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  • Title: Applying a commercial atlas-based synthetic Computed Tomography algorithm to patients with hip prostheses for prostate Magnetic Resonance-only radiotherapy.
    Author: Wyatt J, McCallum H.
    Journal: Radiother Oncol; 2019 Apr; 133():100-105. PubMed ID: 30935564.
    Abstract:
    BACKGROUND AND PURPOSE: Magnetic Resonance (MR)-only prostate radiotherapy has recently been clinically implemented using commercial synthetic Computed Tomography (sCT) algorithms. However patients with hip prostheses have been excluded from all MR-only research to date and assumed to require dedicated sCT algorithms. This study aimed to investigate the dosimetric accuracy of applying a commercial sCT algorithm, based on an atlas of patients without hip prostheses, to patients with prostheses. MATERIALS AND METHODS: 18 patients with unilateral hip prostheses received MR and CT scans in the radiotherapy position. sCTs were generated from the MR using a commercial algorithm. The clinical Volumetric Modulated Arc Therapy (VMAT) plan, consisting of partial arcs which avoided the prosthesis, was recalculated using the sCT and the dose distribution compared. RESULTS: The mean isocentre dose difference was ΔD = (-0.4 ± 0.2)% (mean ± standard error of the mean (sem), range - 1.9%, 1.1%) and the mean differences in Planning Target Volume, bladder and rectum mean doses were ≤0.3%. The 3D global gamma pass rate with dose difference 1% and distance to agreement 1 mm within the body was ΓBody1/1 = (95.0 ± 0.5)% (sem) and within the 50% isodose volume, which excluded the prosthesis, was Γ50%1/1 = (98.5 ± 0.4)% (sem). The pass rate within the PTV was ΓPTV2/2 ≥ 99.7% for all patients, although for PTVs close (≤3.5 cm) to the prosthesis ΓPTV1/1 < 85% for three patients. The sCT did not accurately represent the prosthesis with a mean difference in radiological isocentre depth near the prosthesis of ΔdOutsideRad = (15.8 ± 2.6) mm (sem). However inside the treatment plan arc the difference was ΔdInsideRad = (-1.8 ± 0.5) mm (sem). CONCLUSIONS: Using a commercial prostate sCT algorithm for patients with unilateral hip prostheses is dosimetrically accurate (<0.5%) as long as the routine prosthesis-avoidance treatment planning approach is used and the PTV is >3.5 cm from the prosthesis. This suggests MR-only prostate radiotherapy can be extended to patients with hip prostheses without requiring a specific sCT algorithm.
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