[en] This study aims at the experimental determination of the detector-specific 1D lateral dose response function K(x) and of its associated rotational symmetric counterpart K(r) for a set of high-resolution detectors presently used in narrow-beam photon dosimetry. A combination of slit-beam, radiochromic film, and deconvolution techniques served to accomplish this task for four detectors with diameters of their sensitive volumes ranging from 1 to 2.2 mm. The particular aim of the experiment was to examine the existence of significant negative portions of some of these response functions predicted by a recent Monte-Carlo-simulation (Looe et al 2015 Phys. Med. Biol. 60 6585–607).In a 6 MV photon slit beam formed by the Siemens Artiste collimation system and a 0.5 mm wide slit between 10 cm thick lead blocks serving as the tertiary collimator, the true cross-beam dose profile D(x) at 3 cm depth in a large water phantom was measured with radiochromic film EBT3, and the detector-affected cross-beam signal profiles M(x) were recorded with a silicon diode, a synthetic diamond detector, a miniaturized scintillation detector, and a small ionization chamber. For each detector, the deconvolution of the convolution integral M(x)  =  K(x)  ∗  D(x) served to obtain its specific 1D lateral dose response function K(x), and K(r) was calculated from it. Fourier transformations and back transformations were performed using function approximations by weighted sums of Gaussian functions and their analytical transformation.The 1D lateral dose response functions K(x) of the four types of detectors and their associated rotational symmetric counterparts K(r) were obtained. Significant negative curve portions of K(x) and K(r) were observed in the case of the silicon diode and the diamond detector, confirming the Monte-Carlo-based prediction (Looe et al 2015 Phys. Med. Biol. 60 6585–607). They are typical for the perturbation of the secondary electron field by a detector with enhanced electron density compared with the surrounding water. In the cases of the scintillation detector and the small ionization chamber, the negative curve portions of K(x) practically vanish. It is planned to use the measured functions K(x) and K(r) to deconvolve clinical narrow-beam signal profiles and to correct the output factor values obtained with various high-resolution detectors. (paper)

[en] Purpose: The aim of this study is the measurement of the lateral response function of microDiamonds by comparison with radiochromic film dose measurement. In this study a TM60019 microDiamond (PTW Freiburg, Germany) and a prototype synthetic diamond detector with smaller sensitive volume were investigated. Methods: Two lead blocks were positioned below the gantry head of an Elekta Synergy accelerator using a gantry mount. Between the blocks two sheets of paper were fixed. The water phantom was positioned below the gantry mount, so that the block to water distance was 20 cm. The gap beam profile was measured at 5 cm water depth by radiochromic EBT3 film and diamond detectors. The film was fixed on a RW3 plate, moved by the step motor system of the phantom and digitized by an Epson 10000XL scanner using the red color channel. Results: The lateral response of the prototype diamond detector is comparable to that of film measurements, i.e. has negligible width. This corresponds to the small detector volume of the prototype detector. In contrast to this the FWHM values of the gap-beam dose profiles measured with the TM60019 detector are somewhat larger, which corresponds to the larger sensitive detector volume. Conclusion: This study has illustrated the high spatial resolution of the diamond detectors. In comparison with filmmeasured narrow-beam dose profiles, the TM60019 has a spatial resolution function of about 2 mm FWHM, whereas the FWHM for the prototype is practically negligible. However due to the low signal caused by the small sensitive volume, measurements with the prototype in clinical routine are a challenge. On the other hand the TM60019 is a good compromise between detector volume and signal output and thus a well suited detector for most clinically relevant small field situations