For dosimetry procedures, several dosimetry protocols are available for kilovoltage X-Ray beam therapy. For orthovoltage X-Ray beams, the absorbed dose to water is usually determined with an IC calibrated in air in terms of air kerma.

Following the AAPM TG-61, for orthovoltage X-Ray beams, two different algorithms are recommended:

In-Air Method

An in-air method where the air-kerma is measured in air and then converted to dose to water through the ratio of mass energy absorption coefficient for water to air and a backscatter factor.

M = The instrument reading in air at the reference applicator aperture, corrected for temperature and pressure.

Nk = The air kerma calibration (in grays per scale reading) to convert the instrument reading at the beam quality, concerned to air kerma free in air, at the reference point of the chamber.

Bw = The backscatter factor; defined as the ratio of the water collision kerma at a point on the beam axis at the surface of a full scatter water phantom, to the water collision kerma at the same point in the primary (incident) beam, with no phantom present.

Air = The mass energy absorption coefficient ratio, water to air, averaged over the photon spectrum in air.

This algorithm is recommended for low and medium energy X–Ray beams (tube potential: 40 to 300 kV).

The IPEM code of practice used commonly in the UK [4] allows measurements using a parallel plate chamber in a phantom up to 50 kV. A small phantom is sufficient to provide full backscatter and there is no need for a backscatter factor to be included.

The Kappach factor (kch) accounts for the change in response of the solid phantom and water; for a particular chamber type, between the calibration in air and measurement at the surface of a full-scatter and water equivalent phantom. Initially the Kappach factor was thought to be near unity, but this has now been measured [11]. The IAEA code of practice [5] is unique in that the chamber and the phantom are calibrated as a set in terms of dose to water at the surface, although this dose to water calibration is calculated via an air kerma calibration. The dose to water calibration simplifies the calibration of the users beam, although the calibration is specific to the reference conditions used during the calibration.

In-Phantom Method

An in-phantom method is where the air-kerma, at a reference depth in water (2 cm), is measured under reference condition and then converted to the absorbed dose at the depth of the center of the chamber in undisturbed water using the ratio of mass energy absorption coefficients for water to air and corrections factors.

The kch factors are calculated for commonly used chambers [12, 13]. This algorithm is recommended for tube 100 kV < tube potential < 300 kV.

The choice of the calibration method will depend upon the region of clinical interest. If the region of clinical interest is at depth, then it is more appropriate to use the in-phantom method. The in-air method is the commonly used method in the clinical radiotherapy community, especially in North America. It is more convenient to perform routine beam calibration in air than in a water phantom.

Medium Energy Range 100 kV to 300 kV

Calibration in this range depends upon the region of clinical interest. If the region of clinical interest is at depth it is more appropriate to measure the dose in a water phantom at 2 cm deep. If the region of interest is primarily the surface it is more appropriate to measure the dose in air as for the low energy range.

The dose at depth is given by:

The only addition is the depth at which the water air mass absorption ratios are calculated. The kch factors have been calculated for commonly used chambers [12,13].

The IAEA code of practice [5] is unique in that the chamber in terms of dose to water is at 2 cm deep, although this dose to water calibration is calculated via an air kerma calibration. The dose to water calibration simplifies the calibration of users beam, although the calibration is specific to the reference conditions used during the calibration.

Measurement of Output in a Clinical Beam

Xstrahl 300 Clinical Applicators (Source: Xstrahl.com)
Xstrahl 300 Clinical Applicators (Source: Xstrahl.com)

Treatment fields for kilovoltage treatment machines are defined by an applicator of specific size and optionally, by the addition of cut-out placed on the skin surface.

Low and medium energy X-Rays are readily scattered by air thus to define sharp penumbras and delineate the treatment fields, solid walled applicators which extend to the patients skin surface are used. The walls of the applicator are not required to be very thick because the X-Rays are obliquely incident upon the walls. Low energy applicators are open ended, whereas a medium energy X-Ray beam necessitates the use of applicators, closed with a perspex sheet, to reduce the low energy electron contamination.

Electron contamination will vary between applicators; it is not sufficient to estimate the output of a low or medium energy applicator from the ratio of the back scatter factors between the treatment applicator and the reference applicator. The output factors must be measured by measuring the ratio of the dose to water at the surface or reference depth from the applicator (of size F), to the dose to water at the same depth from the reference applicator.

Extra care must be taken for very small fields, where the electron contamination at the surface and perturbation effects, from the relatively large size of the chamber field, are likely to become significant [14].

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