Determinación de carga

7.1

IntroductionIntroduction

Accurate determination of dose is crucial to the success of radiotherapy. Errors in dose determination can cause failure of control or unacceptable normal tissue damage. Accurate determination is vital in clinical trials where the assumption is made that the observed response has been caused by delivery of a particular dose. Dose measure- ment is difficult but for accurate radiotherapy it needs to be better than 7% accurate and preferably better than 5% accurate.

Radiation emanates from a source, travels a distance and then interacts with the material through which it is travelling resulting in deposition of dose. When a dose measurement is made, the value obtained from the device is an indication of the energy deposited in the device itself and not to the patient or material (e.g. water or other phantom) in which the device sits. It is important to understand how the output of the measuring device (detector) is converted to the dose in the medium in which the detector sits.

7.1.1

Absorbed doseAbsorbed dose

Dosimetry defines a numerical relationship between ionizing radiation and the effect that it produces. It is logical to assume that this effect will vary according to the amount of energy deposited within a material of a given mass. Likewise, for the same amount of energy deposited the effect would differ if the energy were deposited within a large rather than small mass of material. The definition of absorbed dose is simply the expression of these observations.

The unit of absorbed dose is defined by the ICRU (see Chapter 10) as the energy absorbed (E) per unit mass (m).

D = E/m

In SI units this is Joules per kilogram (Jkg-1_{) and is given the special name Gray (Gy).}

When prescribing a dose of 50Gy it could in principle also be prescribed in fundamental units as 50 J kg-1_{. The Gray is small when compared with other more obvious examples}

of energy dissipation. For a tumour of mass 100g (0.1 kg) a total dose of 50Gy results in a deposited energy of only 50 x 0.1 = 5 Joules. This is comparable to that delivered to a 1KW heater in only 5 milli-seconds. The heating effect of ionizing radiation per Gy is thus very small with the temperature rises in irradiated tissues resulting from typical clinical doses being less than 0.001°C (0.00024°C per Gy for water). Calorimetry, the measurement of such temperature rises, although technically challenging is the only

INTRODUCTION 67

method of directly determining absorbed dose. This methodology is discussed later. We know that there is a clear relationship between tumour control probability (TCP) and absorbed dose so it is best to quantify the delivery of radiotherapy by prescribing in terms of absorbed dose. Accurate and traceable dose determination is crucial in maintaining consistency between different treatment equipment within a single department as well as maintaining consistency between equipment in different departments nationally and internationally. Clinical trials and the adoption of clinical protocols implicitly rely on the consistency of dose measurement.

7.1.2

ExposureExposure

The way in which absorbed dose has been quantified has been governed by the radiation energies involved and the technology available to quantify them. The first radiation quantity to be defined was exposure. This concept relies on the number of ionization events measured as an indication of deposited energy in a medium. The greater the energy deposited, the greater the dose. The definition of exposure has undergone many refinements over the years, but the most recent definition is:

The unit of exposure (X) is defined as the quotient Q/m where Q is the total charge of the ions (of one sign) produced in air when all electrons liberated by photons in air of mass m are completely stopped in air.

X = Q/m.

In SI units this is Coulombs per kilogram (Ckg–1 _{) where coulomb is the SI unit of}

charge.

The definition of exposure specifically mentions air as the material in which the ions are produced and completely stopped and concerns only photons. Primary standard free air ion chambers operating in the kilovoltage X-ray range try to adhere to the above definition as closely as possible. It should noted that the effective atomic number (Z) of air (7.64) is similar to that of water and soft tissue (7.42). This is important when considering measur ements in low kV energy ranges when the photoelectric effect is dominant (proportional to Z3 _).

7.1.3

KermaKerma

Photons are not directly ionizing. They do not possess charge and are considered to only transfer energy to the irradiated material via Compton, photoelectric or pair production processes . These interactions produce a charged particle, be it a recoil electron (Compton), liberated electron (photoelectric) or electron and positron pair (pair production). The uncharged photons transfer kinetic energy to these charged particles via these interactions. In the second stage the charged particles then impart energy to the material through collisional losses and a small fraction of radiation losses. This fraction is generally small compared to collision losses and will not be discussed in detail.

Kerma was introduced to describe the first part of this two stage process and is defined as:

RADIATION DOSIMETRY 68

where E_tr is the sum of t he initial kinetic energies of all the charged particles liberated by uncharged particles in a mass m. Kerma is an acronym for Kinetic Energy Released per unit Mass. The unit of kerma is Joules per kilogram (J kg-1_{), which, as for absorbed}

dose, is given the special unit of Gray (Gy). Kerma differs from exposure in that it can be defined for any material not just air. A statement of kerma is not complete without defining the material concerned.

7.2

The relationships between exposure, kermaThe relationships between exposure, kerma