X-rays are produced when an orbiting electron of an atom loses energy, either through deceleration or via the transition of the electron between the different shells of the atom. When utilising X-rays for medical purposes, the X-ray tube is designed to produce electrons and accelerate them using a high voltage, which are then focused to collide with a metal target, causing them to decelerate rapidly. If the electrons have enough energy when they hit the target, they may knock an electron out of an inner shell of the metal atom and as a result, the gap is filled by electrons from higher energy levels. The byproduct of this process is the production of X-rays [1].

X-rays can be used for therapeutic purposes as a result of the process by which they affect the tissues through which they pass; as a beam of radiation is transmitted through a medium, its intensity is reduced. This process is referred to as attenuation. This loss of energy can vary greatly and is dependent on both the initial intensity of the beam before attenuation and the density of the medium through which it is passed.

When an X-ray beam is attenuated by tissue the energy loss is attributable to:
Absorption – the energy is transferred to the surrounding tissue.
Scatter - the photons collide with atomic particles of the surrounding tissues and as a result change direction.

The type and amount of attenuation is characterised by the initial energy of the radiation beam, of which there are four main processes:

Each process occurs at a certain point of the X-ray spectrum. For example, when using low energies (as in diagnostic radiography) photoelectric absorption makes the biggest contribution whereas in high energy therapeutic radiography the dominant attenuation process is Compton scatter [2].

For treatment with superficial and orthovoltage therapeutic X-ray beams the dose at the skin surface is arguably the most important factor, given the rationale of treatment. X-rays at the higher end of the energy spectrum (6-10 mV) are able to penetrate into the deep tissues of the body and are therefore of use when targeting deep seated tumours. When treating superficial tumours such as skin lesions, this level of penetration is unnecessary and would cause unwarranted dose to ‘normal’ tissues. Furthermore, the maximum dose would be delivered to underlying tissues causing tumour under dosage and the potential for recurrence [3].  

Kilovoltage X-ray beams can be subdivided into two energy ranges;
	Low energy 10 kV – 100 kV energy beams; used for superficial therapy, where the intended treatment region is within the first few mm of the dermis.
	Medium energy 100 kV – 300 kV energy beams; used where the intended therapeutic depth is up to 20 mm.

In the United Kingdom code of practice [4] the low energy range is subdivided into the medium 160-300kV, low 50-160kV and very low 8-50kV.

Several methods of absolute dosimetry are used, depending upon the specific point of clinical interest. For low energy X-rays calibration is either performed at the surface of a phantom [4,5] or in-air at the aperture of the reference applicator [4,6], whereas for medium energy X-rays the calibration is either performed at 2cm depth in water [4,5,6], or in-air at the aperture of the reference applicator [4, 6].

The attenuating properties of a kilovoltage X-ray beam are determined both by the accelerating potential and the filtration of the beam; thus merely stating the nominal accelerating potential is insufficient to describe the beam. The beam quality in the kilovoltage energy range is usually described by the amount of material required to attenuate the beam by 50%, or the thickness of the half value layer (HVL). Commonly Aluminium is used in the low energy range and Copper is used in the medium energy range.

Measurement of the HVL for each beam is necessary in order to derive a calibration factor for the secondary standard chamber at the qualities of those beams.

The HVL is defined as the thickness of absorber which reduces the air kerma rate of a narrow X-ray beam to 50% of the air kerma rate of an unattenuated beam.

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The attenuating material is placed between the applicator and a small aperture; just large enough for the narrow beam to cover the measuring chamber. The chamber is positioned so that scatter from the attenuating material and backscatter from material behind the chamber (floor, walls etc) are minimised. Ideally the HVL would be measured at the normal calibration distance, however, in order to minimise scatter from the filters, the source chamber distance is at least twice the length of the applicator.

In reality the HVL for ionisation is being measured; however, as long as the calibration factor of the chamber varies by less than 2% over the quality range being measured, this will not significantly affect the results. Most dosimetry services only offer a calibration in terms of half value layer, although for a given HVL the dose to water calibration factor varies by a little over 2% depending on the accelerating potential and the filtration.

In the low energy kilovoltage range parallel plate chambers specifically designed for low kV dosimetry should be used. The thin entrance window of these chambers is sufficient to remove electron contamination and provide full build-up conditions up to 1 mm HVL (approx 50 kV). If the chambers are used with a beam with a greater HVL plastic foils must be placed in the way of the beam, to ensure full build-up conditions [6]. These foils must be included with the ionisation chamber when it is calibrated. In the medium energy range a cylindrical ion with a volume less than 1.0cm³ must be used, whether the calibration is performed in-air or in-phantom. A typical chamber would be a ‘farmer’ type chamber such as an NE2571 or NE2561.

Historically many dosimetry codes of practice have used an in air calibration in terms of air kerma, where the dose to water, at the water surface is given by:

It is not usually possible to measure the ionisation at the aperture of an applicator because of the finite size of the chamber and its stem. It is usually necessary to measure the ionisation as close to the aperture of the applicator as possible and correct for extra distance from the source. Care must be taken however as the inverse square law may not apply close to an applicator, particularly at short Focus Skin Distances (FSD’s).

Backscatter factors vary with HVL; field size and FSD and have been calculated using Monte Carlo techniques for the purposes of dosimetry [7]. The water air mass energy absorption coefficients vary, depending on the spectrum at the position of measurement; they depend on HVL, field size and depth of measurement. These have also been calculated at the water surface, and 2 and 5 cm deep by Knight [8, 9 & 10].

The UK code of practice [4] allows measurements using a parallel plate chamber in a phantom up to 50 kV. Here a small phantom is sufficient to provide full back scatter and there is no need for a back scatter 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, 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 users beam, although the calibration is specific to the reference conditions used during the calibration.

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 2cm 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 2cm 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.

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].

The range of applicator sizes supplied with a treatment machine is, necessarily, limited to the most common clinically used field sizes. However, in order to minimise irradiation of normal tissue, treatments can be customised to the shape and size of an individual patient’s lesion, using a lead cut-out. The thickness of the lead cut-out is dependent on the X-ray energy; 1mm is suitable for low energy treatments and 2mm is suitable for medium energy treatments. The lead cut-out must be slightly larger than the applicator, with an aperture cut-out according to the lesion, plus a clinical margin.

The cut-out reduces the total dose received at the surface of the lesion, by the ratio of the backscatter factor of the area of the cut-out, to the backscatter factor of the area of the applicator. The lead cut-out factor at beam quality E and aperture size A is given by:

(where F is the size of the applicator used)

If the dose at a depth, other than the reference depth, is required it is necessary to measure the relevant depth dose curve. Depth dose curves can be measured in water, using a waterproofed cylindrical ion chamber, or in solid water, using a low energy X-ray parallel plate chamber. It is not recommended that parallel plate chambers designed for electrons are used.

Example depth dose curves can be found in BJR Supplement 25 [12].

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