Quantifying isocenter measurements to establish clinically meaningful thresholds

Abstract

A dataset 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. Using a laser‐defined isocenter, a total of 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 clinically meaningful isocenter measurement. 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 action and immediate action level were determined for use in analyzing daily isocenter congruency test results in a stereotactic radiosurgery (SRS) program. 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 clinical 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.PACS number: 87.53Ly, 29.20.Ej, 87.56.Fc

Independent evaluation of the effectiveness of IsoCal in improving image center accuracy on Varian TrueBeam and Clinac machines

“…The ranges of gantry sag were 0.56 and 0.59 mm on the TrueBeam machines, which were smaller than those on the Clinac machines, 0.75 and 0.79 mm. The gantry sag measured on the Clinac machines was consistent with the values previously reported in the literature 2, 4, 17…”

“…While the IG systems may vary in physical mechanism, they must provide accurate spatial presentations of the patient in the reference frame of treatment machines. To this end, a key requirement for the IG systems is that the image coordinates should match the treatment coordinates 1, 2. Specifically, the origin of the image coordinates, referred to here as the image center, should coincide with the isocenter of the treatment machines, or the radiation isocenter.…”

“…These imperfections include but are not limited to mechanical sag of image source and image detector, imperfect rotation of gantry, miscalibration of collimator, and misalignment of room lasers 2, 3, 4, 5, 6. As a result, the image centers deviate from the radiation isocenter.…”

3D isocentricity analysis for clinical linear accelerators

“…[1][2][3] The current AAPM TG-142 recommendation for the maximum deviation between mechanical and radiation center is AE 1 mm for SRT-designated linac. [4][5][6] In addition, precise knowledge of the relative location of table and gantry rotation isocenters is important for treatments using noncoplanar beams without repeated verification of patient positioning before beam-on. The misalignment of the two rotation isocenters limits the spatial precision with which dose can be delivered to the patient, possibly impacting necessary margins and dose-volume constraints.…”

Evaluation of the Impact of the Linac MLC and Gantry Sag in volumetric modulated arc therapy

“…1,[3][4][5][6][7][8] Due to the obvious clinical implications of suboptimal linac mechanical performance, the magnitude of this isocenter deviation must be tightly bounded. Denton et al, 9 in a statistically motivated approach, examined the results of 149 Winston-Lutz tests to derive a practical schedule action level of 1.25 mm and an immediate action level of 1.50 mm for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) treatments. These thresholds were defined from the perspective of what is attainable in practice, rather than a clinical perspective.…”