Ramifications of Setup Margin Use During Frameless Stereotactic Radiosurgery/Therapy With Gamma Knife Icon Cone-Beam Computed Tomography (CBCT): A Dosimetric Study.

William N Duggar,Rui He,Claus Chunli Yang,Bart Morris

doi:10.7759/cureus.21996

Abstract

ObjectiveThe objective is to explore the possibility of optimal/rational application of setup margin during treatment planning for frameless stereotactic Gamma Knife radiosurgery/therapy.MethodsUncertainty measurements for frameless Gamma Knife Icon treatment were used to calculate the necessary setup margin via four different published recipes and these margins were subsequently applied to treatment plans of 30 previously treated patients and replans were generated meeting comparable plan quality metrics. All plans were then analyzed based on the ability to maintain normal tissue dose tolerances and the relative increase in target dose coverage probability using a pass/fail scoring system based on published normal tissue dose constraints and an in-house developed optimal scoring method.ResultsGross tumor volume/planning target volume (GTV/PTV) size strongly correlated with both meeting normal tissue tolerances and optimal scores for single fraction plans corroborating published clinical outcomes. The Van Herk Margin Formula (VHMF) and Parker margin formulae were indicated as good candidates for high probabilities of both meeting normal tissue goals and high optimal scores which generally translated to just over 1 mm in GTV to PTV margin.ConclusionFor single fraction treatment, GTV size is highly significant in predicting failure to meet normal tissue goals whereas whether setup margin was used was not a significant predictor. Setup margin can rationally be applied when fraction number is dictated by clinically indicated metrics regarding GTV size of greater or less than 4 cc. 1 mm is a reasonable practical application of margin added to GTV to ensure physical prescription dose target coverage for most cases when clinically desired based on disease type and intended outcome.

Highlights

Margins in radiation therapyEssentially, a setup margin prevents geometric miss of the intended target in part or whole by enlarging the targeted region so that the desired target will fall into the treated area despite instantaneous inaccuracy due to the uncertainty of the system [1,2]
Gross tumor volume/planning target volume (GTV/planning target volumes (PTVs)) size strongly correlated with both meeting normal tissue tolerances and optimal scores for single fraction plans corroborating published clinical outcomes
The Van Herk Margin Formula (VHMF) and Parker margin formulae were indicated as good candidates for high probabilities of both meeting normal tissue goals and high optimal scores which generally translated to just over 1 mm in GTV to PTV margin

Summary

Introduction

Margins in radiation therapyEssentially, a setup margin prevents geometric miss of the intended target in part or whole by enlarging the targeted region so that the desired target will fall into the treated area despite instantaneous inaccuracy due to the uncertainty of the system [1,2]. The trade-off is that the new target volume includes more tissue than just the target leading to prescription dose covering some of the surrounding normal tissue which would not have necessarily occurred without the setup margin. For this reason, instead of adding together all the measured uncertainty of the system to create a setup margin that gives 100% confidence in treating the target, the standard deviations of these measured uncertainties are used mathematically to derive setup margin values. Traditional frame-based radiosurgery has not utilized the concept of a setup margin despite identified uncertainty in the system [3]. This lack of setup margin use has been due to the belief that

Methods

Discussion

Conclusion