[Dose delivered to the patient by megavoltage cone beam computed tomography imaging]. / Dose délivrée au patient lors de l'acquisition d'images par tomographie conique de haute énergie.

PURPOSE: To study the dose delivered by a megavoltage cone beam computed tomography imaging system (MVCBCT) installed on a Oncor Impression linac (Siemens). MATERIALS AND METHODS: The acquisition of MVCBCT images was modelled in a treatment planning system by 67 photon beams (6 MV). A study was conducted to: compare the calculated and measured dose at the centre of a cylindrical phantom; compare the calculated and measured dose distributions in the Alderson-Rando phantom (pelvis); study the influence of MVCBCT image acquisition for the repositioning of a prostate cancer patient treated by 3D conformal radiotherapy (prescribed dose of 74 Gy), on the dose-volume histograms (DVH) for the treatment plus seven MVCBCT (protocol D1-3 and weekly), treatment plus 37 MVCBCT (one for each day of treatment). RESULTS: The difference between calculated and measured doses at the centre of the cylindrical phantom was less than 3%. A deviation of 7% maximum was found between the dose distribution calculated in the Rando phantom and the measured doses normalized at the beam isocentre. The dose delivered at the isocentre was equal to 3,7 cGy for a "5 MU" protocol, with a maximum dose of 6 cGy. In the case of the patient considered, the acquisition of 37 MVCBCT corresponded to an additional mean dose to the PTV of 1.2 Gy for a protocol "5MU" with a significant influence on the DVH. CONCLUSION: In view of this study, it appears that the doses delivered in frequent use of MVCBCT must be taken into account by the radiation oncologist in assessing the therapeutic dose delivered to the target volume and organs at risk.

[Quality control of megavoltage cone beam CT imaging system]. / Contrôle qualité d'un système d'imagerie cone beam mégavoltage.

PURPOSE: This paper presents the development of a protocol for quality control of a megavoltage cone beam CT imaging system (MVCB) mounted on a Siemens Oncor 6MV linear accelerator. MATERIALS AND METHODS: Several parameters were controlled on the MVCB system: (1) the initial geometric calibration of the system; (2) the quality of the images (geometric distortion, uniformity, spatial resolution, low contrast resolution) for various protocols; (3) the correspondence between the intensity of voxels and electronic densities; (4) the dose delivered when achieving a MVCB. These tests were done mainly with two cylindrical phantoms specific to the quality control (QC) of a MVCB system, supplied by Siemens, and with the Catphan 600 phantom (The Phantom Laboratory) and Quasar Multipurpose Body phantom (Modus Medical Devices Inc). RESULTS: The results of the quality control of the images were within the tolerances. The use of the Catphan 600 phantom was inadequate for the QC of MVCB images. These tests also highlighted the need to correct the MVCB images for the "cupping artefact" for dose calculation purpose. CONCLUSION: The initial characteristics of the MVCB imaging system were established. Such testing also provided the assessment of the influence of various parameters on the image quality as well as the associated dose delivered during their acquisition, and emphasized the corrections needed to use MVCB images for dose calculation.

Experimental derivation of wall correction factors for ionization chambers used in high dose rate 192Ir source calibration.

At present there are no specific primary standards for 192Ir high dose rate sources used in brachytherapy. Traceability to primary standards is guaranteed through the method recommended by the AAPM that derives the air kerma calibration factor for the 192Ir gamma rays as the average of the air kerma calibration factors for x-rays and 137Cs gamma-rays or the Maréchal et al. method that uses the energy-weighted air kerma calibration factors for 250 kV x rays and 60Co gamma rays as the air kerma calibration factor for the 192Ir gamma rays. In order to use these methods, it is necessary to use the same buildup cap for all energies and the appropriate wall correction factor for each chamber. This work describes experimental work used to derive the A(W) for four different ionization chambers and different buildup cap materials for the three energies involved in the Maréchal et al. method. The A(W) for the two most common ionization chambers used in hospitals, the Farmer NE 2571 and PTW N30001 is 0.995 and 0.997, respectively, for 250 kV x rays, 0.982 and 0.985 for 192Ir gamma rays, and 0.979 and 0.991 for 60Co gamma rays, all for a PMMA build-up cap of 0.550 gm cm(-2). A comparison between the experimental values and Monte Carlo calculations shows an agreement better than 0.9%. Availability of the A(W) correction factors for all commercial chambers allows users of the in-air calibration jig, provided by the manufacturer, to alternatively use the Maréchal et al. method. Calibration laboratories may also used this method for calibration of a well-type ionization chamber with a comparable accuracy to the AAPM method.

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.

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/).

Monte Carlo calculations of the ionization chamber wall correction factors for 192Ir and 60Co gamma rays and 250 kV x-rays for use in calibration of 192Ir HDR brachytherapy sources.

As in the method for the calibration of 192Ir high-dose-rate (HDR) brachytherapy sources, the ionization chamber wall correction factor A(w), is needed for 192Ir and 60Co gamma rays and 250 kV x-rays. This factor takes into account the variation in chamber response due to the attenuation of the photon beam in the chamber wall and build-up cap and the contribution of scattered photons. Monte Carlo calculations were performed using the EGS4 code system with the PRESTA algorithm, to calculate the A(w) factor for 51 commercial ionization chambers and build-up caps exposed to the typical energy spectrum of 192Ir and 60Co gamma rays and 250 kV x-rays. The calculated A(w) correction factors for 192Ir and 60Co sources and 250 kV x-rays agree very well to within 0.1% with published experimental data (the statistical uncertainty is less than 0.1% of the calculated correction factor value). For the 192Ir sources, A(w) varies from 0.973 to 0.993 and for the 250 kV x-rays the minimum value of A(w) for all chambers studied is 0.983. The calculated A(w) correction factors can be used to calculate the air kerma calibration factor of HDR brachytherapy sources, when interpolative methods are considered, contributing to the reduction in the overall uncertainties in the calibration procedure.

Perturbation corrections for flat and thimble-type cylindrical standard ionization chambers for 60Co gamma rays: Monte Carlo calculations.

The use of an ionization chamber for absorbed dose determinations in a medium requires one to take into account perturbation corrections due to the presence of the chamber cavity in the medium. Evaluation of these corrections for perturbation and their variation with depth in the medium has been performed for a flat cylindrical and a cylindrical (thimble-type) ionization chamber placed in a graphite phantom irradiated by a 60Co gamma beam using Monte Carlo calculations (EGS4 system with correlated sampling variance reduction technique). The results of these calculations agree with published experimental and theoretical data to better than 0.18%, with a statistical uncertainty of less than 0.17%.