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

Volume averaging and lack of electronic equilibrium complicate accurate dos imetry of small photon fields. In this paper the performance of the PinPoin t ion chamber for characterizing small fields used in intensity-modulated r adiotherapy (IMRT) was investigated and the results were compared with thos e obtained using the Markus ion chamber and a diamond detector. Sharp beam penumbras were measured for a 5 x 5 cm field defined using a cerrobend bloc k mounted on the accelerator head. In addition, output factors were measure d for a 6 MV photon beam and a variety of small rectangular fields collimat ed widthwise using the multileaf collimator (MLC) in combination with the b ack-up jaws. From this study, a reference field of 5 x 5 cm and a measuring depth of 5 cm are recommended. This is related to the over-response of the PinPoint chamber to low-energy Compton scattered photons, an effect that w as investigated rigorously and turned out to limit the scope of this ioniza tion chamber. However, taking into account some limitations, the PinPoint c hamber is an excellent detector for output measurements in small fields dow n to 2 cm. In profile measurements the chamber causes a broadening of the m easured penumbras but its spatial resolution is superior to that of the Mar kus chamber.