8.2 - Measurement Of Photon Beams

Measurement of Radiation Quality and Output

Radiation Quality

Radiation quality has numerous meanings, but in the measurement of external photon beams it refers to the spectrum of photon energies that make up the beam. The quality of a megavoltage beam is described as the maximum energy of photons within the beam – hence an ’18 MV beam’ would have photons with a maximum energy of 18 MeV. Kilovoltage beams may be described in a similar way (eg. 100 kV) or by quoting the half value layer of the beam (eg. 10 mm Al).
The quality of a megavoltage beam is typically measured by calculating the dose rate at a depth of 20 cm in water compared with 10 cm in water. This is known as the Tissue Phantom Ratio 20,10 or TPR20,10. A higher TPR20,10 is associated with a more penetrating beam. The TPR20,10 is typically measured using a cylindrical ionisation chamber.

Radiation Output

Radiation output in modern linear accelerators is determined by monitor units. A monitor unit is the amount of charge measured by the ionisation chambers (built in to the treatment head) required to deliver 1 cGy at a reference depth, with a set field size and beam energy. An ionisation chamber is typically used to measure the dose in the phantom.

Dosimetry Protocols

IAEA megavoltage absolute dosimetry protocls (eg. Technical Report Series 398) contain standards for measuring quantities such as the TPR20,10, and for calibrating ionisation chambers. The conditions specified in dosimetry protocols are known as reference conditions. Once calibrated, further corrections usually need to be made in individual departments.

Measurement of Dose Distribution

Beam Profiles

A beam profile is measured at multiple points on a plane perpendicular to the central beam axis. Measurement is usually performed in a water phantom using a cylindrical ionisation chamber. A beam profile can be one dimensional (along one axis) or two dimensional (measuring in the x and y axes).
Beam profile may also be determined by film dosimetry or other dosimeters, particularly TLD or silicon diodes which have a small detection area. This is most beneficial for the penumbra region, which has rapid dose changes that may be above the resolution of a typical cylindrical ionisation chamber.

Depth Dose Curves

Depth dose curves measure radiation dose relative to the point of dose maximum, zmax. This is most easily acheived in a water phantom using a parallel plate ionisation chamber in a water phantom, which gives excellent resolution if the plate is arranged perpendicular to the beam direction.

Isodose Charts

Isodose charts are constructed by combining depth dose curves with the beam profile at multiple depths. This allows depiction of dose in a two or three dimensional plane.
Alternatively, film dosimetry may be performed at varying depths within a phantom, giving dose distribution on a plane perpendicular to the central beam axis. This is beneficial as film dosimetry is accurate in regions of rapid dose changes, such as in the penumbra of the beam.
Gel dosimetry is capable of creating 3D dose profiles.