Average absorbed breast dose in mammography: a new possible dose index matching the requirements of the European Directive 2013/59/EURATOM

Breast cancer is the most common female cancer worldwide and the second most common cancer overall, with more than 1,700,000 new cases diagnosed in 2012, equal to 25% of female cases and 12% of the total [1].

Mammography is considered the most effective imaging technique for the early detection and diagnosis of breast cancer: it uses low-energy ionising radiation to produce a mammogram that is a radiographic two-dimensional projection of the breast. However, the breast is a highly radiosensitive organ: the International Commission on Radiological Protection (ICRP) has varied its estimate of the contribution of breast radiation exposure to total body detriment over time, changing the tissue-weighting factor for this organ from 0.05 in 1991 [2] to 0.12 in 2007 [3]. Thus, it is very important to take into account the dose absorbed by the breast during a mammographic exposure, trying to reduce the radiation dose absorbed by patients in every mammographic examination without impairing its diagnostic quality.

This follows the guidelines contained in the European Directive 2013/59/EURATOM issued on 5 December 2013 [4], which gives challenging targets to all stakeholders in terms of justification and optimisation of the procedures using ionising radiation. In any mammographic quality assurance program, special care is required for evaluating and monitoring radiation doses delivered to the breast. In addition, article 58 of the above mentioned directive [4] requires that patients are informed about the risk associated with ionising radiation and that detailed information on the exposure of the patient is included in the report of the radiological procedure.

The average dose absorbed in breast glandular tissue (considered the most sensitive tissue to the radiation-induced carcinogenesis [5]) is a suitable dosimetry quantity [6]. The average glandular dose (AGD) is the quantity used to compare dose delivered with different techniques. In 1987, the ICRP recommended the AGD as the reference metrics for radiation dose estimation from x-ray mammography, representing the average absorbed dose in the glandular tissue (excluding skin) in a uniformly compressed breast composed of 50% fat and 50% glandular tissue [7]. Therefore, AGD has been adopted as dose index in mammography [8, 9] and it is routinely evaluated in quality assurance programs of mammography units.

AGD is a fundamental quantity because it is directly related to the risk for patients exposed to ionising radiation by mammography. However, a direct calculation of AGD in the breast is not feasible [10]. In fact, the AGD depends on the half value layer (HVL) and on the incident air kerma (k _a,i ) at the upper surface of the breast. These are measurable quantities, but calculations are practically done using tabulated conversion factors, derived from Monte Carlo simulations.

In this article, a new algorithm for the calculation of the average absorbed breast dose (2ABD) is presented. Following this approach, the 2ABD can be directly calculated for each anode-filter combination based on the kVp and mAs used and on the breast thickness.

The experimental points used for the calculation of the parameters α and β by Equation 4 for each anode-filter combination are shown in Fig. 1, where the measured values of k _a,i /mAs are reported on the y-axis as a function of the measured values of kVp. Y _tb  = Y ₀ is the yield of the reference x-ray device evaluated for each anode-filter combination at 28 kVp and d = 0.

The graphical representation of Equation 5 for each anode-filter combination is shown in Fig. 2. The points represent the average values of four measurements of I(x) done by using a 60-cm³ ionisation chamber placed at different depths in a solid-water phantom. Data showed the expected exponential decay trend, thus the experimental points were fitted by Equation 5 to evaluate the energy absorption coefficient μ _en . The results of the fit are reported in Table 3 for each anode-filter combination.

	Mo-Mo
kVp	22	24	26	28	30
I 0	1.45 ± 0.04	2.05 ± 0.05	2.77 ± 0.06	3.55 ± 0.08	4.41 ± 0.09
μ en	1.04 ± 0.05	0.95 ± 0.04	0.89 ± 0.04	0.84 ± 0.04	0.81 ± 0.03
	Mo-Rh
kVp	24	26	28	30	32
I 0	1.59 ± 0.03	2.22 ± 0.03	2.88 ± 0.03	3.62 ± 0.04	4.39 ± 0.06
μ en	0.80 ± 0.03	0.75 ± 0.02	0.72 ± 0.02	0.70 ± 0.02	0.68 ± 0.02
	Rh-Rh
kVp	26	28	30	32	34
I 0	2.08 ± 0.06	2.69 ± 0.07	3.39 ± 0.09	4.1 ± 0.1	4.9 ± 0.8
μ en	0.76 ± 0.04	0.71 ± 0.04	0.67 ± 0.04	0.64 ± 0.03	0.61 ± 0.03
	W-Rh
kVp	26	28	30	32	34
I 0	1.02 ± 0.02	1.25 ± 0.02	1.46 ± 0.02	1.68 ± 0.02	1.90 ± 0.03
μ en	0.61 ± 0.02	0.59 ± 0.02	0.56 ± 0.02	0.54 ± 0.02	0.53 ± 0.02
	W-Ag
kVp	26	28	30	32	34
I 0	1.25 ± 0.02	1.59 ± 0.02	1.90 ± 0.03	2.21 ± 0.03	2.52 ± 0.04
μ en	0.58 ± 0.02	0.55 ± 0.02	0.52 ± 0.03	0.50 ± 0.02	0.48 ± 0.02

The uncertainty on 2ABD considering each error contribution varied in the range 17-33% (Table 5).

The three methods appear to be consistent within the uncertainties. A good agreement can be observed among data in Table 5 for each specific patient.

2ABD, defined as the average value of energy absorbed per unit mass in the breast, is a suitable, easily measurable physical quantity, related to the patient exposure in mammographic procedures. The method presented in this paper requires the knowledge of kVp, mAs, breast thickness d, and anode-filter combination. All these parameters are set on the mammographic device and they are reported also in the DICOM-header file associated to each image. Y _tb can be periodically calculated for each device and anode-filter combination, during the quality controls; FID is a characteristic of the device. Thus, all these parameters are available and could be employed to implement an automatic calculation of 2ABD for each mammographic procedure. Then, 2ABD can be recorded in the report of each exam.

Different anode-filter combinations were characterised in order to evaluate k _a,i by Equation 4 without needing to measure it directly in each procedure. This could avoid inserting expensive measurements systems (which must be periodically controlled and calibrated) in the different mammographic devices.

AGD calculated by the Dance method (Equation 1) or the Wu method (Equation 2) represents the reference standard in mammography quality assurance programs. Even though the AGD does not take into account the fundamental individual radiobiological factors (i.e. radiosensitivity and radiosusceptivity), it has been directly related to the risk of carcinogenesis due to the exposition of the breast tissue to x-ray [5]. Thus, it can be considered as the dose index to be written in the report of the mammographic procedure, as explicitly requested by the 2013 European Council Directive [4]. According to Dance [10], AGD cannot be directly measured. It needs a dedicated detector for the k _a,i evaluation. It is also a function of the HVL, which strictly depends on kVp and anode-filter combination, as well as on other non-easily measurable parameters such as breast density. For these reasons, AGD can be evaluated in breast phantoms, but its calculation for individual patients is inaccurate [19]. As a consequence, the possibility to directly communicate the AGD in the mammographic report is questionable. Conversely, the new proposed dose index 2ABD can be easily calculated from parameters set for each mammogram and stored in the DICOM-header of each patient image, avoiding direct k _a,i measurements. Thus, 2ABD could be easily automatically included in every mammographic report, saving time and facilitating quality assurance.

A comparison between AGD, calculated by Equations 1 and 2, and 2ABD, calculated by Equation 3 (with k _a,i evaluated by Equation 4), was done for 20 procedures selected for different anode-filter combinations and different breast thickness.

Differently from Dance et al. [10–13] or Wu et al. [14, 15], 2ABD does not take into consideration differences in breast density (whose range is 0.93–1.04 g/cm³ [6]). Dance relates breast density to the age of the patient (he considers two ranges of age, 40–49 years or 50–64 years) and to the breast thickness [13]. As reported by Geeraert et al. [20], the evaluation of breast density is a critical factor affecting AGD accuracy. Based on these considerations, the opportunity to consider the breast density in the routinely breast dose calculation for each patient appears to be questionable and should be further discussed.

Anyway, despite the non-consideration of breast density by our 2ABD method, Table 5 does not show evident discrepancy between AGD and 2ABD values, although a different set of radiation exposure and patient-related parameters was involved in each mammographic procedure. The results of the comparison between 2ABD and AGD evaluated by either Dance or Wu methods appear to be consistent within the uncertainties.

Thus, 2ABD could work as surrogate of AGD and could be taken into consideration as a proxy of AGD, to be provided for each mammographic procedure, having the 2ABD calculation algorithm implemented in the software of the mammographic device.