Abstract
Our study aimed to quantify the effect of the Measurement Uncertainty function on planar dosimetry pass rates, as measured and analyzed with the Sun Nuclear Corporation MapCHECK 2 array and its associated software. This optional function is toggled in the program preferences of the software (though turned on by default upon installation), and automatically increases the dose difference tolerance defined by the user for each planar dose comparison. Dose planes from 109 static‐gantry IMRT fields and 40 VMAT arcs, of varying modulation complexity, were measured at 5 cm water‐equivalent depth in the MapCHECK 2 diode array, and respective calculated dose planes were exported from a commercial treatment planning system. Planar dose comparison pass rates were calculated within the Sun Nuclear Corporation analytic software using a number of calculation parameters, including Measurement Uncertainty on and off. By varying the percent difference (%Diff) criterion for similar analyses performed with Measurement Uncertainty turned off, an effective %Diff criterion was defined for each field/arc corresponding to the pass rate achieved with Measurement Uncertainty turned on. On average, the Measurement Uncertainty function increases the user‐defined %Diff criterion by 0.8%‐1.1% for 3%/3 mm analysis, depending on plan type and calculation technique (corresponding to an average change in pass rate of 1.0%‐3.5%, and a maximum change of 8.7%). At the 2%/2 mm level, the Measurement Uncertainty function increases the user‐defined %Diff criterion by 0.7%‐1.2% on average, again depending on plan type and calculation technique (corresponding to an average change in pass rate of 3.5%‐8.1%, and a maximum change of 14.2%). The largest increases in pass rate due to the Measurement Uncertainty function are generally seen with poorly matched planar dose comparisons, while the function has a notably smaller effect as pass rates approach 100%. The Measurement Uncertainty function, then, may substantially increase the pass rates for planar dose comparisons. Meanwhile, the types of uncertainties incorporated into the function (and their associated quantitative estimates, as described in the software user's manual) may not be an accurate estimation of actual measurement uncertainty, depending on the user's measurement conditions. Pass rates listed in published reports, comparisons between institutions or simply separate workstations, or comparisons with the calculation methods of other vendors, should clearly indicate whether or not the Measurement Uncertainty function is used, since it has the potential to substantially inflate pass rates for typical IMRT and VMAT dose planes.PACS number(s): 87.55.Qr, 87.56.Fc
Highlights
Planar diode arrays are a standard tool for myriad external beam quality assurance tests, most popularly for patient-specific dosimetric verification of intensity-modulated radiation therapy fields,(1,2) for both static[3,4] and rotational[5,6] delivery techniques
The types of uncertainties incorporated into the function may not be an accurate estimation of actual measurement uncertainty, depending on the user’s measurement conditions
An additional calculation parameter within the MapCHECK and SNC Patient software, which is often applied but seldom reported in the literature, is the “Measurement Uncertainty” function, which is turned on by default upon software installation and can be toggled in program preferences.[15]. The software manual describes this function as “a supplement to the user-defined acceptance criteria ... added to the percentage acceptance criterion defined by the user” to account for “various uncertainties in measurement.”(15) In other words, this optional function automatically increases the dose difference tolerance selected by the user for DTA or gamma comparison of planar dose distributions, in an attempt to compensate for various sources of presumed uncertainty in MapCHECK or ArcCHECK array measurements
Summary
Planar diode arrays are a standard tool for myriad external beam quality assurance tests, most popularly for patient-specific dosimetric verification of intensity-modulated radiation therapy fields,(1,2) for both static[3,4] and rotational[5,6] delivery techniques. The MapCHECK and SNC Patient softwares allow the user to quantitatively compare measured and calculated dose planes, utilizing the gamma analysis and distance-to-agreement (DTA) techniques described elsewhere in the literature.[12,13,14]. An additional calculation parameter within the MapCHECK and SNC Patient software, which is often applied but seldom reported in the literature, is the “Measurement Uncertainty” function, which is turned on by default upon software installation and can be toggled in program preferences.[15] The software manual describes this function as “a supplement to the user-defined acceptance criteria ... Added to the percentage acceptance criterion defined by the user” to account for “various uncertainties in measurement.”(15) In other words, this optional function automatically increases the dose difference tolerance selected by the user for DTA or gamma comparison of planar dose distributions, in an attempt to compensate for various sources of presumed uncertainty in MapCHECK or ArcCHECK array measurements. Task Group 119 of the American Association of Physicists in Medicine utilized the Measurement Uncertainty function in the analysis of their cross-institutional data, stating that they “applied measurement uncertainty (a presumed measurement error of about 1% is included in the analysis, so that a nominal 3% dose difference can be 4%).”(16)
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