Margin Assessment and Optimization in Prostate Irradiation by Quantitative Analysis of Daily Megavoltage Computed Tomography

Abstract

Purpose/Objective(s): One major challenge in radiation therapy is to determine the appropriate planning target volume (PTV) margin. The ability to secure tumor coverage while reducing normal tissue toxicity is especially critical in treating prostate cancer in the modern dose-escalation era. The goal of this study is to develop a margin assessment and optimization method that takes into consideration the organ deformation, translation and rotation, and the coverage of CTV and the overlapping of organ at risk (OAR). Materials/Methods: We obtained daily mega-voltage computed tomography (MVCT) data from 20 prostate cancer patients who underwent definitive radiation therapy. Each patient received one planning computed tomography simulation scan (CTsim) and 30-38 MVCT. For all scans, one physician delineated the prostate, the seminal vesicles, the rectum and the bladder. Three off-line matching strategies (skin markers, bone, and soft tissue) were performed for the MVCT-CTsim registration, from which translation and rotation registration parameters were collected. After performing the match, contours were exported to Matlab for analysis. By combining contours of all fractions, a 3D probability density rendering for the four organs was produced for each patient and then merged with CTsim. Two margin recipes were created. The first was an isotropic margin with 2 to 10 mm expansion of either the prostate alone or prostate with seminal vesicles and the second was an anisotropic margin with modification of anterior and posterior borders. Finally, both margin recipes were tested for both CTV coverage and OAR overlapping percentage. Results: The mean isotropic margin to achieve 95% of CTV coverage in skin markers, bone, and soft tissue registration were 9.7, 5.9, and 5.8 mm, while the overlapping of rectum/bladder was 5.1/16.3, 5/6.8, and 3.7/7.1 % of total volume. In anisotropic margin, a reduced anterior/posterior margin of 3.9/9.6, 2.4/3.7, and 2/3.7 mm in skin markers, bone, and soft tissue registration can achieve comparable CTV coverage and better OARs sparing than the isotropic margin recipe. Conclusions: Daily 3D IGRT enable analysis of the inter-fractional change of organs in the presence of deformation, translation and rotation. Utilizing this data, we introduced a new margin recipe that optimizes both CTV coverage and OAR sparing. For the patient sample of this study, anisotropic margin with reduced anterior/posterior borders generates optimal PTV. By building a larger database using this technique, we can find a margin recipe that is most suitable for each patient type. Author Disclosure: T. Wang: None. Y. Lee: None. L. Chou: None. J. Chen: None. Y. Liu: None.