Novel Anisotropic Margin Calculation Based On the Cumulative Frequency Distribution of Uncertainties in the Clinical Target Volume

Abstract

Objective: The purpose of this study was to determine the PTV margins from the cumulative frequency of uncertainties of the clinical target volume (CTV) position obtained using hybrid-image guided radiotherapy (H-IGRT) with volumetric-modulated arc therapy (VMAT). Materials and Methods: Study participants were 22 patients with intermediate-risk prostate cancer who underwent VMAT. All patients were treated using the following image-guided (IG) procedure: 1) no-correction skin mark setup (no- IG); 2) bony anatomy registration with 2D-imaging (2D-IG); and 3) organ registration using CBCT images (3D-IG). The systematic (Σtot) and random (σtot) errors were obtained from the no-IG, 2D-IG, and 3D-IG registration images of 198 fractions. The PTV margin was computed using the cumulative frequency distribution of the actual position errors of the CTV, and which was compared with the formulation by MPTV = 2.5 Σtot + 0.7 σtot. Results: The margin size of 3D IG procedure was as follows: 3D-IG: 7.6, 5.4, and 3.5 mm for the AP, SI, and LR axes, respectively. Using MPTV = 2.5 Σtot + 0.7 σtot: 3D-IG: 4.2, 3.7, and 1.3 mm for the AP, SI, and LR axes, respectively. In MPTV: = α Σtot + β σtot, the coefficients α and β were α = 3.4, 2.3, and 4.6 and β = 2.9, 3.0, and 3.0 for AP, SI, and LR, respectively. Conclusion: The margin size (MPTV) calculated by our modified formulation based on the margin direction satisfied 99% of the prescription dose coverage in each direction and a minimum of 95% of the dose coverage to the CTV for all patients using H-IGRT