Planning Target Volume (PTV) Margin Determination from Marker-Based Setup and The ExacTrac 6D X-ray IGRT System in Prostate Cancer Patients
Keywords:
Fiducial marker, Prostate cancer, PTV margin, Systemic error, VMATAbstract
Background: In radiation therapy, setup uncertainty of patients affect the expected treatment outcome and the addition of PTV margin is able to improve treatment accuracy. Objective: To evaluate setup variations and calculated optimal PTV margins for prostate VMAT setup using the ExacTrac image guided system with implanted fiducial marker registration and cone beam computed tomography. Materials and methods: A total number of 923 x-ray pair images and 271 CBCT images from 28 prostate VMAT patients treated with TrueBeamSTx were observed. By daily online localization, the corrections determined from initial laser setup, from the ExacTrac image system, and from CBCT were accumulated. Positioning differences based on fiducial markers registration between Digitally Reconstructed Radiography (DRR) images with the ExacTrac system and internal anatomy matching from CBCT were measured in rightleft (RL), supero-inferior (SI) and antero-posterior (AP) directions. The systematic (Σ) and random (σ) errors were calculated and determined PTV margin using van Herk margin formula (2.5Σ + 0.7σ). Results: The setup uncertainty from laser alignment was 2.65±2.66 mm, 2.96±2.65 mm and 4.83±4.89 mm in right-left (RL), supero-inferior (SI) and antero-posterior (AP) direction, respectively. With marker registration, these uncertainties were reduced to be 0.64±0.77, 0.81±0.95, and 0.90±1.32 mm and the residual error, when rechecked with soft tissue matching from CBCT, was still be 0.65±1.64, 0.80±1.99 and 0.90±2.01 mm in RL, SI and AP directions, respectively. The PTV margins based on implanted markers and ExacTrac daily online correction were calculated to be 1.46, 1.86 and 2.11 mm. in RL, SI and AP directions, respectively. Conclusion: Frequent CBCT imaging verification presented a limitation in patient dose. Using the ExacTrac x-ray system together with fiducial markers was found to be effective tool for daily setup verification. However, the optimal PTV margin was strongly suggested to assess from both the inter and intrafractional setup deviations.
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