Energy dependence investigation for a range of clinically used detectors from 70kV-6MV.

PURPOSE: Accurate measurement of out-of-field dose in radiotherapy directly impacts beam data modelling in treatment planning systems, verification of implanted electronic devices/lens/foetus dose, secondary cancer risk estimation and organ at risk dose reporting. When performing out-of-field dosimetry it is therefore imperative that the response of the detector has been well characterised. Due to the softening of the radiation beam out of field many detectors will exhibit energy dependence. This study investigated the energy dependence of a range of clinical available detectors over typical energies experienced out-of-field.

METHODS: The response of detectors to photon beams from 70 kV to 6 MV was measured. The relative change in response from 6 MV down to 70 kV highlighted the expected deviation in the response of detectors that would typically be calibrated in-field for use out-of-field.

RESULTS: The pinpoint detector displayed the most energy independent response over the energy range investigated. The micro-lion detector was the only detector to show an under-response to all low energy beams relative to 6 MV. The diode type detectors showed the largest energy dependence.

CONCLUSIONS: When considering detectors for use in out-of-field dose measurements, it is important that the energy dependence is investigated over a low energy range as the energy spectra out-of-field comprises of a larger component of photons in the 50-100 keV range. This study highlights the variation in response of a range of clinically available detectors to low energy radiation beams relative to 6 MV for out-of-field dosimetry. The pinpoint detector was the most energy independent detector with a response close to unity over the entire energy range investigated. This article is protected by copyright. All rights reserved.