The calibration of a Scanditronix-Wellhöfer thimble chamber for photon dosimetry using the IAEA TRS 277 code of...

This note investigates the calibration of a Scanditronix-Wellhöfer type FC65-G ionisation chamber to be used in clinical photon dosimetry. The current Adaptation by the Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) of the IAEA TRS 277 dosimetry protocol makes no provision for this type of chamber. The absorbed dose to air calibration coefficient ND was therefore calculated from the air kerma calibration coefficient NK using the formalism of the IAEA TRS 277 protocol and it is shown that the value of the correction factor kmkatt for the FC65-G chamber is identical to that of the NE 2571 chamber. ND was also determined experimentally from a cross calibration against an NE 2571 dosimetry. It was found that there is a good correspondence between the calculated and measured values. To establish to what extent the ACPSEM Adaptation can be used for the FC65-G chamber, values for the ratio of stopping powers in water and air (Sw,air)Q and the perturbation correction factor pQ were calculated using the TRS 277 protocol. From these results it is shown that over the range of beam qualities TPR20,10 = 0.59 to TPR20,10 = 0.78 the Adaptation can be used for the FC65-G chamber.