Abstract
PurposeIn this work, we implemented a method to obtain a nonuniform clinical target volume (CTV) to planning target volume (PTV) margin caused by both rotational and translational uncertainties and evaluated it in the treatment planning system (TPS).Materials and methodBased on a previously published statistical model, the relationship between a target margin and the distance d (from isocenter to target point), setup uncertainties, and significance level was established. For a single CTV, it can be thought as a combination of many small volume elements or target points. The margin of each point could be obtained using the suggested statistical model. The whole nonuniform CTV–PTV margin was determined by the union of all possible margins of the CTV boundary points. This method was implemented in the Pinnacle3 treatment planning system and compared with uniform margin algorithm. Ten vertebral metastases targets and multiple brain metastases targets were chosen for evaluation.ResultsThe combined CTV–PTV margin as a function of d for various initial translational margin and rotational uncertainties was calculated. The combined margin increases as d, rotational uncertainties and translational margin increase. For the same rotational uncertainty, a smaller initial translational margin requires a larger rotational margin to compensate for the rotational error. Compared with the uniform margin algorithm, the advantage of this method is that it could minimize the PTVs volume for given CTVs to obtain same significance level. Using vertebral metastases targets and multiple brain metastases targets, a series of volume difference was obtained for various translational margins and rotational uncertainties. The volume difference of PTV could be more than 17% when translational margin is 2 mm and rotational uncertainty is 1.4°.ConclusionNonuniform margin algorithm could avoid excessive compensation for the CTV boundary points near isocenter. This method could be used for clinical margin determination and might be useful for the protection of risk organs.
Highlights
It is well known that setup uncertainties would be introduced throughout the treatment delivery
We propose a method to obtain a nonuniform clinical target volume (CTV)–planning target volume (PTV) margin caused by setup uncertainties
The location of isocenter is a key issue, and it should be located at the geometric center of CTV in order to obtain the smallest PTV
Summary
It is well known that setup uncertainties would be introduced throughout the treatment delivery These setup errors during treatment are critical to the success of radiotherapy.[1,2,3,4] Without proper attention, these setup errors can cause misalignment of the beams and lead to the radiation dose delivered outside the target area. To solve this problem, a method is provided to account for setup errors in the ICRU 50 report and its supplement.[5] According to ICRU 50, a margin is added to the clinical target volume (CTV), yielding the planning target volume (PTV). Many recipes for CTV–PTV margin have been developed by different groups.[6,7,8,9,10,11]
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