Congress:
EuroSafe Imaging 2016
Keywords:
Action 8 - EuroSafe Imaging Stars network and data collection, Action 3 - Optimisation, diagnostic reference levels, image quality, Action 2 - Clinical audit, Action 6 - Clinical audit tool for imaging, Action 2 - Clinical diagnostic reference levels (DRLs), Action 10 - EuroSafe Imaging Stars
Authors:
C. Tromans, L. Johnston, R. Highnam
DOI:
10.1594/esi2016/ESI-0033
Background/Introduction
X-ray mammography is the primary tool for screening the female population for breast cancer. Since the vast majority of participants are completely healthy,
the ALARA principal,
"As Low As Reasonably Achievable",
is of paramount importance. Glandular tissue in the breast is sensitive to radiation,
to the extent that Yaffe and Mainprize report [1] “for a cohort of 100,000 women,
each receiving a dose of 3.7 mGy to both breasts and who were screened annually from age 40 to 55 years and biennially thereafter to age 74 years,
it is predicted that there will be 86 cancers induced and 11 deaths due to radiation-induced breast cancer”.
The widespread adoption of digital mammography has a fundamental advantage over older film-screen mammography systems in that the image acquisition process is decoupled from that of reader display. The use of software image enhancement algorithms,
such as CLAHE (Contrast Limited Adaptive Histogram Equalisation) and periphery enhancement,
allows the display contrast to be optimised a posteriori,
thereby allowing optimisation of solely the contrast-to-noise ratio,
the fundamental driver of image quality,
during acquisition. In order to exploit this benefit a shift to higher beam qualities has occurred with the introduction of digital mammography. This is achieved using both higher tube voltages,
as well as diversification in the selection of target and filter materials: where Molybdenum and Rhodium were the primary selections in film-screen,
digital has largely replaced these with Tungsten targets and a wide range of filters including Silver,
Aluminium and Rhodium. The higher photon fluence reaching the detector in the case of the harder beam,
reduces quantum noise,
and thus improves the contract-to-noise ratio,
whilst software post-processing is exploited to optimise image contrast at the time of display to the radiologist.
“Diagnostic reference levels” (DRL) are dose levels in radiodiagnostic procedures for typical examinations for groups of standard-sized patients or standard phantoms for broadly defined types of equipment.
These levels are expected not to be exceeded for standard procedures when good practice regarding diagnostic and technical performance is adhered to. Phantoms are often used for establishing DRLs,
however they are fundamentally limited by the realism of the phantom employed. PMMA (Poly Methyl Methacrylate) is widely adopted as the phantom material of choice. Although it approximates the 50/50 adipose/fibrograndular breast,
this ignores the variation in glandularity across the population,
particularly the lower glandularities generally found in larger breasts (an increasingly more common occurrence with the rise in obesity). Further,
a study by Yaffe et al [2] concludes “based on the results obtained from the four groups of women in our study,
the "50-50" breast is not a representative model of the breast composition”.
The UK breast screening programme is one of the subset that sets a national DRL based on patient dose. This is currently 3.5 mGy for patients with a compressed breast thickness between 50-60mm [3] (a small subset centred around the average for the entire screening programme). However,
this limit is over 10 years old and dates from the era of film-screen systems.
A world-wide need exists for an overhaul of techniques to minimise mammographic dose whilst obtaining acceptable diagnostic image quality. This should be based on the dose actual received by the women,
on the basis of the characteristics of their breast tissue.