Direct half value layer measurements in mammography - is near enough good enough?

Transcription

1 Direct half value layer measurements in mammography - is near enough good enough? Poster No.: R-0127 Congress: Type: Authors: 2014 CSM Scientific Exhibit J. Diffey 1, L. Cartwright 2, J. Crocker 1, J. Heggie 3 ; 1 Newcastle/AU, 2 SYDNEY/AU, 3 MELBOURNE/AU Keywords: DOI: Breast, Mammography, Physics, Dosimetry, Dosimetric comparison, Quality assurance /ranzcr2014/R-0127 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply RANZCR/AIR/ACPSEM's endorsement, sponsorship or recommendation of the third party, information, product or service. RANZCR/AIR/ ACPSEM is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold RANZCR/AIR/ACPSEM harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies,.ppt slideshows,.doc documents and any other multimedia files are not available in the pdf version of presentations. Page 1 of 11

2 Aim The accuracy of mean glandular dose (MGD) measurement in mammography depends upon several factors, one of which is half value layer (HVL). This can be calculated from measurements of air kerma with and without aluminium filters in the beam, or measured directly using a solid-state dosimetry system. The aims of this work were to: Compare direct measurements of HVL to calculated values, Assess the agreement within and between dosimetry systems from two vendors, Assess the variation in direct measurements with mas, field size and dosimeter setting, and Examine the effect of these variations in HVL on MGD. Methods and materials HVL measurements were made on two full field digital mammography systems with a range of target/filter combinations: GE Senographe DS with Mo/Mo, Mo/Rh and Rh/Rh Hologic Selenia Dimensions with W/Rh and W/Ag For all exposures, lead rubber was placed on the breast support platform to protect the digital detector. The dosimeter was placed 6 cm back from the chest wall edge in the midline and the compression paddle was always in the beam. The following investigations were carried out: Comparison of Calculated HVL to Direct Measurement For each target/filter combination, HVL was calculated using high purity aluminium (Al) filters (Fig. 1 on page 3) and measured directly using 4 dosimeters (3 from Vendor A; 1 from Vendor B). Measurement of HVL using Al filters requires raising the compression paddle to reduce scatter (Fig. 2 on page 4) and collimating the X-ray beam to the sensitive detector area to achieve narrow beam geometry (Fig. 3 on page 6). A measurement of air kerma is made with no attenuating material in the beam, then Al filters are added in 0.1 Page 2 of 11

3 mm increments (Fig. 4 on page 6) until less than half of the initial unattenuated dose is recorded. All measurements were made using 20 mas. Direct measurements were made using broad beam geometry (no collimation of the X- ray field), the "Paddle" dosimeter setting and 10 mas. Three measurements were made for each target/filter combination to assess repeatability. Agreement within and between dosimetry systems from two vendors The direct measurements from the 4 dosimeters were compared for each target/filter combination to assess consistency within and between dosimetry systems from two vendors. Direct measurement variation with mas Direct measurements were obtained using 10, 20 and 40 mas. Direct measurement variation with field size Direct measurements were made using narrow beam geometry (Fig. 3 on page 6) and broad beam geometry. Direct measurement variation with dosimeter setting When multiple possible dosimetry settings were available, including whether the paddle was in beam or not, these were investigated Mean glandular dose (MGD) was calculated for each measured HVL, assuming typical kv and mas values from mammography quality control tests. The effect of HVL variation on MGD was examined. Images for this section: Page 3 of 11

4 Fig. 1: High purity Al filters of varying thicknesses are used to determine HVL Page 4 of 11

5 Page 5 of 11

6 Fig. 2: Experimental set-up to determine HVL using Al filters. The compression paddle is raised to reduce scatter. Fig. 3: Narrow beam geometry. A manual collimator was placed on the raised paddle to restrict the field of view to the sensitive detector area (illuminated by the light field). Page 6 of 11

7 Fig. 4: Al filters were added in 0.1 mm increments until less than half of the unattenuated dose was recorded. Page 7 of 11

8 Results Comparison of Calculated HVL to Direct Measurement Fig. 5 on page shows the direct HVL measurements and their variation from the calculated values for Mo/Mo, Mo/Rh and Rh/Rh target/filter combinations. All HVL values were within the acceptable range, as defined in the RANZCR spreadsheet. Fig. 5: Comparison of calculated and directly measured HVL on the GE Senograph DS with Mo/Mo, Mo/Rh and Rh/Rh beam qualities References: Hunter New England Imaging - Newcastle/AU The maximum deviations from the calculated values were exhibited by Vendor A for Mo/ Mo (8.5%) and Mo/Rh (10.9%) and by Vendor B for Rh/Rh (-10.8%). The corresponding differences in MGD were 7.6%, 11.9% and -8.7% respectively. For the W target, our initial direct HVL measurements differed from the calculated values by a maximum of 3.9% (W/Rh) and -2.4% (W/Ag) for Vendor A and -22.7% (W/Rh) and -32.0% (W/Ag) for Vendor B. These results suggest that the dosimeter from Vendor B had not been correctly calibrated, although it is interesting to note that the HVL measurements were within the RANZCR-defined acceptable range. Page 8 of 11

9 Our results were shared with Vendor B, who supplied a file to correct the problem. Fortunately, there was no need to send the dosimeter to them for re-calibration. Measurements were repeated and results are shown in Fig. 6 on page for W/Rh and W/Ag. Fig. 6: Comparison of calculated and directly measured HVL on the Hologic Selenia Dimensions with W/Rh and W/Ag beam qualities References: Hunter New England Imaging - Newcastle/AU Despite the improvements, the maximum deviations from the calculated values were still exhibited by Vendor B, but were reduced to -5.6% for W/Rh and -10.2% for W/Ag. The corresponding differences in MGD were -4.2% and -8.3% respectively. Agreement within and between dosimetry systems from two vendors All dosimeters exhibited good repeatability. The coefficient of variation (COV) was dependent upon target/filter combination for Vendor A and ranged from ; COV for Vendor B was for all target/filters. The 3 units from Vendor A gave consistent results for all target/filter combinations, with a typical COV of Page 9 of 11

10 The direct HVL measurements from Vendor A were higher than those for Vendor B, as shown in Fig 7, with the extent of agreement being dependent upon target/filter combination. Fig. 7: Percentage difference in direct HVL measurements between Vendor A and Vendor B References: Hunter New England Imaging - Newcastle/AU It is interesting to note that when compared to the calculated HVL for Rh/Rh and W/Rh, Vendor A over-estimated the value and Vendor B under-estimated the value. Direct measurement variation with mas, field size and dosimeter setting Direct measurements were not significantly affected by mas used (<1%), beam geometry (<2%) or dosimeter setting used (Paddle/No Paddle #2% and W/Rh / W/Rh* <1%). Conclusion Direct HVL measurements exhibit vendor variation and differ from the calculated HVL. However, the variation is within vendor-specification; Vendor A guarantees that their Page 10 of 11

11 direct HVL measurement is accurate to within ±5%, while Vendor B guarantees that their direct HVL measurement is accurate to within ±10%. The effect on resulting MGD calculations is generally not significant considering the other errors and assumptions involved in determining MGD. A 10% increase in the calculated MGD would be very unlikely to cause MGD to exceed the acceptable level, especially for mammography equipment with integrated digital detectors. In Australia, there is no tolerance level for MGD variation with time, although in the UK, MGD must remain within 25% of the baseline value. So near enough is good enough (if dosimeters have been correctly calibrated) and direct HVL measurement saves time during mammography quality control testing. However, we recommend that the accuracy of direct HVL measurements is verified for each target/ filter combination for new solid-state dosimetry systems and after each calibration. Personal information References Sobol WT and Wu X, Parametrization of mammography normalized average glandular dose tables, Med Phys 24 (1997) Boone JM, Fewell TR and Jennings RJ, Molybdenum, rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography, Med. Phys. 24 (1997) Boone JM, Normalized glandular dose.dgn. coefficients for arbitrary x-ray spectra in mammography: Computer-fit values of Monte Carlo derived data Med. Phys. 29 (2002) Page 11 of 11