REFINEMENT OF METHODS FOR THE ASSESSMENT OF ORGAN DOSES, AND POSSIBLE REDUCTION OF PATIENT EXPOSURE

Objectif

THE RESEARCH PROJECT AIMS THROUGH BOTH THE ANALYSIS OF THE RESULTS OF THE PREVIOUS CONTRACT AND NEW INVESTIGATIONS, TO PROVIDE SATISFACTORY ANSWERS TO THE PROBLEM THAT IN SOME RADIOLOGICAL EXAMINATIONS (E.G. SKULL, CHEST, PELVIS, ABOMEN, IVP) AND FOR SOME ORGANS (THYROID, BREAST, TESTES) THE ORGAN DOSES DIRECTLY MEASURED ON PATIENTS DURING THE EXAMINATIONS ARE DIFFERENT UP TO ONE ORDER OF MAGNITUDE FROM THOSE OBTAINED BY APPLYING THE MONTE CARLO CONVERSION FACTORS TO THE VALUES OF ENTRANCE SKIN DOSE. IN ADDITION, SUGGESTIONS FOR THE EFFECTIVE DOSE REDUCTION IN THE PRACTICE OF SOME RADIOLOGICAL PROCEDURES WILL BE SET UP.
The observations on national radiology practice provide only a limited insight into the attitudes adopted by the different countries towards the justification and optimisation of medical radiology. Criteria for selecting both patients and examination techniques will be influenced by the different rates at which developments in diagnostic imaging and clinical management of patients have been introduced and accepted by the radiology professions in the countries concerned, and possibly by different national patterns in the prevalence of disease.

It has not been possible in these surveys to obtain sufficient information to clarify the reasons for the divergences observed in staffing levels, and in the frequency and choice of examinations. Some of the main differences in radiology practice, however, occur for procedures whose value has been reduced by the availability of alternative, less hazardous techniques or a decline in the diseases for which they were initially indicated. The introduction of quality assurance programmes in diagnostic radiology, which is being actively encouraged in Europe by Commission of European Communities (CEC), will help to establish and maintain optimal procedures in this field. The surveys reported have provided a baseline of data concerning practices and doses delivered in diagnostic radiology against which the effects of future quality assurance programmes and reassessments of the need for certain procedures may be judged.

Intravenous urography (IVU) is one of the radiological procedures which mostly contribute to collective doses. In the survey conducted in northeast Italy in 1983 to 1984 this examination accounted for 11.8% of the total per caput effective dose equivalent (EDE) from radiological examinations. Since 1984 significant variations in the methods of conducting IVU have occurred so that a reappraisal of doses and frequencies was thought to be worthwhile. The study was carried out in the major radiological institute of the region where 283 IVU examinations were analysed to assess which films are essential for reporting normal or pathological findings. Moreover, entrance skin doses for each exposure and breast and testis doses for the whole examination were evaluated by using the National Radiological Protection Board's (NRPB) Monte-Carlo conversion factors. As a cumulative index EDE was used.

The mean number of exposures per examination has changed, from 1983 to 1988, from 11.5 to 9 and entrance skin dose for a full length view from 6.9 to 4.8 mGy. Progress in film screen systems accounts for lower entrance skin dose while the increased use of nonionising techniques such as ultrasound is the preminent reason for reduced frequencies.

In medical diagnostic radiology collective doses and associated risks can be estimated from frequencies and types of examinations and related patient organ doses.
A nationwide survey has been conducted in Italy to establish frequency and technical parameters adopted in each type of computerised tomography (CT) examination. Questionnaires were sent to all the facilities (293) to collect relevant information. A tomogram typical of each type of examination was requested as a supplemental source of information to help identify the anatomical landmarks of the irradiated part of the body. The examination types selected were the 10 most frequently performed: head, chest, abdomen, cervical spine, lumbar spine, pelvis, petrous bone, hypophysis, orbits skeletal segments. Finally, a section of the questionnaire was devoted to collect modalities of execution of quality assurance (QA) programmes and frequencies of quality control (QC) tests. Organ doses to patients who undergo CT examinations will be estimated using Monte-Carlo (MC) simulations techniques. Conversion factors relating mean organ doses to the free in air dose at the centre of rotation are being calculated by National Radiological Protection Board, United Kingdom United Kingdom fora variety of make and type of CT scanners. Dose profiles have to be measured for each selection of the scanning parameters commonly used for the different examinations.

The spread of examination techniques between installations is very large. The distributions of technical parameters are characterised by high coefficients of variation and positive asymmetry. As a consequence of any given examination the complete range of patient doses received in practice will extend as widely as 1 or more orders of magnitude.

Differences between computerised tomography dose index's (CTDI) per unit tube workload may or may not affect patient doses. Where these differences are due to different levels of peak dose in the profile, it can be reliably s upposed that a number of mAs inversely proportional to peak dose is needed to obtain images at the same noise level. On the contrary, dose enhancement factors greater than 1 adversely affect patient dose without improving image quality.

Since on the average 94 examinations per scanner are carried out weekly in Italy, the total annual number of CT examinations can be estimated in 1.5 million, corresponding to 2.5% of all types of X-ray examinations. Patient doses are much higher than those delivered by conventional radiography concerning the same anatomical compartment. For instance, the tentative estimate of the effective dose equivalent for a CT examination of the head, 2.95 mSv, should be compared with that for the conventional examination of the head, 0.224 mSv. These first data confirm that CT procedures are assuming an increasing importance as contributors to population doses in medical diagnostic radiology, although the most valuable information obtained by CT examinations has to be accounted for in any risk benefit analysis.

Quality Assurance (QA) programmes assess the performance of radiological equipment by verifying that all parameters believed to influence either the image quality (IQ) or patient dose (PD) lie inside the tolerance levels.
IQ PD are critical terms in QA. Producing images of consistently high quality with the lowest possible radiation exposure is the goal of QA. Although IQ represents the ultimate justification of QA, no single objective measurable index of IQ dose exist. Other methods are used to evaluate IQ. These methods are based on radiologist impression, visibility of anatomical landmarks and observer performance.

IQ and PD are opposing characteristics in a radiological examination. Generally, a compromise between IQ and PD is necessary, which takes into account the type of information needed and does not put the patient at risk through misdiagnosis.
Every single QC procedure consists in checking the efficiency of a part of the radiological equipment using a test tool whose result must be compared with standards. The characteristics of equipment form the basis for any relation between the results of the test and the status of the tested equipment. Other concepts of particular use are those concerning the physical quantities which are observed during test procedureand equipment control.

The expert system (ES) can be conceived as a guide to perform QC tests correctly, evaluate results, take corrective action if necessary.
Tests may be divided into 2 main categories, those which assess the imaging performance of an imaging system through the measure of one of more parameters of the image or evaluate the patient risk through the measure of one or more doses. (IQ and PD tests) and those concerned with the measurement of a single aspect of equipment performance per se. This aspect certainly affects both IQ and PD, but no attempt is made to correlate the latter 2 with the former. Tests of the first group tell something about the final result of the radiologica l examination, but tracing back to the single causes of that results is very different. Tests of the second group ascertain changes or drifts in single parameters, but not the extent to which they affect IQ and PD.

As a consequence of the factorisation of the search space an incremental approach to the development of the ES is possible. The first prototype will be devoted to the management of the phantom test in mammography.
THE RESEARCH WILL HAVE THREE MAIN PARTS :
A. THE DATA COLLECTED DURING THE FORMER CONTRACT WILL BE ANALYSED ACCORDING TO THE FOLLOWING STEPS:
- PERFORM A COMPARISON BETWEEN DOSES MEASURED WITH TLDS AND THOSE CALCULATED FROM ENTRANCE SKIN DOSES BY USING MONTE CARLO CONVERSION FACTORS. THE EXPECTED RESULTS OF THIS COMPARISON SHALL VERIFY THE RELIABILITY ON MONTE CARLO METHODS FOR THE EVALUATION OF DOSES TO COMPACT ORGANS AND THE NECESSITY OF CARRYING OUT DIRECT MEASUREMENTS OF DOSES TO COMPACT ORGANS, WHEN THE GENETICALLY AND SOMATICALLY SIGNIFICANT DOSES OF A POPULATION HAVE TO BE ASSESSED;
- GIVE NEW CONSIDERATION TO CORRELATIONS AMONG ENTRANCE SKIN DOSE, EXPOSURE-AREA PRODUCT AND BEAM AREA;
-EXTEND POSSIBILITIES, BOTH TECHNICAL AND RELATED TO PROCEDURE, FOR PATIENT DOSE REDUCTION THROUGH THE COMPARISON WITH ANALOGOUS DATA OF THE GROUPS AT NRPB, CHILTON (UK) AND CEPN, FONTENAY-AUX-ROSES (FRANCE).

B. THROUGH BOTH NEW MEASUREMENTS AND THE ANALYSIS OF PREVIOUS RESULTS THE INFLUENCE OF VARIOUS PARAMETERS ON PATIENT EXPOSURE WILL BE EVALUATED, INCLUDING NUMBER, FIELD SIZE AND PROJECTION OF EXPOSURES, APPROPRIATE TECHNICAL FACTORS AND SHIELDS; IN THIS CONNECTION SPECIAL CONSIDERATION WILL BE GIVEN TO THE DATA ESTABLISHMENT FOR THE PROFESSIONAL TRAINING OF RADIOLOGICAL WORKERS. IN ADDITION, ALTERNATIVE RADIOLOGICAL PROCEDURES WHICH DELIVER LOWER DOSES WITH THE SAME DIAGNOSTIC INFORMATION WILL BE TESTED IN SOME RADIOLOGICAL DEPARTMENTS AND ORGAN DOSES WILL BE COMPARED. THIS WORK WILL BE CARRIED OUT IN CLOSE COLLABORATION WITH THE RADIOLOGISTS OF THE UNIVERSITY OF TRIESTE.

C. A METHOD FOR THE DETERMMINATION OF THE RISK TO PATIENTS UNDERGOING CT EXAMINATIONS WILL BE DEVELOPED IN COLLABORATION WITH THE GROUPS NRPB, CHILTON (UK), CEPN, FONTENAY-AUX-ROSES (F) AND GSF, MUENCHEN (FRG). THE ENERGY IMPARTED TO THE PATIENT DURING CT SCANNING WILL BE EXPERIMENTALLY EVALUATED FOR EACH BEAM, SLICE THISCKNESS AND PART OF THE BODY, BY IRRADIATING TLDS MERGED INTO A TISSUE-EQUIVALENT PHANTOM. THESE MEASUREMENTS WILL ALLOW THE ASSESSMENT OF THE COLLECTIVE SOMATIC RISK, WHERE A FREQUENCY SURVEY OF CT-EXAMINATIONS INCLUDING NUMBER, THICKNESS AND LOCATION OF SLICES IN THE BODY, WILL HAVE BEEN CARRIED OUT.

Champ scientifique

Programme(s)

FP1-RADPROT 6C - Multiannual research and training programme (Euratom) in the field of radiation protection, 1985-1989

Thème(s)

Data not available

Appel à propositions

Data not available

Régime de financement

CSC - Cost-sharing contracts

Coordinateur

Unità Sanitaria Locale

Contribution de l’UE

Aucune donnée

Adresse

Via Colugna 50
33100 Udine
Italie

Coût total