Forward-scattered radiation from the compression paddle

Forward-scattered radiation from the compression paddle

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Description: Estimation of forward scatter factors (FSF), Measurements of forward-scatter, Dosemeters with high FS sensitivity, HVL measurements in mammography, Dosemeters with low FS sensitivity, Forward-scattered radiation.

 
Author: Bengt Hemdal (Fellow) | Visits: 1441 | Page Views: 1459
Domain:  Medicine Category: Equipment Subcategory: Radiology 
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Short URL: https://www.wesrch.com/medical/pdfME14GWSY8HWHG
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Contents:
Forward-scattered radiation from the compression paddle should be considered when average (or mean) glandular dose is estimated
Bengt Hemdal
Department of Medical Radiation Physics, Lund University Sk�ne University Hospital Malm� Malm�, SWEDEN bengt.hemdal@med.lu.se

Introduction
� From major protocols on dosimetry in mammography there is no doubt that the incident air kerma should be evaluated without backscattered (BS) radiation to the dosemeter. However, forward-scatter (FS) from the compression paddle is neglected. � This has resulted in confusion and unnecessary differences in dosimetry methods and results. � A recent thesis [1] and paper [2] made it clear that FS should be included as for an ionisation chamber in contact with the compression paddle and the contribution reported to the incident air kerma was a factor of 1.06 and 1/0.929=1.076, respectively. � It is suggested that this factor should be called forward-scatter factor (FSF) [1]; cf. backscatter factor (BSF).
1. Hemdal B, PhD Thesis Lund University, Malm� (2009) 2. Dance D R et al. Phys Med Biol 54 4361-4372 (2009)
2

The purpose of this work is to ...
� ... further investigate the contribution from FS for typical incident air kerma measurements. � ... compare with backscatter data. � ... discuss some practical consequences that should be considered in dose protocols.

3

Materials and Methods 1 (2)
� Measurements of forward-scatter were performed with
� a plane-parallel ionisation chamber (PTW) with volume 0.2 cm3, diameter 13 mm and depth 1.5 mm at a 60 mm distance from the breast support edge to its center.

� Four mammography units were used, each one with a compression paddle selected based on clinical relevance.
� A = Senographe Essential (GE)
� 19x23, flex, high edge, 5144833

� B = Mammomat Inspiration (Siemens)
� 24x30, no flex, low edge, 10139964

� C = DM 1000 (Agfa), which is based on Lorad M-IV (Hologic)
� 18x24, flex, high edge, FAB-00207

� D = Mammomat Novation (Siemens)
� 18x24, no flex, high edge, 10048515
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Materials and Methods 2 (2)
Estimation of forward scatter factors (FSF) for an ionisation chamber from air kerma (K) measurements, as FSF=Ka / Kb

Ka

Kb

Compression paddle in Compression paddle up, contact with the chamber beam well collimated Note: The figures only shows the principle, not one of the units in this work.

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Results 1 (2)
1,15 Scatter factors
A: 2.72 mm C: 2.13 mm BSF B: 2.43 mm D: 1.51 mm

1,10

1,05

1,00 0,20

0,30

0,40

0,50

0,60

0,70

Half-value layer (mm Al)

Measured FSF values for mammography units A-D. The measured compression paddle thickness (mm) is indicated in each case, A-D. BSF values from the European dose protocol [3] are also presented.
3. Zoetelief J et al. EUR 16263 (1996)
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Results 2 (2)
Forward-scatter factor (FSF) 1,15
A: 2.72 mm C: 2.13 mm FSF=1.076 B: 2.43 mm D: 1.51 mm FSF=1.06

1,10

1,05

1,00 0,20

0,30

0,40

0,50

0,60

0,70

Half-value layer (mm Al)

FSF results in the present work compared to literature data
-- FSF=1.076 from Monte Carlo calculations, 2.4 mm compression paddle [2]. - - FSF=1.06 from a 6 cm3 6M chamber (RADCAL ), MDM unit (Sectra), 2.6 mm compression paddle ~40 cm up (no collimation) compared to close contact in an experimental setup [4].
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4. Hemdal B et al. Radiat Prot Dosim 114 (1-3), 436-443 (2005)

Discussion 1 (2)
� The FSF was here found to be 1.02-1.10 and increased with compression paddle thickness, but also with HVL. � The FSF could have the same value as the BSF and is consequently as relevant to consider in dose protocols. � FSF=1.076 [2] is an example and not a standard value; this Monte Carlo calculated value is in reasonable accordance with the present experimental results. � FSF=1.06 [1] is also an example and probably an underestimation as no collimator was used (experimental setup); the purpose was not to determine FSF [4]. � Based on the results in this work, FSF=1.06 happens to be the best choice of a standard FSF, with a maximum error of about 8 4%.

Discussion 2 (2)
� Some practical consequences for output measurements.
� Dosemeters with high FS sensitivity, e.g. ionisation chambers, which for a long time has been the standard dosemeter for output and HVL measurements in mammography.
� The compression paddle should be in contact with the chamber. Note: For a well collimated beam (slide 5, Kb), a FSF must be used.

� Dosemeters with low FS sensitivity, e.g. dosemeters based on wellcollimated semiconductors, which are increasingly used and now are dominating the usage in e.g. Sweden.
� FSF must be used. FS contribution is small regardless of geometry and can be minimized with a high position of the compression paddle; collimation should not be necessary. � Suggested strategies to determine the FSF to be used � Compare with an ionisation chamber or � use a FSF that depend on paddle thickness and possibly HVL, etc or 9 � use a standard FSF, e.g. 1.06.

Conclusions
� It can be as important to consider FS as BS radiation. � Neglection of FS can underestimate AGD/MGD by 10%. � Procedures for output measurements in evaluation of incident air kerma
� Ionisation chambers in contact with the compression paddle and no FSF is then needed. � Dosemeters with low FS sensitivity; correct with a FSF.

� Based on the results in this work, FSF=1.06 will lead to a maximum error of about 4% and might be a suitable standard FSF.

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