[The EQUAL-ESTRO external quality control laboratory in France]. / Le laboratoire de contrôle de qualité externe EQUAL-ESTRO en France.

PURPOSE: The EQUAL-ESTRO laboratory was set up in 1998 to perform external audits of radiotherapy beams for all the European centres. Until the end of the year 2003, it was funded by EC projects. The external quality control is based on measurements performed with thermoluminescent dosimeters (TLD) sent by postal mail to the participating centre in order to be irradiated on axis in reference conditions and in conditions close to clinical conditions for photon and electron beams. The EQUAL-ESTRO laboratory also started in 2002, a new TLD control allowed to check the dosimetry of complex clinical fields for photon beams with multileaf collimator (MLC). New geometric and dosimetric checks have also been developed for brachytherapy. MATERIAL AND METHODS: The participating centre irradiates TLDs (7LiF: Na, mg, Ti, TLD 937 [Philitec]) in water at a dose of 2 Gy, calculated with the TPS used clinically, following the EQUAL-ESTRO protocol. RESULTS: Since the beginning of the activities, 46% of the French radiotherapy centres and 55% of the European radiotherapy centres applied to participate to the EQUAL quality control programme. In France, the EQUAL-ESTRO laboratory checked, from 1998 and June 2004, 283 photon beams, 180 electron beams and 61 photon beams with MLC. For all the French beams checks, the following results have been observed: for the photon beams, the results show that about 1% of the measured doses in the reference conditions on axis have been detected outside the tolerance level (deviation between the measured dose and the stated dose > +/- 5%) after a first or a second check. For points checked in photon beams with wedge filter, 2.5% of the beams checked show a deviation > +/- 5% after a first or a second check. For the electron beams, the check has been set up in January 1999. For the 180 electron beams checked, 5% of the measured doses in the reference conditions have been found outside the tolerance level (> +/- 5%). CONCLUSION: These results show clearly the importance of the quality control in radiotherapy in the frame of an external audit. This audit should now be a part of a quality assurance programme in the radiotherapy centres in addition to the internal quality control as planned in the French law (02/03/2004) concerning the external quality control conditions of the radiotherapy treatment units.

The ESTRO-EQUAL Quality Assurance Network for photon and electron radiotherapy beams in Germany.

BACKGROUND: In 1998 an ESTRO Quality Assurance Network for radiotherapy (EQUAL) has been set up for 25 European countries for photon and electron beams in reference and non-reference conditions. MATERIAL AND METHODS: Measurements are done using LiF powder (DTL937-Philitech, France) that is processed with the PCL3 automatic reader (Fimel-PTW). The participating centers irradiate the TLDs with an absorbed dose of 2 Gy according to the clinical routine. RESULTS: Until September 2000 EQUAL has checked 135 photon beams (including the beams rechecked) from 51 radiotherapy centers in Germany out of 86 accepted centers. The results show that 2% of the beam outputs in reference conditions and 3% of the percentage depth doses are outside the tolerance level (deviation > +/- 5%). 6% of the beam output variations and of the wedge transmission factors show deviations > +/- 5%. The global analysis of results shows deviations > +/- 5% in at least one parameter for 18 beams out of the 135 beams checked. Five rechecked beams present one "real dosimetric" problem in one or more parameters, corresponding to 4% of the 114 beams for which the deviations cannot be attributed to set-up errors.--The EQUAL network has checked 89 electron beams in Germany. The results show that all beam outputs checked are within the tolerance level. The standard deviation for the beam output in reference conditions is 2.0% and 2.2% for the beam output for the others field sizes. The percentage of deviations > 3% and < or = 5% for the reference beam output is higher for electron beams than for photon beam checks. Therefore the electron beam calibration and the TPS algorithms should be improved to increase the accuracy of the patient dosimetry for radiotherapy. CONCLUSION: EQUAL program demonstrates a consistency in radiotherapy dosimetry for photon and electron beams resulting in a satisfying accuracy of the dosimetry in Germany.

External audits of electron beams using mailed TLD dosimetry: preliminary results.

AIM: A feasibility study has been performed to investigate the possibility of using mailed thermoluminescence dosimetry (TLD) for external audits of clinical electron beams in Europe. METHODS: In the frame of the EC Network Project for Quality Assurance in Radiotherapy, instruction sheets and mailing procedures have been defined for mailed TLD dosimetry using the dedicated holder developed by a panel of experts of the International Atomic Energy Agency (IAEA). Three hundred and thirty electron beam set-ups have been checked in the reference centres and some local centres of the EC Network Project and in addition through the centres participating to the EORTC Radiotherapy Group trial 22922. RESULTS: The mean ratio of measured dose to stated dose is 0.2% and the standard deviation of measured dose to stated dose is 3.2%. In seven beam set-ups, deviations greater than 10% were observed (max. 66%), showing the usefulness of these checks. CONCLUSION: The results of this feasibility study (instruction sheets, mailing procedures, holder) are presently endorsed by the EQUAL-ESTRO structure in order to offer in the future to all ESTRO members the possibility to request external audits of clinical electron beams.

Energy correction factors of LiF powder TLDs irradiated in high-energy electron beams and applied to mailed dosimetry for quality assurance networks.

Absorbed dose determination with thermoluminescent dosimeters (TLDs) generally relies on calibration in 60Co gamma-ray reference beams. The energy correction factor fCo(E) for electron beams takes into account the difference between the response of the TLD in the beam of energy E and in the 60Co gamma-ray beam. In this work, fCo(E) was evaluated for an LiF powder irradiated in electron beams of 6 to 20 MeV (Varian 2300C/D) and 10 to 50 MeV (Racetrack MM50), and its variation with electron energy, TLD size and nature of the surrounding medium was also studied for LiF powder. The results have been applied to the ESTRO-EQUAL mailed dosimetry quality assurance network. Monte Carlo calculations (EGS4, PENELOPE) and experiments have been performed for the LiF powder (rho = 1.4 g cm3) (DTL937, Philitech, France), read on a home made reader and a PCL3 automatic reader (Fimel, France). The TLDs were calibrated using Fricke dosimetry and compared with three ionization chambers (NE2571, NACP02, ROOS). The combined uncertainties in the experimental fCo(E) factors determined in this work are less than about 0.4% (1 SD), which is appreciably smaller than the uncertainties up to 1.4% (1 SD) reported for other calculated values in the literature. Concerning the Varian 2300C/D beams, the measured fCo(E) values decrease from 1.065 to 1.049 +/- 0.004 (1 SD) when the energy at depth in water increases from 2.6 to 14.1 MeV; the agreement with Monte Carlo calculations is better than 0.5%. For the Racetrack MM50 pulsed-scanned beams, the average experimental value of fCo(E) is 1.071 +/- 0.005 (1 SD) for a mean electron energy at depth Ez ranging from 4.3 to 36.3 MeV: fCo(E) is up to 2% higher for the MM50 beams than for the 2300C/D beams in the range of the tested energies. The energy correction factor for LiF powder (3 mm diameter and 15 mm length) varies with beam quality and type (pulsed or pulsed-scanning), cavity size and nature of the surrounding medium. The fCo(E) values obtained for the LiF powder (3 mm diameter and 15 mm length) irradiated in water, have been applied to the EQUAL external audit network, leading to a good agreement between stated and measured doses, with a mean value of 1.002 +/- 0.022 (1 SD), for 170 beam outputs checked (36 electron beam energies) in 13 'reference' radiotherapy centres in Europe. Such fCo(E) data improve the accuracy of the absorbed dose TLD determination in electron beams, justifying their use for quality control in radiotherapy.

The ESTRO-QUALity assurance network (EQUAL).

BACKGROUND AND PURPOSE: ESTRO has set up a Quality Assurance network (EQUAL) to check the dose delivered on axis in reference and non-reference conditions for external radiotherapy. The external audits covered by the network are based on measurements made with mailed thermoluminescent dosimeters (TLD). MATERIAL AND METHODS: The TLD consist of LiF powder type DTL 937 read with a PCL 3 automatic TLD reader. The participating centres are instructed to deliver to the TLDs absorbed doses of 2 Gy calculated with the Treatment Planning System used in clinical routine. A maximum of three photon energies by participating centre have been checked with 10 on-axis points per beam. The quantities checked include the reference beam output, beam output variation with collimator opening, depth dose data and wedge transmission factor. RESULTS: During the 1998 EQUAL programme 102 centres have been checked corresponding to 235 beams (28 (60)Co beams and 207 X-ray beams). About 3% of the outputs in reference conditions show deviations outside tolerance level (>+/-5%). A similar rate of deviation is noted for the percentage depth doses. A rate of deviation (6%) has been observed for the beam output variation (open and wedged beams) and the wedge transmission factor. The analysis of the results shows that for 24 out of the 102 centres, a deviation outside tolerance level is observed at least in one point, mainly for the large and rectangular field sizes and for the wedged beams. CONCLUSIONS: The results for the EQUAL programme show the importance of a quality assurance network in Radiotherapy especially for the non reference points even if they are only located on the beam axis (In order to participate in this network, please contact EQUAL secretariat or download the attached application form ESTRO web site: Dr I.H. Ferreira or Mrs Aline Mechet, EQUAL-ESTRO, Physics Department, Institut Gustave-Roussy 39 Rue Camille Desmoulins, F-94805 Villejuif Cedex, France. e-mail:equal@igr.fr or http://www.estro.be/).

Variation of relative transit dose profiles with patient-detector distance.

BACKGROUND AND PURPOSE: In view of using portal images for exit dosimetry, an experimental study is performed of relative transit dose profiles at different distances behind patients (and phantoms) and of their relation to the exit dose profile. MATERIALS AND METHODS: Irregular, homogeneous polystyrene phantoms with a variable thickness to simulate head and neck (H&N) treatments (6-MV photon beam) are investigated by ionization chamber measurements performed close to the exit surface and at various distances behind the phantom (10, 20 and 30 cm). Similar measurements are performed for a rectangular phantom with large inhomogeneities (A1 and air). For one irregular homogeneous phantom and an irregular phantom containing an A1 inhomogeneity, ionization chamber measurements are performed at the exit surface, and a portal film image is taken at 30 cm behind the phantom. Portal films of a patient treated for a head and neck malignancy are evaluated for different air gaps behind the patient. RESULTS: For the irregular phantoms, deviations up to 15% and more are observed between the exit dose profile (along the shaped surface of the phantom) and the transit profile close to the phantom (perpendicular to the beam axis). There is, however, a good agreement--within 3%--between the exit profile and the transit profile at 30 cm. For the rectangular, inhomogeneous phantom, the deviation between the exit profile and the transit dose profile at 30 cm does not exceed 5%; transit dose profiles overestimate the exit dose for the air cavity and underestimate the dose for the A1 inhomogeneity. Measurements on portal films of a H&N patient for different air gaps confirm the order of magnitude of the difference observed between transit dose profiles close to the patient and transit dose profiles at some distance behind the patient. CONCLUSIONS: For 6-MV photon beam treatments with significant thickness variations (H&N), large variations (> 10%) are observed in transit dose profiles as a function of the air gap between the patient and the portal film. For this energy, a good agreement is found between the exit profile and the transit profile at about 30 cm behind the patient.

Radiotherapy of pelvic malignancies: impact of two types of rigid immobilisation devices on localisation errors.

BACKGROUND AND PURPOSE: To determine the distribution of set-up errors for patients treated with and without two rigid partial immobilisation devices for pelvic malignancies. MATERIALS AND METHODS: 30 patients receiving pelvic irradiation with a four field technique underwent a total of 524 portal films. The patients are divided into 3 cohorts of 10 patients. The first cohort is treated on a standard treatment couch without immobilisation device (NI); the second and third cohorts are treated with a custom-made immobilisation device used in an attempt to improve set-up accuracy: an Alpha-Cradle mattress (AM) or an Orfit cast (OC). Set-up deviations are analysed in the X, Y, Z directions of a fixed coordinate system, corresponding to the lateral, cranio-caudal and antero-posterior direction, respectively. RESULTS: Considering the percentage of discrepancies < or = 5 mm between the simulation films and the portal films as a measure of set-up accuracy, immobilisation devices seem to increase accuracy: 88.5% (X) 79% (Y) and 100% (Z) with AM; 84% (X-Y), 97.5% (Z) with OC and only 76.5% (X), 40% (Y) and 65.5% (Z) for NI. The distribution of the systematic set-up errors for the three patient cohorts, defined as the mean patient displacement for the treatment course, had a mean and a standard deviation of (0.7 +/- 2.7) mm in the X-axis, (-5.5 +/- 2.6) mm in the Y-axis and (-0.9 +/- 2.2) mm in the Z-axis when no immobilisation is added; (0.8 +/- 1.7) mm, (-2 +/- 2.7) mm and (0.3 +/- 0.4) mm for the Alpha-Cradle group; (0.3 +/- 1.4) mm, (0.5 +/- 1.1) mm and (0.5 +/- 0.6) mm for the Orfit cast group. The distribution of random errors about the mean approximated a normal distribution and the standard deviations are 4.4 mm (X), 4.2 mm (Y) and 4.8 mm (Z) for NI; 3.3, 3.5 and 2.5 mm for the AM; 3.4, 3.3 and 2.7 mm for the OC. CONCLUSIONS: The two rigid immobilisation devices improve the reproducibility of a given pelvic field but there is a small benefit comparative to the cost and the cumbersome place of the devices.

Methods for beam data acquisition offered by a mini-phantom.

Mini-phantoms are an important tool for measurement of basic head scatter parameters in high-energy photon beams, and recently they have also been used for beam quality specification. Therefore the feasibility and reliability of basic beam parameter acquisition using only a mini-phantom is checked in 6, 18 and 25 MV photon beams. These parameters include head scatter correction factors, phantom scatter correction factors, total scatter correction factors, wedge factors, off-axis ratios, as well as beam attenuation coefficients and beam hardening coefficients. In order to specify beam quality variations and beam quality modifications by a wedge, two different methods are compared: the first method uses a constant source to chamber distance of 1 m, the second method refers to narrow beam geometry. Mu values derived with two different beam quality specification methods show a systematic deviation. However, relative variations of the attenuation coefficient within the beam and the associated beam quality modifications observed with the two methods show good agreement in open and wedged beams. Phantom scatter correction factors are calculated from measured head scatter correction factors and total scatter correction factors as well as from attenuation coefficients. Measured and calculated phantom scatter correction factors agree within 1% with the values given in literature. For 18 and 25 MV photon beam, wedge factors measured in water or in the mini-phantom agree within 0.5%, but maximum deviations of approximately 1.5% are observed at 6 MV for the largest field sizes. It is demonstrated that the determination of several beam data related to full scatter conditions does not necessarily require the availability of a full scatter phantom. The mini-phantom is a reliable but very cheap and simple tool. It offers versatile possibilities to measure, check and verify basic beam parameters in high-energy photon beams.

A quality assurance network in Central European countries--radiotherapy infrastructure.

A survey of the infrastructure in radiotherapy centres in three Central European countries has been performed as a first step in the development of a quality assurance network. Data concerning radiotherapy equipment, staff and number of patients treated in most of the radiotherapy centres from Czech Republic, Poland and Hungary were collected at the beginning of 1994. Equipment data have shown that 35% of 182 treatment units are conventional x-ray units, 35% 60Co units, 19% linear accelerators, 7% 137Cs units and 4% betatrons. About 47% of high energy units are older than 12 years and about 20% older than 21 years. An important number of centres still have no simulator which would constitute an important handicap to carry out adequate radiotherapy. The number of treatment planning systems has also been registered; 44% being PC-based systems with locally developed software. Large variations are observed in the number of patients treated per year, per high energy unit, but 12/47 centres treat more than 700 patients per year and unit. On the average, staffing seems adequate in numbers though there are wide variations. The main limitation of radiotherapy infrastructure in the Central European countries is the low number of linear accelerators and simulators and the advanced age of therapy units.

Assessment of dose inhomogeneities in clinical practice by film dosimetry.

AIM: To use portal images acquired in routine circumstances for assessment of midplane dose variations in the patient. MATERIAL AND METHODS: Optical density readings are performed on routinely acquired Verification films of breast and ear-nose-throat (ENT) cancer patients and these readings are converted into relative doses with the sensitometric curve. ( 1 ) The impact of redistribution is evaluated on films taken close to the patient exit surface and at routine focus film distance. (2) Midplane doses are estimated from film readings to assess dose variations in the patient. The influence of wedges is evaluated. Film measurements doses are compared with calculated exit doses. RESULTS: (1) In regions with large variations in the distance between the patient exit surface and the film but without inhomogeneities in tissue density, the relative doses distributions read on films acquired at large focus-film-distance (FFD) are proportional to exit doses. In regions with flat exit surfaces but with inhomogeneities in tissue density, the redistribution has only a small impact. (2) Large variations in relative midplane doses were found in both breast (85-115%) and ENT (-3.6 to +15%) patients. The application of a wedge was shown to increase dose homogeneity in the midplane. A good agreement (differences < 3%) was found between exit doses obtained from film readings and exit doses calculated with the treatment planning system (TPS). CONCLUSION: Films acquired in routine circumstances at large FFD can be used to obtain information on exit doses and to assess midplane doses in breast and ENT, without the use of a TPS. Film dosimetry can also provide a quality assurance tool to check actually delivered doses in patients by comparing exit doses estimated on film to expected exit doses calculated by the TPS.

Dosimetric comparison of an integrated multileaf-collimator versus a conventional collimator.

The dosimetric characteristics of both a conventional GE collimator (CC) and a GE multileaf collimator (MLC) are compared for different photon beam energies. The integrated GE MLC consists of 32 pairs of tungsten leaves, replacing the lower pair of jaws of the conventional collimator. Measurements were performed with the conventional collimator before this collimator was replaced by the MLC. All parts of the accelerator except the collimator remained the same. Leakage and transmission measurements show good agreement with the manufacturer's specification, stating a leakage between leaves of less than 1% for all energies and a transmission through leaves of less than 0.5%. The dosimetric characteristics of both collimators are very similar for square and rectangular fields. No significant change in beam quality, beam attenuation and depth of maximum dose could be detected within the measurement accuracy. The MLC output ratio variation is smaller than the one measured with the CC. The penumbra difference in the Y direction is less than 0.5 mm at a depth of 5 cm in phantom; in the X direction the penumbra is 1 mm larger for the MLC due to the rounded leaf fronts. As the two leaf banks replace the lower pair of collimator jaws the distance from the collimator end to the isocentre is similar for the two collimators, therefore the MLC does not reduce the flexibility of the treatment unit. For symmetrical and regular collimator settings the MLC can be treated as the CC.

Output ratios in a miniphantom for asymmetric fields shaped by a multileaf collimator.

The integrated GE multileaf collimator (MLC) provides the ability to achieve 'double' asymmetric fields: each of the 64 leaves allow an over-axis travel of 10 cm and the Y-jaws allow 20 cm. A formalism has recently been proposed by the authors to calculate the output ratio in a miniphantom for this type of MLC by the product of independent leaf and jaw correction factors. The original proposed formalism was restricted to regular or irregular fields including the collimator rotational axis. Introducing 'reduced coordinates' for the correction factors in the present work this formalism is extended to asymmetric fields where central leaves or jaws overlap the collimator axis. The extended formalism is applied to asymmetric square, rectangular and irregular fields. For all fields checked at a given off-axis position, measured and calculated output ratios agree within 1% for 6, 18 and 25 MV photon beams. To relate output ratios normalized to off-axis points with output ratios on-axis, off-axis ratios are derived from film and miniphantom measurements. Both off-axis ratios agree to within 1% for 6 and 25 MV photon beams; a maximum deviation of 1.3% is observed at 18 MV. Calculated products of output ratios and off-axis ratios derived from films are compared with measurements for asymmetric square, rectangular and irregular fields, and agree mostly within 1% for all energies checked; maximum deviations of 1.3 and 1.6% are observed for 6 and 18 MV photon beams.

Measurements of basic parameters in wedged high-energy photon beams using a mini-phantom.

Basic dosimetric quantities necessary to specify wedged beans (beam quality, wedge factors, output ratios) are obtained by measurements performed in a narrow coaxial mini-phantom for 6, 18 and 15 MV photon beams. To express beam quality, an attenuation coefficient mu is derived from measurements in a mini-phantom at 20 and 10 cm depth. Wedge factors and output ratios are measured as a function of field size at 10 cm water-equivalent depth. In open beams one observes beam softening with increasing distance from the collimator axis for all energies. With an inserted wedge a beam hardening is observed at 6 MV. This beam hardening decreases at 18 MV while at 25 MV a slight beam softening is detected. Larger variations of output ratios with field sizes are observed with a wedge than without a wedge. An equivalent square formula for head-scatter factors can be used with a good accuracy for rectangular wedged fields. For irregular wedged fields a method is proposed to calculate the product of the output ratio and the wedge factor. Measurements and calculations agree within 1% for all irregular wedged fields checked.

External audit of photon beams by mailed film dosimetry: feasibility study.

A feasibility study for mailed film dosimetry has been performed. The global reproducibility of the method is better than 2%. It is shown that the normalized sensitometric curve does not depend on photon beam quality in the range from Co-60 gamma-rays to 18 MV x-rays, although the dose per optical density decreases when the energy increases. The fading of the latent image before film processing is only 3% per month and the normalized sensitometric curve is not modified after a period of 51 days between irradiation and processing. Sets of films were mailed to three different institutes for irradiation and returned for processing and evaluation after more than two months in order to verify that mailing of irradiated and unprocessed films does not produce unwanted artefacts. Finally the feasibility of external audits with mailed film dosimetry is illustrated by comparison of beam profiles measured with films and ionization chambers in a polystyrene phantom.

A formalism to calculate the output ratio in a mini-phantom for a GE multileaf collimator.

A GE multileaf collimator (MLC) has been recently installed on a Saturne 43 and is used with 6, 18 and 25 MV photon beams. In the integrated GE MLC the lower pair of jaws in the X direction is replaced by 32 pairs of computer controlled opposed tungsten leaves. The influence of each set of leaves on the output ratio is smaller than the influence of the jaws it replaces (8% instead of 10%). For irregular fields it is necessary to evaluate the influence on the output ratio of each independent leaf. It is assumed that each leaf and each jaw have an independent influence on the output ratio. According to this assumption leaf correction functions are derived from measurements as a function of their X position. A second-order correction (less than 1%) has to be applied for the jaw positions. The output ratio in a mini-phantom for a given irregular field can be calculated by the product of the 64 leaf and two jaw correction factors. The formalism is applied to symmetric square and rectangular fields, asymmetric and irregular fields. For all fields checked, the calculated and the measured output ratios agree within 1%. Furthermore the simple formula suggested by Vadash and Bjarngard for square field sizes equivalent to rectangular fields can be used with a good accuracy with an A value of 1.6 for the three energies used. The proposed formalism to calculate the output ratio in a mini-phantom is restricted to fields including the collimator axis with a minimum distance of 2 cm between any leaf and the collimator axis.

Quality assurance in radiotherapy: the importance of medical physics staffing levels. Recommendations from an ESTRO/EFOMP joint task group.

The safe application of ionising radiation for diagnosis and therapy requires a high level of knowledge of the underlying processes and of quality assurance. Sophisticated modern equipment can be used effectively for complicated diagnostic and therapeutic techniques only with adequate physics support. In the light of recent analyses and recommendations by national and international societies a joint working group of representatives from ESTRO (European Society for Therapeutic Radiology and Oncology) and from EFOMP (European Federation of Organisations for Medical Physics) was set up to assess the necessary staffing levels for physics support to radiotherapy. The method used to assess the staffing levels, the resulting recommendations and examples of their practical application are described.

Quality assurance in brachytherapy: the role of the ICRU in achieving uniformity in dose and volume specification for reporting.

A quality assurance programme in brachytherapy implies a general consensus on the method of dose and volume specification for reporting. This in turn implies a consensus on certain definitions of terms and concepts. For several decades, the ICRU (International Commission on Radiation Units and Measurements) has been actively involved in an effort to reach a consensus between different brachytherapy centres worldwide, and to improve uniformity in reporting. The ICRU has prepared two reports containing recommendations for reporting brachytherapy treatments. The first, report #38 published in 1985, deals with intracavitary therapy in gynecology. The second deals with interstitial therapy, and is now in press. A summary of these two ICRU reports is presented here. Some definitions of terms and concepts are first recalled and discussed: Total Reference Air Kerma (TRAK), gross tumor volume (GTV), clinical target volume (CTV), treated volume, mean central dose, dose uniformity parameters, etc. Specific recommendations for reporting interstitial and intracavitary brachytherapy are then presented.

The influence of the IAEA standard holder on dose evaluated from TLD samples.

The EROPAQ quality assurance project started in 1994 with TLD postal dose intercomparisons for photon beams used in 47 radiotherapy centres in the middle and eastern European countries. The photon dose intercomparisons include beam output checks and beam quality checks. Since an acceptance limit of +/- 3% was chosen for the EROPAQ intercomparisons, any systematic error in dose evaluation by the measuring centre should be minimized. The standard IAEA TLD holder is used in the intercomparisons. In this work the magnitude of the holder attenuation is evaluated and holder corrections derived both for output and photon beam quality checks for beam qualities ranging from Co-60 up to 18 MV. The correction of the dose at the depth of reference does not exceed 1% and decreases with increasing beam energy. A correction of the beam quality ratio of the order of 1% and that is independent of the photon energy has been determined by both calculation and measurements. These corrections are consistent with the preliminary data published by the IAEA and EC QA network.

A European quality assurance network for radiotherapy: dose measurement procedure.

In the frame of the experimental implementation of a European quality assurance network for external radiotherapy, the methodology in the European Measuring Centre (MC) is presented. Mailed TL dosimeters are used for the check of the beam output and of the beam quality of photon beams. The thermoluminescent material is PTL 717 LiF powder. The readings were first performed on a manual, and then on an automatic reader, with standard deviations of the mean of 0.7% for one dosimeter. Corrections for supralinearity and for the energy dependence of the dosimeter response are applied. An original method has been developed to correct for the variation of the LiF response as a function of time. It is shown that the sensitivity of the powder changes during storage, leading to a kind of 'inverse fading'. The global uncertainty of the TL postal measurement procedure is estimated to be about 1.5% for the 60Co beams and 2% for the x-ray beams. Intercomparisons with the IAEA and with the EORTC have shown an agreement better than 2% for all energies. It can be concluded that the results of the MC are suitable for the requirements of a European quality assurance network.