Quality Control in Mammography: Results from a study in Recife, Brazil. H. J. Khoury, V. S. de Barros, M. Sampaio

Transcription

1 Quality Control in Mammography: Results from a study in Recife, Brazil H. J. Khoury, V. S. de Barros, M. Sampaio Grupo de Dosimetria e Instrumentação, Universidade Federal de Pernambuco, Rua Prof. Luiz Freire, 1000, Recife, Pernambuco, Brazil. khoury@ufpe.br Abstract. A radiological screening or diagnostic unit has the primary aim of providing images with a maximum amount of diagnostic information while keeping breast radiation dose as low as reasonably achievable (ALARA). For this purpose it is essential that the whole imaging system be fully optimized. Optimization of image quality is particularly important in mammography. This paper presents results of a quality control (QC) program implemented in a mammography center of Recife, NE Brazil, with focus on patient dose and image quality. The QC program consisted of four different components: a) equipment testing, b) reject analysis, c) sensitometric variability analysis of the automatic processor, and d) dose evaluation to the standard phantom. Sources of radiological quality test protocols were the Brazilian Ministry of Health and the European Guidelines for Quality Assurance in Mammography Screening. Reject analysis was divided in periods between maintenances. Films used for diagnosis were also screened (artifacts, patient movement, film - too dark, too bright). The Entrance Surface Air Kerma and Mean Glandular Dose were determined with termoluminescent dosimeters for a 47mm PMMA phantom under automatic exposure mode and a loaded cassette in place for target film optical density measurements. Significant improvements were achieved with both simple corrective actions and some more technical interventions, such as tube voltage maintenance of the mammography unit by a local engineer. Reject analysis revealed a reduction from 16.1% to 0.6% of films rejected during the period July- December Results of phantom dose measurements were found to be mgy, higher than the current Brazilian recommended limits of 10 mgy, mainly because of AEC system inadequacies. 1. Introduction Quality assurance applied to the radio diagnostic practice is intrinsically related to medical ethics. It s primary goal is to guarantee the fulfillment of three basic criteria: a) Be necessary and appropriate to solve the clinical problem; b) Be able to produce images with enough information to solve the clinical problem; and c) Be optimized, in order that the screening or diagnostic examination results in the lowest possible radiation exposure, lower costs, and inconvenience to the patient. Brazilian National Ministry of Health has determined a series of quality criteria under Portaria 453 [1], to regulate and optimize public and private radio diagnostic installations in the country. It establishes that all medical X-ray equipment should be routinely verified to ensure adequate performance. Patient doses should also be monitored and reference values for incident air kerma at the skin position are provided for various radiological modalities. Assuming the importance of reducing patient doses and optimizing image quality, this paper describes the results of a quality control program (QC) implemented in a mammography center located in Recife, NE Brazil, with focus on patient dose and image quality. 2. Materials and Methods The medical center surveyed uses a mammography unit model Mk MCP (Emic Eletro Medicina Ind. Com. Ltda / Limex) equipped with a Varian M-113R tube, double focus (0.1 e 0.3 mm), molybdenum anode - molybdenum filter (Mo-Mo). Although this model has an Automatic Exposure Control System, it is not equipped with a post-exposure display of the tube load (mas). This facility, as many others in the region, uses a dedicated Macrotec MX-2 model film processor, not specifically designed for mammography film processing. The QC program consisted of four different components: a) Equipment testing, b) Reject analysis, c) Sensitometric variability analysis of the automatic processor; and d) dose evaluation for the standard phantom. In the first part, the Brazilian Ministry of Health protocols were used [1,2], along with the International Atomic Energy Agency (IAEA) ARCAL protocol for mammography [3] and the 1

2 European quality assurance protocol (CEC, 2001). The following tests were performed on the mammography unit: a. Collimation of the X-ray beam (edge of film-edge of bucky distance) b. Reproducibility and exactness of the tube voltage (KVp) c. Reproducibility of the exposure time d. Half-value layer measurement e. Reproducibility and linearity of the incident air kerma rate f. Automatic Exposure Control (AEC) System g. Maximum compression force h. Image quality evaluation using a mammography phantom Image quality evaluation was performed using the Phantom Mama mammography phantom (Centro de Diagnóstico Mamário, Santa Casa da Misericórdia, Rio de Janeiro/RJ), which is similar to the ACR accreditation phantom and is the most commonly available in the region. This test artifact simulates a 45 mm thick breast, with 50%-50% adipose-glandular tissue content. It is fitted with a low contrast scale, metal grids, high contrast microcalcifications, and low contrast structures which simulate masses and fibers. Automatic processor evaluation was done using a 21 step sensitometer and a Clamshell densitometer (Victoreen Nuclear Associates). Tolerance values were taken from the CEC protocol (2001), ± 0.30 DO (± 0.20 DO recommended) for contrast and speed indexes and ± 0.03 DO (± 0.02 DO recommended) for base+fog. Entrance surface air kerma (ESAK) was measured using TLD-100 termoluminescent dosimeters placed on the test phantom (equivalent to a 45 mm, 50:50, breast) and exposure made using AEC. Incident air kerma (INAK) was derived from ESAK using the following equation: Kae, Kai, = (1) B where B backscatter factor (KRAMER et al., [4]) K a,e entrance surface air kerma (including backscatter) incident air kerma (free in air) K a,i 3. Results Results for equipment testing, presented in Table I, shows correspondence with adopted tolerances for most quality control tests, except for irradiation geometry and AEC tests. Table I. Results of the quality control tests of the mammography unit. Description Results Tolerance Adequate Geometry of irradiation (X-ray field to film border distance) 5.8 mm 3 mm No Reproducibility of the tube voltage (CV 1 ) 0.25 % 2% Yes Exactness of the tube voltage (kv) -1.1% 5% Yes Reproducibility of exposure time (CV 1 ) 0.047% 10% Yes Half-value layer (at 28 kv) 0.33 mm Al mm Al Yes Reproducibility of the incident air kerma (CV 1 ) 1.98% 10% Yes Linearity of the incident air kerma (CL 2 ) 0.81% 10% Yes Reprodutibilidade of the AEC system (CV 1 ) 5.9% 5% No Film OD difference, varying PMMA thickness, with AEC 2.34 DO 0.30 DO No Maximum force of compression 120 N N Yes 1 Variability Coefficient (MS, 2003 [2]; CEC, 2001 [4]). 2 Linearity Coefficient (MS, 2003 [2[). A mammographic phantom image was used do evaluate image quality, which was scored by an expert member of the medical staff. Results, in Table II, show minimum detectable structures 2

3 below tolerance limits established by the Ministry of Health and by phantom manufacturer. Based on these results it was concluded that the system has adequate image quality. Table II. Minimum size of the objects correctly visualized in the phantom image and tolerance limits. Results Tolerance Adequate Fibre diameter (mm): a Yes Microcalcification, diameter (mm): a Yes Masses, diameter / height (mm) / (mm): 4 / 2 4 / 2 b Yes Low contrast scale (%): b Yes a Portaria 453 (MS, 1998).[1] b Phantom manual. Reject analysis and investigation of films for defects (artifacts, stains, too dark, poor contrast, etc) were performed on a sample of 5078 films, during the 6 months of the QC programme. Results showed 169 films (3.33%) were rejected by radiographer and 28 films (0.57%) were rejected by the radiologist. The reasons for rejection were primarily due to inadequacies of irradiation parameters (tube voltage, mas) and inadequate film processor temperature. Figure 1 shows a diagram of the rejected films by the radiographer, radiologist and of the quality if the images used for diagnosis. Compared to the total number of films used, a very small amount of films have been rejected. On the other hand, our investigation of the quality of the films used shows that only 8.95% of the films used for diagnosis are actually defect-free. Primary causes are artifacts (75.13%), stains (39.99%), and image of the grid (34.75%). Other defects found were poor contrast films, light in the cassette, and scratches. FIG 1. Reasons for film rejection and number of defective films used for diagnosis. The survey was conducted during intervals between equipment maintenances and corrective actions taken, such as kvp calibration, installation of water filtering, and change of the viewing box light bulbs. The success of the quality program can be seen in Figure 2, showing the relative number 3

4 of films rejected in each period. This Figure shows a reduction in the number of films rejected from 16.1% to 0.6%. The number of films with defects was also reduced as a consequence of the corrective actions taken during the QC program. Figure 3 shows this reduction along the time for films too dark, too light, and with stains. 18% 16% 14% % rejected 12% 10% 8% 6% 4% 2% 0% Períod FIG 2. Percentage of rejected films by period between equipment maintenances. % rejected 7% 6% 5% 4% 3% 2% 1% Too dark Too Light Stains 0% Period FIG 3. Relative number of films rejected due to: film too dark, too light, and stains. Sensitometry was performed daily, before the beginning of the service. Sensitometry indexes kept between acceptable limits, except for 3 days for Speed and 7 days for Contrast index, after equipment maintenances. Incident air kerma at the entrance of the breast position was estimated over a 40 mm PMMA phantom. Results showed INAK to be 17.1 mgy for this institution, which is 70% higher than the 10 4

5 mgy reference value established by the Ministry of Health [1]. Film optical density of 2.43 OD was also found to be higher than the suggested optimal range of 1.40 to These results reflect the AEC system test failure and also the low kvp (26 kv) used for 45 mm patient breasts. 4. Conclusion Equipment testing found inadequate the performance of the AEC system and collimation of the X-ray field. All other quality control tests performed were found to be adequate. Image quality, evaluated using phantom images, was found adequate. Daily sensitometric control of the automatic processor showed large variations after maintenances. Further investigations will be necessary to establish the adequacy of this type of processor for mammography, since it was not specifically designed for this purpose. Reject analysis revealed a reduction from 16.1% to 0.6% in the number of films rejected, consequence of corrective actions, such as correct film storage, film processor maintenances, adjustment of the mammography unit tube voltage, installation of water filtering, and change of the viewing box light bulbs. The number of films used for diagnosis but containing defects was also reduced. Incident air kerma at the phantom surface position was found to be 70% higher than the established reference value. The results indicate that the quality control programme needs to be extended in the studied centre, in order to reduce breast doses during mammographic screenings. 5. References 1. Ministério da Saúde (MS). Diretrizes de Proteção Radiológica em Radiodiagnóstico Médico e Odontológico - Portaria 453 de 01 Junho de Diário Oficial da União, Brasil (1998) 2. Ministério da Saúde (Ms). Agência Nacional de Vigilância Sanitária (Anvisa). Annex of the Resolution Re N. 64 de 04 de Abril de 2003, Brasil (2003). 3 Acuerdo Regional de Cooperacion para la Promocion de la Ciencia y la Tecnologia Nucleares en America Latina y el Caribe (ARCAL) / International Atomic Energy Agency (IAEA). Protocolo de Garantía de Calidad en Mamografia. Aspectos Físicos. Havana (1999). 4. Commission of the European Communities (CEC). European Guidelines for Quality Assurance in Mammography Screening. 3. Ed. Luxemburg: Office for Official Publications of the European Communities (2001). 5. Kramer, R.; Drexler, G.; Pentoussi-Henss, N.; Zankl, M.; Regulla, D.; Panzer, W. Backscatter Factors for Mammography Calculated with Monte Carlo Methods. Physics in Medicine and Biology, v. 46, p (2001). 5