Source:Practical Radiation Oncology
Author(s): Jenghwa Chang
PurposeThe single isocenter for multiple targets (SIMT) technique has become a popular treatment approach for multiple brain metastases. However, the rotational error it introduces is usually not considered in PTV expansion. We have developed a statistical model that takes into account both translational and rotational uncertainties. In this study, the author incorporated the rotational error into PTV margin expansion for clinical use of SIMT technique.Methods and Materials.In the statistical model, both translational and rotational are assumed to follow the three-dimensional independent normal distribution with a zero mean, and standard deviations of respectively σS and σR, where σR = 0.01424σD (rotation uncertainty in degree)×dI⇔T (distance in mm from isocenter to target). Based on this model, the author derived in this study the additional PTV margin, ∆M, required to maintain the same coverage probability when the rotational uncertainty is present as a function of MS (initial PTV margin), σD and dI⇔T. The maximal allowable dI⇔T, C and σD, C were also calculated as a function of user-specified ∆Mc/MS, the fraction of MS below which the extra PTV margins can be ignored.ResultsCombined PTV margin, ME, and additional PTV margin, ∆M, were plotted for commonly encountered clinical parameters including dI⇔T, MS or σD. Unlike other reported margin recipes, ∆M is not a linear function of any of these three parameters. In addition, the rate of increase for ∆M is quite slow for small dI⇔T and becomes more significant for larger dI⇔T. Cutoff values dI⇔T, C and σD, C are also plotted for various ∆Mc/MS, which can be used to determine if additional PTV margin is needed for the SIMT technique.ConclusionsThe presented data provide a convenient way for clinics to determine the appropriate PTV margin for the SIMT technique.
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