Letters to the Editor Comment on The Radiological Physics Center's standard dataset for small field size output factors (J Appl Clin Med Phys. 2012;13(5):282‐89)

Dear Editor, A standard dataset for small field output factors was published by the Radiological Physics Center (RPC).(1) The data are to be used as part of the quality control in other radiotherapy departments. As part of commissioning a new linear accelerator, we performed a comparison of our data against the original RPC dataset and found relative large deviations. We, therefore, conducted a comparison of the RPC dataset with measurements performed at four oncology departments in Denmark in order to validate the RPC dataset. The measurements were performed on Varian TrueBeam accelerators (Varian Medical Systems, Palo Alto, CA) using three different detectors all suited for small field dosimetry: a pinpoint chamber (PTW 31014; PTW, Freiburg, Germany), a prototype scintillator system developed at Technical University of Denmark,(2) and a diamond detector (PTW 60003). In addition we included some data measured on a Varian Clinac 2100C/D accelerator. Furthermore, we included output factors calculated by the treatment planning systems of the participating centers. Comparing our data to the original RPC dataset revealed a significant difference for all energies and a difference in output factors of about 2.5%. Recently an error in the original dataset was discovered, which lead to a major change in the output factors (see Followill, et al.: Erratum; Vol.15 #1). In Figs. 1 to 4, a plot of all measured and calculated values is seen for 6, 10, 15, and 18 MV beam qualities, respectively. All RPC data are taken from the corrected dataset and the error