. Megavoltage imaging is therefore often used to complement CT information and will show, for instance, if a prosthesis is solid or hollow, and can also be used for exit dose measurements. Furthermore, the presence of high-Z material in a photon beam causes a dose enhancement and reduction at the proximal and distal prosthesis interfaces, respectively. Only Monte Carlo simulation has the ability to calculate precisely the impact of a hip prosthesis on the dose distribution, whereas superposition and pencil beam algorithms do not. However, further from the prosthesis, the difference between these dose calculation algorithms diminishes.
Introduction to References
The AAPM Report
is intended to reflect the scientific understanding and technical methodology in clinical dosimetry for radiation oncology patients with high-Z hip prostheses. Its purpose is to make the radiation oncology community aware of the problems arising from the presence of these devices in the radiation beam, to quantify the dose perturbations they cause, and to provide recommendations for treatment planning and delivery.