Poster — Wed Eve—48: An Opposed Matched Beam IMRT Technique for Prostate Cancer Patients with Bilateral Hip Prosthesis

When intensity modulated radiation therapy (IMRT) is used to treat prostate patients with bilateral metal hip implants, beams which would otherwise be oriented in a lateral direction must be angled toward a posterior direction in order to avoid incidence through the prostheses. This has the undesirable effect of increasing the volume of rectum subtended by these beam, thereby either increasing rectal dose or reducing target coverage. We report a new technique which exploits the use of opposed fields that are asymmetric and matched along a plane that is tangent to the most anterior aspect of the implant on one side and the most posterior aspect of the implant on the other. This geometry allows the posterior beams to be aligned more closely to the lateral directions since the asymmetric beam subtends only the posterior portion of the target. The remaining anterior portion of the target is subtended by the opposed asymmetric beam. Neither beam of the opposed matched pair is incident through an ispilateral prosthesis. IMRT plans were developed for a representative patient using both the conventional and the opposed matched techniques. Analysis of the dose volume histogram indicates better rectal sparing and target coverage by the opposed matched technique. Calculation and measurement of dose at a point along the match line indicates sensitivity to misalignment of 2.5% per mm per beam. Thus, care must be taken to ascertain opposed matched beam alignment and to minimize the time between treating each beam of the opposed matched pair.