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

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). 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. 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%. 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).