Investigation of the spatial resolution of an online dose verification device

The aim of this work is to characterize a new online dose verification device, COMPASS transmission detector array (IBA Dosimetry, Schwarzenbruck, Germany). The array is composed of 1600 cylindrical ionization chambers of 3.8 mm diameter, separated by 6.5 mm center-to-center spacing, in a 40 × 40 arrangement.

The line spread function (LSF) of a single ion chamber in the detector was measured with a narrow slit collimator for a 6 MV photon beam. The 0.25 × 10 mm(2) slit was formed by two machined lead blocks. The LSF was obtained by laterally translating the detector in 0.25 mm steps underneath the slit over a range of 24 mm and taking a measurement at each step. This measurement was validated with Monte Carlo simulation using BEAMnrc and DOSXYZnrc. The presampling modulation transfer function (MTF), the Fourier transform of the line spread function, was determined and compared to calculated (Monte Carlo and analytical) MTFs. Two head-and-neck intensity modulated radiation therapy (IMRT) fields were measured using the device and were used to validate the LSF measurement. These fields were simulated with the BEAMnrc Monte Carlo model, and the Monte Carlo generated incident fluence was convolved with the 2D detector response function (derived from the measured LSF) to obtain calculated dose. The measured and calculated dose distributions were then quantitatively compared using χ-comparison criteria of 3% dose difference and 3 mm distance-to-agreement for in-field points (defined as those above the 10% maximum dose threshold).

The full width at half-maximum (FWHM) of the measured detector response for a single chamber is 4.3 mm, which is comparable to the chamber diameter of 3.8 mm. The pre-sampling MTF was calculated, and the resolution of one chamber was estimated as 0.25 lp∕mm from the first zero crossing. For both examined IMRT fields, the χ-comparison between measured and calculated data show good agreement with 95.1% and 96.3% of in-field points below χ of 1.0 for fields 1 and 2, respectively (with an average χ of 0.29 for IMRT field 1 and 0.24 for IMRT field 2).

The LSF for a new novel online detector has been measured at 6 MV using a narrow slit technique, and this measurement has been validated by Monte Carlo simulation. The detector response function derived from line spread function has been applied to recover measured IMRT fields. The results have shown that the device measures IMRT fields accurately within acceptable tolerance.