Synthetic computed tomography based dose calculation in prostate cancer patients with hip prostheses for magnetic resonance imaging-only radiotherapy

BACKGROUND AND PURPOSE: Metallic hip prostheses cause substantial artefacts in both computed tomography (CT) and magnetic resonance (MR) images used in radiotherapy treatment planning (RTP) for prostate cancer patients. The aim of this study was to evaluate the dose calculation accuracy of a synthetic CT (sCT) generation workflow and the improvement in implant visibility using metal artefact reduction sequences. MATERIALS AND METHODS: The study included 23 patients with prostate cancer who had hip prostheses, of which 10 patients had bilateral hip implants. An in-house protocol was applied to create sCT images for dose calculation comparison. The study compared prostheses volumes and resulting avoidance sectors against planning target volume (PTV) dose uniformity and organs at risk (OAR) sparing. RESULTS: Median PTV dose difference between sCT and CT-based dose calculation among all patients was 0.1 % (−0.4 to 0.4%) (median(range)). Bladder and rectum differences (V(50Gy)) were 0.2 % (−0.3 to 1.1%) and 0.1 % (−0.9 to 0.5%). The median 3D local gamma pass rate for partial arc cases using a Dixon MR sequence was Γ(20%)(2mm/2%) = 99.9%. For the bilateral full arc cases, using a metal artefact reconstruction sequence, the pass rate was Γ(20%)(2mm/2%) = 99.0%. CONCLUSIONS: An in-house protocol for generating sCT images for dose calculation provided clinically feasible dose calculation accuracy for prostate cancer patients with hip implants. PTV median dose difference for uni- and bilateral patients with avoidance sectors remained <0.4%. The Outphase images enhanced implant visibility resulting in smaller avoidance sectors, better OAR sparing, and improved PTV uniformity.