The radiation oncologist decides optimum radiation dose distribution for a patient prior to commencing treatment planning. The aim of treatment planning is to model that dose distribution as closely as possible using a number of radiation beams of appropriate modality and energy. The direction and size of each beam has to be determined, together with radiation dose to be given and any modification that may be required to the inherent dose distribution of the beam. To achieve this aim, it is necessary to know the precise dose distribution that will result from any treatment beam. This is obtained from prior radiation measurements in unit-density tissue-equivalent material, made available as a set of charts or tables (known as the depth dose distribution). These data specify the dose at any point in the beam relative to a reference point (usually the point of maximum dose on the central axis of the beam). The depth of the reference point increases with increasing beam energy and is in the range 1-3 cm for commonly available megavoltage (MV) treatment beams.
Dose distributions are required for a whole range of clinical settings available on a treatment machine. The relative dose distribution for the patient is obtained by summing the individual distributions for all beams and is displayed as a map showing lines of constant dose, known as isodose lines. The radiation oncologist prescribes an absolute dose to an isodose line and to achieve this dose each individual beam must be calibrated in terms of the number of machine units required to give a specified dose to its reference point.
The machine units control the quantity of radiation given in an individual exposure. This absolute calibration is again carried out by prior radiation measurements, provided in tabular form as a set of output factors. These also vary with the possible clinical machine settings and must be measured over the complete range to be used.
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