Room: Exhibit Hall
Purpose: Patients with bilateral hip implants are generally treated with beams avoiding entrance through the hip implants which makes it difficult to achieve acceptable dose constraints for OARs and dose conformity for the target. Treatment of such patients in a MR-linac would provide a bi-fold advantage with improved imaging for soft tissue (MRI) and high density implants (EPID) as well as improved dosimetry. This study was designed to assess the dosimetric implications of treating a patient with a bilateral hip implant in a 7 MV MR-linac (Elekta AB,Stockholm,Sweden).
Methods: A patient with a bilateral titanium hip implant was chosen for hypo-fractionated SBRT treatment of the prostate with a prescription of 35Gy in 5 fractions. The electron density of the metal implant was overridden in the Monaco TPS (Elekta, research version 5.19.03). Concentric rings with 2, 4, and 8 mm radial distances from the hip implants were created to record D0.1cc to the bone surrounding the implant. Plans were optimized both with and without the presence of the magnetic field using a conventional beam arrangement, avoiding the prosthesis, as well as an alternative beam arrangement with two beams going through the prosthesis.
Results: For the plans optimized using beams entering through the prosthesis, the D0.1cc was 15.3, 15.5, and 16.2Gy at 2, 4 and 8 mm radial distances from the implant, respectively. The D0.1cc values were similar for plans optimized with and without the presence of the magnetic field. The conformity index (CI) for the conventional versus alternative beam arrangements were 1.21 versus 1.04.
Conclusion: The maximum dose to the bone (D0.1cc) surrounding the prosthetic implant, for the alternative beam arrangement, was recorded to be well within the published dose limit (30Gy for a 5-fraction regimen) for bone necrosis. A better conformity index was achieved using the alternative beam arrangement.