Comparison of Safety Margin Generation Concepts in Image Guided Radiotherapy to Account for Daily Head and Neck Pose Variations

• Published in: PloS one 2016-12, Vol.11 (12), p.e0168916-e0168916
• Main Authors: Stoll, Markus, Stoiber, Eva Maria, Grimm, Sarah, Debus, Jürgen, Bendl, Rolf, Giske, Kristina
• Format: Article
• Language: English
• Subjects: Archives & records; Automatic control; Biology and Life Sciences; Biomechanical Phenomena; Biomechanics; Cancer therapies; Care and treatment; Cohort Studies; Comparative analysis; Computed tomography; Computer science; Conformation; Deformation mechanisms; Dosimetry; Finite element method; Formability; Head; Head & neck cancer; Head and neck; Head and neck cancer; Head and Neck Neoplasms - diagnostic imaging; Head and Neck Neoplasms - radiotherapy; Health physics; Humans; Ionizing radiation; Mechanical properties; Medical research; Medicine and Health Sciences; Models, Biological; Oncology; Organs; Organs at Risk - radiation effects; Patient Positioning; Pattern recognition; Physical Sciences; Physics; Principal components analysis; Radiation; Radiation therapy; Radiotherapy; Radiotherapy, Image-Guided - adverse effects; Radiotherapy, Intensity-Modulated - adverse effects; Research and Analysis Methods; Retrospective Studies; Safety; Safety and security measures; Safety margins; Simulation; Spinal cord; Tomography, X-Ray Computed; Tumors; Uncertainty
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0168916

Intensity modulated radiation therapy (IMRT) of head and neck tumors allows a precise conformation of the high-dose region to clinical target volumes (CTVs) while respecting dose limits to organs a risk (OARs). Accurate patient setup reduces translational and rotational deviations between therapy planning and therapy delivery days. However, uncertainties in the shape of the CTV and OARs due to e.g. small pose variations in the highly deformable anatomy of the head and neck region can still compromise the dose conformation. Routinely applied safety margins around the CTV cause higher dose deposition in adjacent healthy tissue and should be kept as small as possible. In this work we evaluate and compare three approaches for margin generation 1) a clinically used approach with a constant isotropic 3 mm margin, 2) a previously proposed approach adopting a spatial model of the patient and 3) a newly developed approach adopting a biomechanical model of the patient. All approaches are retrospectively evaluated using a large patient cohort of over 500 fraction control CT images with heterogeneous pose changes. Automatic methods for finding landmark positions in the control CT images are combined with a patient specific biomechanical finite element model to evaluate the CTV deformation. The applied methods for deformation modeling show that the pose changes cause deformations in the target region with a mean motion magnitude of 1.80 mm. We found that the CTV size can be reduced by both variable margin approaches by 15.6% and 13.3% respectively, while maintaining the CTV coverage. With approach 3 an increase of target coverage was obtained. Variable margins increase target coverage, reduce risk to OARs and improve healthy tissue sparing at the same time.