Incorporating the rotational setup uncertainty into the planning target volume margin expansion for the single isocenter for multiple targets technique

Publication Date


Journal Title

Pract Radiat Oncol


© 2018 Purpose: The single isocenter for multiple targets (SIMT) technique has become a popular treatment approach for multiple brain metastases. However, the rotational error that is introduced is usually not considered in planning target volume (PTV) expansion. We have developed a statistical model that takes into account both translational and rotational uncertainties. In this study, we incorporated the rotational error into PTV margin expansion for the clinical use of the SIMT technique. Methods and materials: In the statistical model, both translational and rotational errors are assumed to follow the 3-dimensional, independent, normal distribution with a zero mean and standard deviations of σS and σR, where σR = 0.01424σD (rotational uncertainty in degree)×dI ⇔ T (distance in mm from isocenter to target). Based on this model, we derived in this study the additional PTV margin, ∆M, that is 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 maximum 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. Results: Combined 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 3 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 were also plotted for various ∆Mc/MS, which can be used to determine if an additional PTV margin is needed for the SIMT technique. Conclusions: The presented data provide a convenient way for clinics to determine the appropriate PTV margin for the SIMT technique.

Volume Number


Issue Number



475 - 483

Document Type





School of Medicine

Primary Department

Radiation Medicine





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