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Ionizing radiation in diagnostics and therapy

Ionizing radiation is not visible and cannot be perceived with the senses which humans have such as, e.g., the sense of smell or the sense of hearing. Exposure to ionizing radiation can damage people's health and lead to serious diseases. On the other hand, ionizing radiation has become an indispensable tool of modern medicine. At the right dose, it is used in diagnostics and in radiation therapy for the patient's good, either to detect diseases and decide upon the treatment, or for the targeted destruction of tumours.

In medicine, imaging diagnostic methods such as MRI, ultrasound, endoscopy - and the most frequently used one X-ray diagnostics - are applied. In hospitals, X-ray radiation is used for approx. 63 % of the imaging diagnostics (see Fig.).

Fig.: Frequency of use of different methods in imaging medical diagnostics in hospitals1



Special radionuclides having particularly well-suited decay properties are used to complement the traditional imaging methods based on X-rays which penetrate the body from the outside. The patients are administered these radionuclides in the form of a radiopharmaceutical. By selecting the appropriate chemical compound, the radionuclides gather in certain organs or tissues of the body. The radiation properties of the atoms decaying inside the body are then exploited for imaging, e.g. by means of gamma cameras or positron emission tomography (PET).

In radiation therapy, the damaging effect of ionizing radiation is exploited to kill cancerous cells; this effect depends on the dose applied. At approx. 25 %, cancer is one of the most frequent causes of death in the industrialized countries. 50 % to 60 % of all cancer patients are treated by means of radiation therapy within the course of their treatment; in approx. 50 % of all long-term tumour cures, it is one of the components of therapy or sometimes even the only treatment methods2.

In radiation therapy, one differentiates (apart from interoperative irradiations) between external radiation therapy and internal radiation therapy. In external radiation therapy, the patient (or rather the tumour) is irradiated from outside the body. Hereby, it is unavoidable that the healthy tissue above the tumour is also exposed to radiation along with the tumour which is located somewhat deeper. In routine clinical practice, external radiation therapy is performed with high-energy photon radiation (which is generated by clinical electron linacs) due to its greater penetration depth. All developments in radiation therapy are aimed at attaining a sufficiently high and uniformly distributed dose in the tumour while only applying a small dose to the healthy tissue surrounding the tumour in order to prevent damage. The key term is "tumour-matched dose distribution". One of the latest developments in external radiation therapy is Intensity Modulated Radiation Therapy (IMRT), where the patient is irradiated from different directions by means of a large number of narrow radiation fields that have irregular shapes and are adapted to the tumour's geometry.

When using ionizing radiation in medicine, the dose and its uncertainty are very important. The so-called "ALARA" principle applies meaning "as low as reasonably achievable". In diagnostics, the challenge resides in applying a dose to the patient which will be as small as possible, but high enough to obtain meaningful imaging - and thus, a reliable diagnosis. In radiation therapy, the dose deposited in the tumour has to be high enough to kill cancerous cells, but at the same time, the dose should not be too high in order not to cause unnecessary damage to the surrounding healthy tissue. To summarize: for the patientxs good, reliable and traceable dosimetry with a correspondingly low measurement uncertainty – and whose quality assurance is guaranteed – is of paramount importance both in diagnostics and in radiation therapy. Report 24 by ICRU (International Commission of Radiation Units and Measurements)3 requires, for optimum radiation therapy, that the uncertainty of the dose deposited in the tumour be no more than +/- 2.5 % (= 1). In X-ray diagnostics, Report 74 of ICRU4 requests a total uncertainty of the dose measurement of 7 % (= 2). Today, these two requirements are, unfortunately, not always attained; especially in radiation therapy, measurement uncertainty still represents a huge challenge. In radiation therapy, the dose measurand is the absorbed dose to water, in X-ray diagnostics, the base measurand of dosimetry is the air kerma. Both measurands are expressed in the unit "gray" (abbreviated to "Gy"; Gy = J/kg).

For the realization of the units of these measurands, PTB operates various primary standards (WG 6.25 Dosimetry for Diagnostic Radiology). Through the calibration of secondary standards, the units are disseminated and traceability in dosimetry is, thus, ensured. In X-ray diagnostics, the use of dosimeters having a type-approval certificate is prescribed by the quality-assurance requirements for certain measurement applications. In the field of dosimetry in radiation therapy, the regulations of the Medical Devices Act and of the Medical Devices Operator Ordinance apply.

The secondary standards used in X-ray diagnostics and radiation therapy are nearly exclusively ionization chambers (WG 6.21 High-energy photon and electron radiation). These exhibit high metrological stability and reliability, good measuring characteristics (e.g. low energy dependence of the response), and their properties are easy to describe by means of numerical simulations. Ionization chambers of the most differing shapes (cylindrical, spherical or flat and coplanar chambers) and air volumes (volumes from 0.015 cm3 to 0.6 cm3 in radiation therapy and up to approx. 100 cm3 in X-ray diagnostics) are used, depending on the requirements of the measurement.

Besides direct dose measurement, radiation transport calculations based on Monte Carlo simulations are an important tool to determine dose values, dose distributions, correction factors, the response of detectors, etc. Such calculations can be used, on the one hand, to understand measurement results and draw the correct conclusions from them, and, on the other hand, to determine parameters which cannot be determined by means of measurements. Radiation transport calculations must categorically be considered as a complement to measurements.

The technical developments in radiation therapy and X-ray diagnostics keep presenting dosimetry with new challenges. Examples are - in radiation therapy - dose measurement in small fields and - in X-ray diagnostics - dosimetry in the broad radiation field of modern CT devices. Research and development activities for new detectors, measurement procedures, etc. play an important role in realizing traceable dosimetry in these novel fields. Due to the small size, to the low dependence of the response on the radiation quality and to the material properties, the alanine dosimeter is an essential piece of measuring equipment for such investigations (WG 6.21 High-energy photon and electron radiation). There is close national and international cooperation between medical physicists in hospitals and experts from other national metrology institutes, calibration laboratories, and industry. PTB transposes the research results into national and international standardization as well as into internationally applied technical documents (e.g. IAEA Reports).

1 BMU-2005-660: Erfassung der Häufigkeit bildgebender Diagnostik, insbesondere strahlendiagnostischer Massnahmen und der Altersverteilung der Patienten, Schriftenreihe Reaktorsicherheit und Strahlenschutz, Bundesministerium für Umwelt und Reaktorsicherheit, 2005

2 Die blauen Ratgeber: Strahlentherapie, Antworten, Hilfen, Perspektiven, Band 53, Deutsche Krebshilfe e.V.

3 International Commission on Radiation Units and Measurements, Determination of Absorbed Dose in a Patient Irradiated by Beams of X or gamma Rays in radiotherapy Procedures, ICRU Report 24, 1991

4 International Commission on Radiation Units and Measurements, Patient Dosimetry for X-rays used in Medical Imaging, ICRU Report 74, Journal of the ICRU, Vol. 5, No.2 (2005), Oxford University Press