Evaluation of a biplanar diode array dosimeter for quality assurance of step-and-shoot IMRT

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 x 20 cm(2) 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 +/- 1 degrees angular intervals centered on the detector boards. This variation is reduced to less than +/- 2% outside of the four similarly centered +/- 5 degrees 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 gamma (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.