Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_18_3293 - Additional PTV margin for compensating rotational error is not a linear function of the distance between isocenter and treatment target.

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Additional PTV margin for compensating rotational error is not a linear function of the distance between isocenter and treatment target.
J. Chang; Radiation Medicine, Northwell Health, Lake Success, NY; Department of Physics and Astronomy, Hofstra University, Hempstead, NY

Purpose/Objective(s): The single isocenter for multiple targets (SIMT) technique is a popular treatment approach for multiple brain metastases but there is no consensus on how to compensate for the rotational error it introduces. Some suggested that the PTV expansion should be increased linearly with the distance between the isocenter and treatment target. In this study the author challenged these published margin recipes and hypothesized that the additional PTV margin is not a linear function as suggested. The purpose of this study was to derive the correct margin recipe for the SIMT technique using a previously developed statistical model that considers both translational and rotational uncertainties.

Materials/Methods: In the statistical model, both translational and rotational errors are assumed to follow the three-dimensional independent normal distribution with a zero mean, and standard deviations of respectively σS and σR, where σR = 0.01424 σD (rotation uncertainty in degree)× dI⇔T (distance in mm from isocenter to target). Based on this model, the author derived in this study the formulas for combined PTV margin, ME, and additional PTV margin, ΔM=ME-MS, 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. Formulas for the maximal allowable dI⇔T and σD were also derived as a function of user-specified ΔMC/ MS, the fraction of MS below which the extra PTV margins can be ignored.

Results: Unlike other reported margin recipes, ΔM increases linearly with dI⇔T and σD only when there is no translational setup error, i.e., MS=0. When MS≠0, ΔM is a non-linear function of dI⇔T and σD but asymptotically approach a linear function for large dI⇔TσD. Using the derived formulas, values of ME and ΔM were calculated and plotted for commonly encountered clinical parameters including dI⇔T, MS or σD. It is observed from the plot that ΔM is insignificant when MS is dominant, and becomes more pronounced for larger dI⇔TσD. Cutoff value (dI⇔T σD)C for user specified ΔMC /MS is a linear function of σS with slope kMC/ MS), which is a function of ΔMC/ MS and is independent of the coverage probability. The following table lists kMC/ MS) values for a few selected ΔMC/MS with corresponding (dI⇔T σD)C (in mm·degree) for σs=0.71 mm. (dI⇔T)C or the maximally allowable dI⇔T (in mm) is also tabulated for two (0.3° and 0.45°) commonly encountered σD in clinics.
ΔMC/ MS 1% 5% 10% 15% 20% 30% 40% 50%
kMC/ MS) 10.0 22.5 32.2 39.9 46.6 58.3 68.8 78.5
(dI⇔T σD)C in mm·degree for σs=0.71 mm 7 16 23 28 33 41 49 56
(dI⇔T)C in mm for σD=0.3°, σs=0.71 mm 24 53 76 94 110 138 163 186
(dI⇔T)C in mm for σD=0.45°, σs=0.71 mm 16 35 51 63 73 92 109 124

Conclusion: The additional PTV margin, ΔM, is not a linear function of dI⇔T and σD, and can be ignored when MS is dominant. Cutoff value (dI⇔T σD)C is independent of coverage probability if it is specified as a function of ΔMC /MS. The presented data provide a convenient way for clinics to determine if additional PTV margin is needed for the SIMT technique.

Author Disclosure: J. Chang: None.

Jenghwa Chang, PhD

Biography:
Dr. Jenghwa Chang is currently a Senior Medical Physicist and the Director of Medical Physics Residency Program at the Department of Radiation Medicine of Northwell Health. He also holds the positions of Associate Professor in the Donald and Barbara Zucker School of Medicine at Hofstra/Northwell, and Adjunct Associate Professor in the Hofstra College of Liberal Arts and Sciences. Dr. Chang earned his Ph.D. degree in Electrical Engineering from the Polytechnic University of New York in 1995 and has been practicing medical physics for over 20 years. He was an Assistant/Associate Attending Physicist of Medical Physics at Memorial Sloan-Kettering Cancer Center from 1997-2008, an Associate Professor and the Director of Physics Research of Radiation Oncology at NYU Langone Medical Center from 2008-2010, and an Associate Professor and the Director of Centralized Treatment Planning of the Combined Department of Radiation Oncology at NewYork Presbyterian Hospital from 2010-2016. Dr. Chang was a member of the AAPM Therapy Physics Committee from 2006-2008 and served as the president of RAMPS in 2011. He is a site surveyor for ACR Radiation Oncology Practice Program and a reviewer for multiple international journals. Dr. Chang has been involved in the training of technologists, medical residents and physics residents since 2000. He was the co-founder of the medical physics residency program at NewYork Presbyterian Hospital and served as the Program Director at the Weill Cornell Medical College campus from 2010-2016. His research interest involves applying engineering and physics principles to radiology and radiation oncology to improve cancer target definition, treatment setup accuracy, and critical organ avoidance.

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