Why glandular dose?
In mammography, patient dose is expressed not simply as "entrance dose to the breast" but as the mean glandular dose (MGD). (Both are in mGy; the difference is which dose quantity is reported.) The reason is clear: the most radiosensitive — i.e. the most at-risk for cancer — component of the breast is the fibroglandular tissue. So the goal of dosimetry is to estimate the absorbed dose to the fibroglandular tissue, averaged over the whole breast — not to the skin or fat (AAPM TG-282/EFOMP).1
How MGD is computed
MGD cannot be measured directly — there is no way to measure the dose inside the fibroglandular tissue in place. Instead it is estimated by multiplying a measurable quantity (incident air kerma) by a conversion coefficient.1 Bushberg writes the classic form:2
Here KESAK is the entrance skin air kerma (ESAK; mGy) and DgN is the coefficient converting air kerma to average glandular dose. Depending on the formalism, whether backscatter is included must be stated explicitly; incident air kerma and entrance skin air kerma may differ on this point. DgN is computed by Monte Carlo simulation and depends on beam quality (HVL, kV), target–filter material, breast thickness and tissue composition.2 For example, for Mo/Mo at 26 kV and 0.35 mm Al HVL, the average glandular dose is about 19% of the entrance kerma (DgN ≈ 0.19).2
AAPM TG-282 modernizes this: using standard breast models, conversion coefficients (γ) computed for monochromatic energies, an incident-air-kerma correction, and a realistic amount/distribution of fibroglandular tissue, Dg is estimated more accurately.1
What it depends on
At the same entrance kerma, the main factors that set MGD:
- Breast thickness: at constant beam quality, DgN decreases as thickness rises (deep glandular tissue receives less dose due to attenuation). But a lower DgN does not mean a thick breast receives less dose — the required kerma also rises.2
- Glandularity (fibroglandular fraction): how much of the tissue is glandular directly changes the dose estimate.1
- Beam quality (HVL, kVp): a harder beam (higher HVL/kV) raises DgN and lowers contrast.2
Target–filter combinations
The mammographic spectrum is shaped by the choice of target (anode) and filter; typical tube voltage is ~25–35 kVp. Mo and Rh targets produce useful K-characteristic peaks; the filter, via its K-absorption edge, transmits a narrow energy band.2
| Target (anode) | Filter | Typical use |
|---|---|---|
| Molybdenum (Mo) | Mo 0.030 mm | Thin/medium breast (classic) |
| Molybdenum (Mo) | Rh 0.025 mm | Thicker/denser breast |
| Rhodium (Rh) | Rh 0.025 mm | Thick/dense breast |
| Tungsten (W) | Rh / Ag 0.05 mm; Al 0.7 mm | Digital systems (common) |
Bushberg notes that digital mammography systems now mostly use a W target; and a Mo filter should not be used with an Rh target.2
The role of compression
Compression matters not only for image quality but for dose. Adequate, correctly applied compression spreads overlapping tissue, reduces motion blur, and can lower the required exposure by reducing breast thickness.2 So compression is one of the most direct practical ways to lower mean glandular dose.
Tomosynthesis (DBT) dosimetry
In digital breast tomosynthesis (DBT) the tube moves along a small arc to acquire many low-dose projections. AAPM TG-282 defines its formalism not only for standard mammography but also for DBT and the contrast-enhanced (CEDM/CEDBT) variants under a single framework, enabling consistent glandular-dose calculation across exam types.1
Acceptance levels for a standard phantom
References
- Sechopoulos I, et al. Joint AAPM Task Group 282 / EFOMP Working Group Report: Breast dosimetry for standard and contrast-enhanced mammography and breast tomosynthesis. Medical Physics, 2023. (Modern ortalama glandüler doz formalizmi ve dönüşüm katsayıları.)
- Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The Essential Physics of Medical Imaging, 3rd ed. Lippincott Williams & Wilkins, 2011. Bölüm 8 (Mammography). Atıflardaki sayfa numaraları bu baskıya aittir.
- U.S. FDA. Mammography Quality Standards Act (MQSA), 21 CFR 900 — standart fantom için poz başına ortalama glandüler doz üst sınırı 3,0 mGy. fda.gov
- European Commission / EUREF. European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis (4th ed.) — standart memede (4,5 cm PMMA) kabul edilebilir/ulaşılabilir glandüler doz seviyeleri. op.europa.eu