Encrypted login | home

Program Information

Quantifying Isocenter Measurements to Establish Clinically Meaningful Thresholds


T Denton

T Denton1,2*, L Shields3,4, J Howe1,2, A Spalding3,4,5,(1) The Norton Cancer Institute Radiation Center, Louisville, Kentucky, (2) Associates In Medical Physics, LLC, Greenbelt, MD, (3) Norton Neuroscience Institute, Louisville, Kentucky, (4) The Brain Tumor Center, Norton Healthcare, Louisville, Kentucky, (5) Kosair Children's Hospital, Louisville, KY

PO-BPC-Exhibit Hall-4 Saturday  Room: Exhibit Hall

Purpose: A range of isocenter congruency verification tests have been examined from a statistical perspective for the purpose of establishing tolerance levels that are meaningful based on the fundamental limitation of linear accelerator isocentricity and the demands of a high-precision stereotactic radiosurgery program.

Methods: Using a laser-defined isocenter, 149 individual isocenter congruency tests were examined with recorded values for ideal spatial corrections to the isocenter test tool. These spatial corrections were determined from radiation exposures recorded on an electronic portal imaging device (EPID) at various gantry, collimator, and treatment couch combinations. The limitations of establishing an ideal isocenter were quantified from each variable which contributed to uncertainty in isocenter definition. Individual contributors to uncertainty, specifically, daily positioning setup errors, gantry sag, multileaf collimator (MLC) offset, and couch walkout, were isolated from isocenter congruency measurements to determine a meaningful isocenter measurement.

Results: Variations in positioning of the test tool constituted, on average, 0.38 mm magnitude of correction. Gantry sag and MLC offset contributed 0.4 and 0.16 mm, respectively. Couch walkout had an average degrading effect to isocenter of 0.72 mm. Considering the magnitude of uncertainty contributed by each uncertainty variable and the nature of their combination, an appropriate schedule and immediate action level were determined for use in analyzing daily isocenter congruency test results in a stereotactic radiosurgery (SRS) program.

Conclusion: The recommendations of this study for this linear accelerator include a schedule action level of 1.25 mm and an immediate action level of 1.50 mm requiring prompt correction response from medical physicists before SRS or stereotactic body radiosurgery (SBRT) is administered. These absolute values were derived from considering relative data from a specific linear accelerator and, therefore, represent a means by which a numerical quantity can be used as a test threshold with relative specificity to a particular linear accelerator.

Microsoft OLE DB Provider for ODBC Drivers error '80040e14'

[Microsoft][ODBC SQL Server Driver][SQL Server]The multi-part identifier "db2012.amos.dbo.AbstractID" could not be bound.

/meetings/2014SCM/PRAbs.asp, line 283