Room: Exhibit Hall
Purpose: There exists the uncertainty in target positioning using cone-beam computed tomography (CBCT) without fiducial marker due to low contrast images. The goal of this study was to evaluate the uncertainty and planning target volume (PTV) margins in CBCT image-guided target-based patient positioning for prostate cancer.
Methods: The uncertainty was evaluated by the residual error defined as the difference between positioning errors, which were manually and computationally obtained. Twenty patients, who underwent intensity modulated radiation therapy (IMRT) for prostate cancer, were selected for this study. The manually obtained positioning (MOP) errors were estimated using manual target-based positioning after a bone-based automatic registration between pre-treatment CBCT images and the treatment planning CT (PCT) images. The computationally obtained positioning (COP) errors were calculated as a distance between two centroids of prostate contours delineated on the PCT and CBCT images, which were performed the bone-based automatic registration. Systematic and random errors in the residual errors were evaluated from the differences between the MOP and COP errors in anterior-posterior (AP), superior-inferior (SI) and left-right (LR) directions. PTV margins were calculated from the systematic and random errors using van Herk formula.
Results: The systematic and random errors of the residual errors in target-based patient positioning in AP, SI, and LR directions were 0.7 mm, 1.1 mm, 0.5 mm and 0.8 mm, 1.1 mm, 0.4 mm, respectively. Based on the systematic and random errors of the residual errors, the PTV margins in AP, SI, and LR directions were 2.4 mm, 3.5 mm and 1.6 mm, respectively.
Conclusion: For target-based patient positioning using the CBCT images, these uncertainties should be considered in the IGRT era in the treatment planning for prostate cancer.