Abstract
In this paper, we describe and characterize a novel biplanar diode array, and demonstrate its applicability to dosimetric QA of step‐and‐shoot IMRT. It is the first commercially available device of its kind specifically designed for performing measurements at varying gantry angles. The detector consists of a cylindrical PMMA phantom with two orthogonal detector boards. There are a total of 1069 p‐type 1 mm wide diode detectors covering the measurement area of 20×20cm2 in each of the measurement planes. The orthogonal detector arrays ensure that the dose modulation information is not lost regardless of the beam incidence angle. For absolute calibration, the dose to the reference detector is calculated at the appropriate SSD and radiological depth by the treatment planning system and is scaled by the measured accelerator output. The directly measured rotational response on the central axis shows the maximum variation of approximately ± 3% in the narrow ±1º angular intervals centered on the detector boards. This variation is reduced to less than ± 2% outside of the four similarly centered ± 5% angular intervals. For all detectors, the difference between the measured and the calculated dose for a plan with 12 equally spaced beams is −0.2±0.9%. Of eleven IMRT plans, ten passed the γ (3%,3 mm) criterion at or above 95%, while one passed at 92%. The biplanar diode array is a useful tool for IMRT QA, allowing for essentially instantaneous online analysis of absolute dose errors in 3D.PACS number: 87.55Qr
Highlights
On in the development of clinical IMRT processes it was well understood that “at the heart of acceptance testing and commissioning procedures are dose measurements and their comparison with IMRT planning system calculations”.(1) While historically this function was performed with an ion chamber for absolute dosimetry at a few points and with radiographic film for dose distribution verification, a number of alternative approaches emerged in recent years
A hypothetical ideal dosimeter for IMRT QA was described by Nelms et al[2] It should consist of very small, isotropic, absolute dose detectors arranged in a highdensity three-dimensional array in a water-equivalent phantom
The Delta4 device is capable of quickly providing a large amount of absolute, three-dimensional dose data, which makes it a robust tool for QA of IMRT dosimetry
Summary
On in the development of clinical IMRT processes it was well understood that “at the heart of acceptance testing and commissioning procedures are dose measurements and their comparison with IMRT planning system calculations”.(1) While historically this function was performed with an ion chamber for absolute dosimetry at a few points and with radiographic film for dose distribution verification, a number of alternative approaches emerged in recent years. Among the three popular commercially available devices, one is diode-based[12,13] and the other two utilize ionization chamber arrays.[14,15] The common tradeoff with such devices compared to film is the ease of absolute two-dimensional dose measurements at the expense of resolution loss due to both detector size and interdetector spacing. A novel diode-based detector (Delta, ScandiDos AB, Uppsala, Sweden) was introduced and briefly characterized,(21-23) indicating favorable day-to-day reproducibility, dose rate independence, and linearity. It is the first commercially available dosimeter array designed for three-dimensional dose verification of all currently available IMRT treatments, whether utilizing static gantry angles or dynamic rotational delivery. We further examine the most clinically relevant properties of the Delta system, and describe the use of the detector for IMRT QA
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