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).