Comments on 'The influence of a novel transmission detector on 6 MV x-ray beam characteristics'

In their recent paper (Venkataraman et al 2009 Phys. Med. Biol. 54 3173-83) the authors reported on photon beam attenuation and secondary electron production in the novel transmission detector COMPASS, to be placed in the accessory holder of the linac treatment head. In the interest of IMRT patient safety, space-resolved measurements by transmission detectors analysing the MLC-shaped photon fluence pattern in real time are in fact an urgent item for equipment designers. However, there are some constraints for the construction of such devices. The COMPASS system, at its present stage of development, has difficulties in complying with the constraints that the spatial sampling rate should fit the desired task and that the enhanced secondary electron contamination of the photon beam due to the presence of the device should be minimized. The authors also missed to mention a forerunner in this field, the DAVID transmission detector (Poppe et al 2006 Phys. Med. Biol. 51 1237-48), serving for the real-time supervision of the MLC aperture during patient treatment and ever since proven in clinical practice. The DAVID system, a transparent multiwire ionization chamber placed in the accessory holder, will be shortly described.