some steps towards establishing a tertiary standard dosimetry laboratory at a radiotherapy department

Background: in order to deliver the precise dose to the target in radiotherapy, absorbed dose to water at the reference point should be assessed. when the calibration procedure is performed for a reference dosimeter in the 60co beam of a secondary standard dosimetry laboratory (ssdl), the total uncertainty in absorbed dose to water (dw) is estimated to be approximately 1.5%. this study attempts to re-measure the nd,w factors for all available field chambers at reza radiotherapy & oncology centre (rroc). materials and methods: consistency and linearity checks were performed for a range of available chambers using a check source. the nd,w factors were also measured for the ionization chambers. all cylindrical chambers have been cross calibrated at 6 mv photon beam using a siemens primus plus linac. the plane parallel chamber has been cross calibrated at the highest available electron beam and the nd,w factor has been measured. results: the tolerance of consistency and linearity checks has been reported to be within 0.3%. the nd,w value for field farmer chamber was found in agreement with certificate within 1%. in contrast for small active volume chambers, the deviation from the ssdl reports was 2.3%. for the plane parallel chamber, the difference between ssdl and home measurements are found to be 12%. conclusion: although the calibration of reference chambers used for absolute dosimetry through a primary standard dosimetry laboratory (psdl) or ssdl is recommended, for field chambers this can be done at home department as a tertiary standard dosimetry laboratory (tsdl).