Abstract
Objective:The reconstruction kernel used for a CT scan strongly influences the image quality. This work investigates the changes in Hounsfield units (HUs) which can arise when altering the image reconstruction kernel for planning CT images and the associated changes in dose in the radiotherapy treatment plan if the treatment planning system (TPS) is not re-calibrated.Methods:Head and neck, prostate and lung CT images from four centres were used. For a specific scan, the base image was acquired using the original reconstruction kernel (used when the TPS was calibrated) and the treatment plan produced. The treatment plan was applied to all images from the other reconstruction kernels. Differences in dose-volume metrics for the planning target volume (PTV) and organs at risk (OARs) were noted and HU differences between images measured for air, soft tissue and bone.Results: HU change in soft tissue had the greatest influence on dose change. When within ±20 HU for soft tissue and ±50 HU for bone and air the dose change in the PTV and OAR was within ±0.5% and ±1% respectively.Conclusions: When imaging parameters were changed, if HU change was within ±20 HU for soft tissue and ±50 HU for bone and air, the change in the PTV and OAR doses was below 1%.Advances in knowledge:The degree of dose change in the treatment plan with HU change is demonstrated for current TPS algorithms. This adds to the limited evidence base for recommendations on HU tolerances as a tool for radiotherapy CT protocol optimization.
Highlights
The CT images used for radiotherapy treatment planning must be of good geometric fidelity and of sufficiently high image quality to allow accurate outlining of tumour volumes and organs at risk
For each image, when compared against a baseline image, the values of maximum Hounsfield unit (HU) change for air, bone and soft tissue were plotted alongside the maximum change in planning target volume (PTV) dose quantities and maximum change in either volume at specified dose or in dose level for two OARs
The tolerances used, ±50 HU for air and bone and ±20 HU for soft tissue, when compared against the values in the baseline images, are those proposed in the introduction as corresponding to a dose change of within ±1% and are based on a detailed literature review.[20]
Summary
The CT images used for radiotherapy treatment planning must be of good geometric fidelity and of sufficiently high image quality to allow accurate outlining of tumour volumes and organs at risk. The quality of a CT image is primarily dependent on the scan protocol settings.[1] Parameters in all CT scan protocols should be set to ensure that optimal levels of image quality and imaging dose are delivered It is usual practice for the protocols on a diagnostic CT scanner intended for imaging different clinical conditions or body regions to have different settings for the various scan parameters. Radiotherapy CT protocols should be set to provide levels of contrast and spatial resolution which are appropriate for the particular body region imaged and size of patient.[2] The task of accurately outlining the tumour and organs at risk on radiotherapy CT planning images is a demanding one and variability and inaccuracies are known to be a key source of uncertainty in the treatment planning process.[3,4,5,6] The quality of radiotherapy CT images should support this process. The CT reconstruction kernel, which is selectable by the operator, is an important part of the image production process and can have a significant effect on the quality of the final CT image.[6,7,8,9] Some CT scanners have many reconstruction kernels available for selection, the kernels are usually developed with a specific imaging task in mind, such as imaging soft tissue or bony detail, or imaging in the head or body regions.[10,11]
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