Study of the Spatial Distribution of Scattered Radiation Dose Around a Surgical C-arm. Assessment of Radiation Exposure to Staff.

Transcription

1 Study of the Spatial Distribution of Scattered Radiation Dose Around a Surgical C-arm. Assessment of Radiation Exposure to Staff. D. Jurado, R. Pallerol, N. Jornet, J. Martin-Viera, P. Carrasco, T. Eudaldo, M. Ribas Servei de Radiofísica i Radioprotecció, Hospital de la Santa Creu i Sant Pau, Sant Antoni Maria i Claret 167, 825 Barcelona, Spain. djurado@hsp.santpau.es Abstract. The use of C-arm mobile fluoroscopy systems in operating rooms has increased these last years. A study of the spatial distribution of the scattered radiation dose around a surgical C-arm was performed in order to design procedures to minimize radiation exposure to staff. Dose rate maps were measured for different C-arm angles and two different kvp-ma combinations at 1 and 1.5 m height from the floor. The uncertainty associated to these measurements was studied, as well as an estimation of the time needed to reach the body, thyroid and crystalline lens annual limits in order to study the need of using protection devices. From these dose rate measurements, some recommendations of C-arm orientation and staff location were drawn: for a fixed C-arm orientation, the tube should be located in the lowest extreme of the arm, and for an oblique C-arm orientation staff s best locations are around the intensifier, avoiding working near the tube. Following these recommendations and with the workloads and characteristics of our centre, the use of protection aprons is justified, but the use of shielding glasses is unnecessary. The use of thyroid protectors depends on the type of exploration and the time spent in it. 1. Introduction These last years there has been a growth of interventional radiology in operating rooms using C-arms mobile fluoroscopy systems, mainly by non-radiologist specialists. It has been therefore necessary to measure the distribution of the scattered radiation dose around these C-arms in order to design procedures to minimize staff exposure. This study also served to reconsider the need of protection aprons, thyroid protectors and shielding glasses for the staff. 2. Material and Method To study the spatial distribution of scattered radiation dose around a surgical C-arm, a Philips BV-25 C-arm was used as radiation source. A PMMA slab phantom of 3 cm x 3 cm x 2 cm was used to simulate the patient, and was placed on the radio-transparent part of the surgical table with its centre at 1 m height from the floor (see FIG. 1). Several lines were traced radially on the floor each 45º, converging on the centre of the phantom. Measurement points were marked on each of these lines at distances from the centre (d) of 25, 5, 1, 2 cm. Dose rate measurements were performed using a N.E. PDM 1A ionisation chamber, locating the chamber over these points for different C-arm angles, kvp-ma combinations and heights. FIG. 1. Measurement arrangement. C-arm angles were defined by the C-arm orientation and the tube position (e.g. tube up at º and tube 1

2 down at 18º), and measurements were carried out at º, 45º, 135º and 18º C-arm angles. Two kvpma combinations were measured for each C-arm angle: 8 kvp-2.8 ma and 1 kvp-3 ma. For each C-arm angle and kvp-ma combination, measurements were performed at 1 and 1.5 m height from the floor (z). The chamber reading was corrected to take into account the different quality between the measured radiation and that from the calibration (Cs 137 ). This correction factor had a value of 1.7 and was obtained from the chamber response curve with photon energy, provided by the manufacturer. A study of the uncertainties associated to these measurements was carried out. Different factors were taken into consideration for each dose rate measurement: the chamber calibration factor uncertainty, the measurement scale precision, the energy and angular responses of the chamber, and the chamber positioning uncertainty. The fluoroscopy time needed to reach the body, thyroid and crystalline lens annual limits according to the Spanish Royal Decree 783/21 [1] (same annual limits as in 96/29/EURATOM [2] and ICRP-6 [3]) was calculated. Measurements at z = 1 m were used for the calculations involving the body annual limit, whereas measurements performed at z = 1.5 m were used for the thyroid and the crystalline calculations. The need of using protection aprons, thyroid protectors and shielding glasses was studied from the fluoroscopy times needed to reach the body, thyroid and crystalline lens annual limits respectively. Only those tube angulations and staff positions that followed the recommendations derived from dose rate measurements were considered. The criteria to decide if a protection device is necessary or not depends on the characteristics of each centre: exploration types, time spent in each exploration, workloads, etc. According to the workloads and characteristics of our centre, a threshold fluoroscopy time of 7 h per year was chosen. So, if the fluoroscopy time needed to reach the annual limit in a certain position was lower than 7 h, the use of the protection device was justified. 3. Results and Discussion Measurements were represented as dose rate maps for a given C-arm angle, kvp-ma combination and z using the 3DField software (freeware). Colour fill dose rate levels (calculated using the Kriging interpolation algorithm [4]) were also represented in these maps in order to have a more graphical visualization of measured data. The arrangement of the C-arm, the tube, the intensifier and the phantom is shown in a scheme attached to the right part of each map. Due to the paper length constraint, only the dose rate maps for the 8 kvp-2.8 ma combination are shown in this paper, but the same behaviour and conclusions are valid for the 1 kvp-3 ma combination. Representative values of the measurement uncertainty were assigned as a function of d, due to the fact that the chamber positioning uncertainty was the most important contribution to the total error. Supposing a chamber positioning uncertainty of 5 cm, the resulting measurement uncertainties (k = 1) were about 4% for d = 25 cm, 2% for d = 5 cm, 15% for d = 1 cm and 1% for d = 2 cm. In order to show better the influence of the tube and the image intensifier positioning influence for a fixed C-arm orientation, dose rate maps for º and 18º C-arm angles, as well as for 45º and 135º, are presented consecutively using the same dose rate level colour scales for both configurations. 2

3 Dose rate maps for º C-arm angle are presented in FIG. 2 and FIG. 3: FIG. 2. Dose rate map for º C-arm angle, 8 kvp-2.8 ma, z = 1 cm FIG. 3. Dose rate map for º C-arm angle, 8 kvp-2.8 ma, z = 15 cm.. 3

4 Dose rate maps for 18º C-arm angle are presented in figures FIG. 4 and FIG. 5: FIG. 4. Dose rate map for 18º C-arm angle, 8 kvp-2.8 ma, z = 1 cm FIG.5. Dose rate map for 18º C-arm angle, 8 kvp-2.8 ma, z = 15 cm.. 4

5 Dose rate maps for 45º C-arm angle are presented in figures FIG. 6 and FIG. 7: FIG. 6. Dose rate map for 45º C-arm angle, 8 kvp-2.8 ma, z = 1 cm FIG. 7. Dose rate map for 45º C-arm angle, 8 kvp-2.8 ma, z = 15 cm

6 Dose rate maps for 135º C-arm angle are presented in figures FIG. 8 and FIG. 9: FIG. 8. Dose rate map for 135º C-arm angle, 8 kvp-2.8 ma, z = 1 cm FIG. 9. Dose rate map for 135º C-arm angle, 8 kvp-2.8 ma, z = 15 cm

7 Dose rate maps for 9º C-arm angle are presented in figures FIG. 1 and FIG. 11: FIG. 1. Dose rate map for 9º C-arm angle, 8 kvp-2.8 ma, z = 1 cm FIG. 11. Dose rate map for 9º C-arm angle, 8 kvp-2.8 ma, z = 15 cm

8 Dose rates for º and 18º C-arm angles were very similar at z = 1 m. At z = 1.5 m, dose rates for º C-arm angle doubled the previous ones while for 18º they were much lower. Therefore, the 18º C- arm angle configuration is recommended in order to have lower doses at the organs found at the head level (crystalline, thyroids). Dose distributions presented a roughly radial symmetry at both planes. Therefore, there is no recommendation concerning staff positioning for a vertical C-arm orientation. Dose rates for 45º and 135º C-arm angles were quite different at z = 1 m. For 135º C-arm angle and a fixed d, there was a high gradient, with the highest rates near the tube and the lowest rates near the intensifier. For 45º C-arm angle, dose rates were lower and the dose rate distribution did not present this gradient. At z = 1.5 m dose rates were much lower for 135º C-arm angle in comparison to that at z = 1 m, whereas for 45º C-arm angle increased about 1%. Therefore, the 135º C-arm angle configuration is recommended in order to have lower doses at z = 1.5 m (head level), and staff s best locations are near the intensifier. Dose rates for 9º C-arm angle at z = 1m had a high gradient, with the highest rates near the tube and the lowest rates near the intensifier. At z = 1.5 m dose rates were lower, as well as the dose rate gradient in comparison with z = 1 m. Therefore, recommended staff locations are near the intensifier. Dose rates decreased with d for all configurations, having that at d = 2 m dose rates were very low in all cases. The fluoroscopy time needed to reach the body, thyroid and crystalline lens annual limits was calculated from these dose rates, performing the study of the need of using protection aprons, thyroid protectors and shielding glasses. Only those C-arm angles and staff positions that followed the previous recommendations were considered (i.e. 18º, 135º, 9º C-arm angles; positions near the intensifier for the 135º and 9º C-arm angles), having that for the particular workloads and characteristics of our centre: The use of protection aprons is justified for all the configurations at d 5 cm. The use of thyroid protectors is justified for the 9º C-arm angle and 1 kvp-3 ma configuration at d 25 cm (7 h to reach the thyroid annual limit at d = 25 cm). The use of shielding glasses is unnecessary due that in the worst situation 5 h are needed to reach the crystalline annual limit (9º C-arm angle and 1 kvp-3 ma configuration at d = 25 cm). 4. Conclusions For a fixed C-arm orientation, the tube should be located in the lower extreme of the arm. For an oblique C-arm angle, staff s best locations are around the intensifier, avoiding working near the tube. According to the specific characteristics of our centre, if these recommendations are followed the use of shielding glasses is unnecessary and the use of protection aprons is justified. The use of thyroid protectors depends on the type of the exploration and the time spent in it. 5. References 1. Real Decreto 783/21 of 6 July 21, por el que se aprueba el Reglamento sobre protección sanitaria contra radiaciones ionizantes. BOE, No. 178, 26 July European Council Directive 96/29/EURATOM of 13 May 1996, amending the Directives laying down the basic safety standards for the health protection of the general public and workers against the dangers of ionising radiation. Official Journal of the European Communities, 39, No. L 159, 29 June International Commission on Radiological Protection, 199 Recommendations 199 of the International Commission on Radiological Protection. Publication 6. Annals of the ICRP, 21, No. 1-3, Pergamon Press, Oxford (1991). 4. Olea, R.A., Optimal contour mapping using universal kriging. J. Geophys. Res., Vol. 79, No. 5: , (1974). 8