PATIENT DOSIMETRY IN INTERVENTIONAL RADIOLOGY

Transcription

1 2009 International Nuclear Atlantic Conference - INAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: PATIENT DOSIMETRY IN INTERVENTIONAL RADIOLOGY Mauro Wilson O. da Silva 1, Bárbara Beatriz D. Rodrigues 2 and Lucía V. Canevaro 1 1 Instituto de Radioproteção e Dosimetria, IRD CNEN/RJ Av. Salvador Allende s/n Recreio dos Bandeirantes - Rio de Janeiro - RJ maurowilson@gmail.com canevaro@ird.gov.br 2 Universidade Federal do Rio de Janeiro Instituto Alberto Luiz Coimbra de Pós-Graduação e Pesquisa de Engenharia Programa de Pós-Graduação em Engenharia Nuclear - COPPE/UFRJ Av. Horácio Macedo, 2030, Bloco G - Sala Centro de Tecnologia Cidade Universitária, Ilha do Fundão - Rio de Janeiro - RJ bbdr@terra.com.br ABSTRACT Dosimetric methods used for interventional radiology are reviewed and evaluated, including terms, quantities, equipment, calibration and measurements. Measurement of local skin dose and estimation of maximum local skin dose are emphasised. A method for the evaluation of patient doses in interventional radiology procedures is the slow film, named EDR2 (Extended Dose Range) by Kodak. The EDR2 film kvp dependence is negligible and the processor conditions can be standardized to obtain skin dose estimations. The linear range for accurate dose measurements is from 50 mgy to 500 mgy. Kodak EDR 2 film was calibrated across the range of beam qualities and exposure rates typically used in cardiac catheterisation laboratory. Its dose response curve was modelled up to its saturation point of 1000 mgy. Dose responses are a function of facility dependent factors including processing conditions (processing time, processing temperature, processing equipment, processing chemistry), the density sampling (digitizer equipment and calibration), and exposure monitoring equipment. The response relationship should be measured and verified for the local conditions. 1. INTRODUCTION Medical applications of ionizing radiation are accepted worldwide as essential tools for protecting and improving human health [1]. However, they also represent by far the largest human-made source of radiation exposure. Furthermore, there is certain to be a continuing increase in the prevalence of medical applications of radiation, including high dose procedures, as in the following case: Radiological interventional procedures, which are increasingly being used to replace surgery, can lead to very high radiation doses in some cases exceeding thresholds for deterministic effects [2]. Interventional radiology procedures offer substantial health care benefits. However, associated with the increasing complexity as well as a lack of quality control programmes and specific training in radiation protection, there is an increase in the occurrence of deterministic effects in both patients. Visible effects of radiation can still be noticed on patients after exposures during interventional radiology procedures, in the form of rashes, with doses that cannot be considered as low. In interventional cardiac procedures can be complex and involve extensive use of long time of fluoroscopy and high dose rate in image acquisition mode.

2 Reference levels in radiodiagnostics are a requirement stated by the Council Directive 97/43/EURATOM. Reference levels are also relevant for interventional procedures, in accordance with this Directive, which claims special attention to quality assurance programmes, including quality control and patient dose evaluations for special practices such as interventional radiology, to assess the convenience of corrective action depending on the measured doses. Since the development of interventional radiology, the number and complexity of procedures has increased significantly and continues to grow. Interventional radiology is a technique with significant benefits for patient care, and its use is increasing rapidly. It involves, however, relatively high doses and, in a number of cases, deterministic effects are reported to have been produced in patients as a result of radiation exposure. In addition to the protection of patients is an important issue requiring a comprehensive approach and standardized methods of exposure monitoring. Areas of the body of particular concern in this respect is the skin of patients. To improve the radiological protection of patients, there is a need to develop standardized methods for determining the doses, especially to the patient s skin, during interventional radiology. Dose measurements, and especially skin dose measurements, are therefore increasingly important. Methods and acceptable dosemeters are, however, not clearly defined. Radiation exposure during interventional procedures is relatively high compared with other types of radiological procedures. In order to estimate the integral radiation dose absorbed by the patient. Dosimetric methods used for interventional radiology are reviewed and evaluated, including terms, quantities, equipment, calibration and measurements. Measurement of local skin dose and estimation of maximum local skin dose are emphasised. A method for the evaluation of patient doses in interventional radiology procedures is the slow film, named EDR2 (Extended Dose Range) by Kodak. 2. MATERIALS AND METHODS Kodak cm, EDR2 film (Eastman Kodak Co., Rochester, NY, USA) has been used to visualize and estimate doses from the different patient irradiation fields. This film can used for verifying the orientation and the approximate patient doses in interventional radiology procedures. The process of calibration of the film Kodak EDR2 consists of relating the density optics registered in the film with the entrance surface air kerma, through a calibration curve. This curve, and provides the relationship between density optics and entrance surface air kerma, allows to know the uncertainties associates to the calibration process who will happen in the final measure of the dose in the film. The film Kodak EDR2 was calibrated in the apparatus Siemens Polymat 50 in Laboratory of Medical Physics (LMPh) of Institute of Radioprotection and Dosimetry (IRD) measuring entrance surface air kerma with a dosimetric system of Radcal reference, made up of an electrometer model 9015 connected to an ionization chamber model 10x5-6 with sensible volume of 6 cm 3, and 4 blocks of PMMA of 5 cm of thickness each one. These blocks had been used to simulate the presence of a patient and to produce a scattering similar what this would produce during real an interventional procedure. For the calibration process of the film are set of different irradiation techniques (kvp, ma, time), similars to real conditions in which the patients are submitted during the interventional procedures. With the defined techniques, some expositions on the

3 Optical Density PMMA blocks are made and with the ionization chamber the register of the corresponding values of entrance surface air kerma is made. After that, the ionization chamber is removed and the slow film Kodak EDR2 is located in the same place where it was the ionization chamber. Finally, with a set of 24 pairs of values of entrance surface air kerma input versus optical density, it was possible to construct the calibration curve and to get the equation represents that it. The EDR2 film kvp dependence is negligible and the processor conditions can be standardized to obtain skin dose estimations. The linear range for accurate dose measurements is from 50 mgy to 500 mgy. Its dose response curve was modelled up to its saturation point of 1 Gy. 3. RESULTS AND DISCUSSION From the results gotten of the process of calibration in the laboratory according to described, was built the response curve of the Kodak EDR2 film (FIGURE 1). 4,5 4 3,5 3 2,5 2 1,5 1 0, Entrance surface air kerma (mgy) Figure 1. Response curve of the slow Kodak EDR2 film. (kvp = 80). The points of the curve shows the shape of the slow response of the Kodak EDR2 film. As it can be seen this film saturates approximately in 1 Gy. The X-ray equipment was reproducible during the calibration process, since all the readings taken with the camera monitors the meter for kerma-area product were played for all the irradiations performed with the reference ionization chamber (Radcal) of the slow films. With the curve of response was possible to get an equation that directly supplies the values of entrance surface air kerma from the optical density recorded on film. Using some mathematical algorithms and programs could get an equation of degree 4 that fit the curve, however this equation was not feasible since it was not practical to obtain the values of entrance surface air kerma from the optical density. The best solution was to use the equation by Morrell and Rogers [3], given by equation 1. (1)

4 where DO is the optical density, DO max is the optical density which if the saturation of the film registers, DO min is the base + fog the film, D is the dose in mgy, and α is a constant. For each dose, the uncertainty was ± 8% for the doses measured. For the case, was a careful adjustment of the parameters of equation 1: DO min : 0.20 and DO max : Working equation 1, the value of the constant can be gotten α, so has the equação2: (2) Organizing equation 2 and using the appropriate parameters, an expression is gotten directly to find the dose from the density optics. (3) It should be noted that this equation was considered valid for the values of entrance surface air kerma between 0 and 1400 mgy, which were the values obtained experimentally. 4. CONCLUSIONS The definite methodology proved satisfactorily for performing measurements on a safe and effective. The use of films during the procedure does not intervene with the accomplishment of the same, and identify the irradiated fields and estimate the exposure in the skin of the patient. The method is effective to quantify the dose and to identify to the distribution of the fields, thus allowing a possible ( follow up ) of the patient in order to investigate the appearance of injuries. ACKNOWLEDGMENTS This work was partially supported by the CNEN and IAEA. REFERENCES 1. Technical Reports nº 457 Dosimetry in Diagnostic Radiology: An International Code of Practice 2. Radiological Protection of Patients in Diagnostic and Interventional Radiology, Nuclear Medicine and Radiotherapy

5 3. MORREL R.E., ROGERS, A.T. Film dosimetry for fluoroscopic procedures: potencial errors. Radiation Protection Dosimetry, vol 114, nº 1-3, pp , 2005.