Absorbed Dose to Water Standards for Megavoltage Therapy Beams

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Objective: Develop a national standard of absorbed dose to water for therapy level megavoltage x-ray beams

Background: Megavoltage x-ray beams are widely used by external beam radiation therapy machines for the treatment of cancer. The success of radiation treatments relies on knowing with high accuracy the dose delivered to patients during treatment. For this the measurements of dose delivered by megavoltage x-ray beams at hospitals need to be traceable to the national standard of absorbed dose to water. The traceability of measurements performed in hospitals is established through the use of reference ionization chambers that are calibrated at one of the Accredited Dosimetry Calibration Laboratories (ADCLs) of the American Association of Physics in Medicine (AAPM) in cobalt 60 (60Co) gamma ray beams. The measurements provided by the ADCLs are in turn directly traceable to the national standard held at the National Institute of Standards and Technology (NIST). Due to differences between 60Co gamma ray and megavoltage x ray beams, calculated corrections that account for these differences are applied in transferring the national standard to the clinics for use in megavoltage x-ray beams. These corrections are available through published protocols such as those written by the AAPM task group 51 (AAPM TG 51 1999) and are specific to the ionization chambers used, the beam characteristics and differences in the energy spectra between cobalt 60 (60Co) and x-ray beams. While the implementation of these protocols has been the norm for almost 20 years now, a new standard for absorbed dose to water needs to be developed to address the increase of new external beam therapy devices based on megavoltage x-ray beams with peak voltages available between the 60Co energy to 50 MV (reference to websites with different external beam therapy units)

Needs: A new standard needs to be developed to provide traceability measurements for the doses delivered by modern external beam therapy devices that make use of megavoltage x-ray beams. The standard should consist of megavoltage x-ray reference beams such as those delivered in clinics throughout the country. As opposed to the time when the AAPM TG51 was published, currently NIST and the ADCLS have access to clinical accelerators that are capable of producing these reference beams. This enables the possibility of developing a new additional standard specific to these new type of devices and establishing traceability measurements through the AAPM ADCLs to clinics nationwide.

Action items:

  • Measurement methods for determining dose delivered in water by megavoltage x-ray beams need to be identified for two purposes: One for the development of a national standard and for the dissemination of the standard through the AAPM ADCLs.
  • Calorimetric methods have been used in the past for 60Co and are being currently considered as one of the possible options for determining an absolute value of dose (from the definition of dose).
  • Investigations need to be conducted to evaluate what are best dosimeters to transfer standard measurements of dose delivered by megavoltage x-ray beams in water to the clinics.