TU-C-BRA-02: Calibrating the Output of a Linear Accelerator - TG-51 Updated

Abstract

Although dosimetry protocols tend to be fairly long lived it is more than ten years since the AAPM's TG‐51 protocol was published. With the developments in the technology for dose delivery and advances in the measurement and calculation of ion chambercalibration coefficients it is reasonable to question the validity of TG‐51 for the determination of reference (absolute) dose in 2011. Four areas are immediately obvious: 1. The majority of chambers available today do not have kq factors listed in TG‐51. 2. Users can obtain linac‐based absorbed dose calibration coefficients for ion chambers from calibration laboratories on a routine basis. 3. Monte‐Carlo radiation transport algorithms now allow accurate modelling of ion chamber geometries. 4. Linacs with very different beam parameters (e.g., flattening‐filter‐free) are now being introduced into clinical practice. 5. In light of these issues the AAPM set up a working group to review TG‐51 and provide clarification. The group would focus on accelerators capable of reproducing standard reference conditions (i.e., 10 cm × 10 cm field at 1 m) and the output would be an addendum that updates the TG‐51 protocol to the situation in 2011. This presentation will deal primarily with megavoltage photon beams and will discuss the content of the proposed report: i) Calculated k Q factors for new chambers developed after the publication of TG‐51 ii) A comparison of these calculations with measured k Q factors obtained at primary standards laboratories worldwide. iii) An in‐depth discussion of uncertainties with guidance on how the clinical physicist can affect the overall uncertainty in absorbed dose to water iv) Best practice recommendations to minimize errors and ensure consistent dosimetric reporting In addition there will be some discussion of the issues to be faced in the update for electron beamdosimetry. Development of electron beam standards has lagged behind that for photon beams and further experimental and theoretical work is required before an update to the electron part of TG‐ 51 can be considered. Issues include: i) status of primary standards worldwide; ii) validity of a 60Co‐based calibration (direct versus cross‐ calibration); iii) choice of chamber, cylindrical or parallel‐plate; iv) correct values of kQ factors in light of recent determinations of chamber perturbation factors. Learning objective: 1. Rationale for an update to TG‐51 2. Content of photon beam addendum 3. Timeline and research requirements for electron beam update.