Water or Medium: Dose Specification in Trials and Real Life

Radiation dose is the therapeutic agent in radiotherapy where the objective is to maximise radiation dose to a target while minimising the dose to surrounding healthy tissues. Dose in this context is typically associated with the quantity "absorbed dose" as energy deposited per unit mass and measured in J/kg of tissue. However, even if high doses are delivered (no stochastic distribution considered) and photon or electron radiation is considered (no neutrons or heavy charged particles), there will be differences in the actual dose delivered to different tissue types as the stopping power for the electrons that deliver the vast majority of dose varies with elemental composition. Historically, radiation beam calibration and dose calculations were performed in water as a readily available, easily standardised material that closely matches the radiation properties of many human tissues. However, many superior dose calculation algorithms that have recently become available due to improved computer power (Monte Carlo Calculations, Acuros) calculate dose as deposited in the medium. The present paper examines arguments for both and proposes that based on the current scientific and political developments specification of dose as dose to medium would be the more robust and future proof choice.