PinPoint Chamber Research Articles

The value of the PinPoint ion chamber for characterization of small fieldsegments used in intensity-modulated radiotherapy

Volume averaging and lack of electronic equilibrium complicate accuratedosimetry of small photon fields. In this paper the performance of thePinPoint ion chamber for characterizing small fields used inintensity-modulated radiotherapy (IMRT) was investigated and the results were comparedwith those obtained using the Markus ion chamber and a diamond detector. Sharpbeam penumbras were measured for a 5×5 cm field defined using a cerrobendblock mounted on the accelerator head. In addition, output factors weremeasured for a 6 MV photon beam and a variety of small rectangular fieldscollimated widthwise using the multileaf collimator (MLC) in combination withthe back-up jaws. From this study, a reference field of 5×5 cm and a measuringdepth of 5 cm are recommended. This is related to the over-response of thePinPoint chamber to low-energy Compton scattered photons, an effect that wasinvestigated rigorously and turned out to limit the scope of this ionizationchamber. However, taking into account some limitations, the PinPoint chamber isan excellent detector for output measurements in small fields down to 2 cm. Inprofile measurements the chamber causes a broadening of the measured penumbrasbut its spatial resolution is superior to that of the Markus chamber.