STUDY THE EFFICIENCY OF PINPOINT AND SEMIFLEX CHAMBERS FOR MEASURING THE SMALLEST FIELD SIZE REQUIRED FOR INTENSITYMODULATED RADIOTHERAPY (IMRT) TECHNIQUE

Background: IMRT is the logical continuation of conformal radiotherapy in the sense that, over the high-dose volume, the dose distribution itself rather than its geometry is actively controlled in three dimensions. Volume averaging and lack of electronic equilibrium complicate accurate dosimetry of small photon fields. Objective: To evaluate the performance of the PinPoint ion chamber for characterizing small fields used in intensity-modulated radiotherapy (IMRT) technique. Method: A 6 MV photon beam (Siemens Primus Linac) has been employed. We used the PTW dosimetry system and MP3 water tank to compare beam character of PinPoint and Semiflex chambers. Results: The build up region in pinpoint is 1.6 cm but in Semiflex is 1.7 cm. The penumbra regions in pinpoint is smaller than PinPoint. Conclusion: The pinpoint chamber has a high resolution. The smallest field size 2x2 cm or less should be measured using PinPoint chamber rather than semiflex chamber.