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

Abstract

In this paper, we described and characterized a novel biplanar diode array, and demonstrated its applicability to dosimetric QA of the step-and-shoot IMRT. It is the first commercially available device of its kind specifically designed for 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 cm2 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 degree 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 calculated dose for a plan with twelve equally spaced beams is -0.2+/-0.9%. Of eleven IMRT plans, ten passed the gamma(3%,3mm) criterion at or above 95%, while one passed at 92%. Delta4 is a useful tool for IMRT QA, allowing for essentially instantaneous on-line analysis of absolute dose errors in 3D.

Figure 1

Delta4 dosimeter in the measurement position (a); CT scan of the device (b) with an axial slice through the center of the measurement region (top) and an oblique reconstruction through the midplane of the main detector board (bottom); the Delta4 phantom with film along the main board and the wings removed (c) with PMMA slab shown half‐inserted, for clarity; assembled calibration phantom (d) with the main detector board inserted.

Figure 2

Orthogonal relative dose profiles for EDW fields obtained with film and Delta4 for the main detector (a) and wings (b). Dose maps below corresponding graphs show the areas where dose‐difference exceeds 2% (mostly dark “frames” on the periphery), and indicate the location of the profiles. “G” and “T” mark the gantry‐target direction.

Figure 3

Relative dose comparison between film and Delta4: (a) Isodose overlay; (b) profile directions; (c) transverse profile ‐ Delat4 vs. film (green); (d) longitudinal profile (film is red).

Figure 4

Relative sensitivity variation vs, the angle to the normal to the black side of the main board. Raw readings with PMMA replacing the wings, with wings in place, and readings normalized to the TPS‐predicted values on the CT dataset.

Figure 5

Radial beam profiles for a single beam: measured with PMMA replacing the wings, measured with the wings in place, and calculated by the TPS. Beam incidence is perpendicular to the main board (along the wings, when applicable). All profiles are normalized to the calculated value at the central axis.

Figure 6

Histogram of relative measured vs. calculated (on synthetic and physical CT datasets) point dose differences for a single beam at 45° incidence angle to both detector planes.