The question is addressed to people, who use methodology described in the "European Guidelines 4th Edition" to calculate average glandular dose.
The dose is calculated as K*g*c*s, where 'K' stands for kerma, and g/c/s are factors dependent on characteristics of breast and beam quality. Now an example: 50 mm of PMMA is equivalent to 60 mm of typical breast (there are some assumptions on breast composition behind it). We can assume, that the exposure will be made at the same conditions for 50 mm of PMMA and for 60 mm of breast (let's forget about spacers for a while). In both cases we have e.g. Mo/Rh, 30 kV, 100 mAs, so in both cases kerma at some specified point will be identical. G/c/s factors are the same for "typical breast simulated with 5.0 cm PMMA" and for "6.0 cm breast", thus the only will be in 'K':
a) typical breasts simulated with PMMA: in that case 'K' is kerma "at the upper surface of the PMMA";
b) clinical breast doses: 'K' is kerma "at the upper surface of the breast", closer to the source than in case (a)
Because of that difference, the 'clinical' dose will be slightly higher than the 'phantom' dose. Is there any good reason for that?
Dear,
The linear attenuation coefficient of PMMA is higher than that of breast so the energy absorbed by PMMA is more than that absorbed by breast tissue. The PMMA is good for the simulation of glandular tissue only.
the same focus surface distance could be established, if spacers are used. The influence of that imprecision is very smal and neglectable. The accuracy of any dose measurement with commercially available dosemeters is within 5%. That uncertainty is much greater than the influence of a different focus detector distance. The Mean Glandular Dose according Dance or the EUREF protocol is a rough value to check if an adequate dose for imaging.was used. If an individual dose should be calculated the glandularity of the breast has to be be analysed with different soft ware tools which can analyse the image.
I would like to follow the discussion. But it would be helpful to explain your factors.
As for the source surface distance it could easily be corrected by Christians proposal, the remaining problem is the different composition of breast tissue (it´s not homogenious) and PMMA. The reason for this remark is the extreme dependence of the attenuation from the atomic number Z in this low energy region. The mean energy in the used 30 kV mammography spectrum Mo/Rh is at 18 keV. Therefore you will find a different depth dose curve dependent from kV, filter and kind of transmitted tissue.
"the 'clinical' dose will be slightly higher than the 'phantom' dose. Is there any good reason for that?". This slightly overestimation of the mean glandular dose without the proposed corrections has to be brought into relation to other uncertainties, e.g. the used kV or better the measured Al-HVL which respects the hardening/roughness of the target and the used filtering or the uncertainty of the accuracy of the used dosemeter. The ISO handbook of error estimations can help you to perform that calculation.
Without any doubt the depth dose curve is different: see comment of Hanno.
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