Premium
SU‐E‐J‐64: Margins for Single‐Fraction Treatments: Stereotactic Radiosurgery Brain Cases
Author(s) -
Zhang Q,
Chan M,
Burman C,
Amols H,
Ling C,
Mageras G
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3611832
Subject(s) - isocenter , radiosurgery , margin (machine learning) , nuclear medicine , standard deviation , radiation treatment planning , dosimetry , fraction (chemistry) , image guided radiation therapy , residual , linear particle accelerator , mathematics , radiation therapy , medicine , imaging phantom , algorithm , statistics , physics , computer science , radiology , optics , beam (structure) , chemistry , organic chemistry , machine learning
Purpose: To determine the appropriate margin expansion from clinical target volume (CTV) to planning target volume (PTV) for single‐fraction stereotactic radiosurgery (SRS). Setting a proper margin is crucial for delivering the right dose to the CTV and sparing of organs at risk. Methods: Whereas in conventional fractionated treatment a Gaussian function is used to represent systemic errors, for single‐fraction treatment with a specific machine, the delta function is more appropriate. As input to the function, we measured the differences between image isocenter and radiation isocenter for a Varian Trilogy linac over a period of six weeks. To approximate the residual setup errors for a population of patients, we adopted a Gaussian function with different standard deviation. A phenomenological formula for calculating the CTV‐PTV margin for single‐fraction SRS brain cases is proposed. For 95% success (CTV received prescribed dose) of patientsˈ treatments, the margin for a single‐fraction treatment is obtained which is a function of standard deviation of the residual setup errors and system errors. Results: The differences between the image isocenter and radiation isocenter remain constant to within sub‐ millimeter during the typical treatment period of six weeks. This confirms our assumption on the system errors distribution as being a delta function and machine‐specific. This definition is especially important for a single‐fraction treatment. Our study found that human factors could also attribute to the reduction of the margin of CTV‐PTV in addition to stability of the treatment machine. Conclusions: A margin formula is proposed for the SRS brain case that attempts to help physicists determine the proper margin of CTV‐ PTV. This margin derivation could also extend for multi‐fraction cases because the system errors do not change from time to time unless system upgrades. R01‐CA126993