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

Abstract

PurposeIntensity 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.Materials and MethodsIn 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.ResultsThe 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.ConclusionVariable margins increase target coverage, reduce risk to OARs and improve healthy tissue sparing at the same time.

Application of an automatic, uncertainty model‐guided, target‐generating algorithm to lung stereotactic body radiotherapy

“…Techniques such as thresholding then used to the final target volume. 4,20,23 Although the αTarget algorithm uses the mixture model in a similar way to prioritize which regions to incorporate into the target volume, a critical difference is the combination of the subsequent steps of evaluating the coverage for each individual voxel and increasing the PDF weighting coefficient of those that are inadequately covered. As a result, the αTarget algorithm considers not only the probability that a particular point in space is occupied by tumor voxels, but also considers which tumor voxels they are.…”

“…The approach of many probabilistic margins can be interpreted as describing the probability that a particular point in space will be occupied by tumor voxels. Techniques such as iso‐probability thresholding are then used to determine the final target volume 4,20,23 . Although the αTarget algorithm uses the mixture model in a similar way to prioritize which regions to incorporate into the target volume, a critical difference is the combination of the subsequent steps of evaluating the coverage for each individual voxel and increasing the PDF weighting coefficient of those that are inadequately covered.…”

“…Increasingly, the limitations of the conventional margin approach have been recognized, and concepts of probabilistic margins are being considered. 4,[20][21][22][23][24][25] The targets created with the αTarget algorithm can appropriately be considered an example of such a probabilistic margin. However, a couple of key differences exist between αTargets and those of other probabilistic methods.…”

A review of Image Guided Radiation Therapy in head and neck cancer from 2009–2019 – Best Practice Recommendations for RTTs in the Clinic

“…Five-point fixation is recommended with fixation at head, neck and shoulders as standard to minimise sub regional variations in set up errors in the lower neck and shoulders [22][23][24]. Open face masks are growing in popularity as they can minimise distress in claustrophobic patients [25,26].…”

“…3-5 mm margins have been reported as appropriate only where daily IGRT [45,48], or alternating day IGRT [49,43] is routine practice. Where daily IGRT is not practiced studies recommended CTV-PTV margins of greater than 5 mm due to potential sub region set up variabilities in mandible, nasal septum and neck [50][51][52][53]24,54,55] and unstable set ups in patients with a high Body Mass Index (BMI) [50]. Random errors of up to 2-5 mm have been reported which could result in geometric miss if not imaged and corrected online where standard CTV-PTV margins are used [13,56,57].…”