Session 5 mammography

Master in Medical Physics 2015 to 2016

Quality control in Mammography

Francisco J.Hernández Flores∗

International Centre for Theoretical Physics

[email protected]

August 3, 2015

Abstract

This report it is about the quality control in mammography, the importance inaccurately diagnosing breast cancer, particularly in earlier stage cancers, reducingits high mortality rate in women, it is essential that all mammograms be performedand interpreted with the highest possible quality standards. The existence of andstrict adherence to quality assurance (QA) and quality control (QC) measuresand guidelines must be practiced in all mammography facilities in order to assurethe most accurate diagnoses for all patients. The following report will discusscurrent QC and QA measures in mammography, including the Phantom exposurefor determine the mean glandularly dose, determination of incident air Kermaand determination of Half Value Layer. In the case of mean glandular dosewas evaluated for the manufacturer this measure was compared with the meanglandular dose obtained in practice using ki and HVL, the difference obtainedwas 14.2%.the value obtained for HVL during the practice was 0.64 mmAl using28KVand 74mAs, the incident air kerma was 5.026 mGy.

I. Introduction

Breast dosimetry is an importantpart of mammographic quality con-trol and is an essential element ofthe optimization of ionizing-radiation-based breast imaging procedures. Forconventional projection mammogra-phy there are various standard proto-cols for the estimation of breast dosewhich provide conversion factors torelate the incident air kerma at the up-

per surface of the breast to the meandose to the glandular tissues withinthe breast (mean glandular dose). Thedetermination of the mean glandu-lar dose and HVL was made using x-ray spectra from a W/Ag target/filtercombination at 28 kV using ionizationchamber and electrometer PTW forobtain the measured and comparedwith the manufacturer value.

∗Physics of diagnostic X ray 2

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II. Theory

I. Half-Value Layer

The half-value layer (HVL) is definedas the thickness of material requiredto reduce the intensity (e.g., air kermarate) of an x-ray or gamma-ray beamto one half of its initial value. TheHVL of a beam is an indirect mea-sure of the photon energies (also re-ferred to as the quality) of a beam,when measured under conditions ofnarrow- beam geometry. The HVL ofa diagnostic x-ray beam, measured inmillimeters of aluminum under nar-row beam conditions, is a surrogatemeasure of the penetrability of an x-ray spectrum. It is important to un-derstand the relationship between µ

and HVL. In Equation 1, N is equalto No/2 when the thickness of the

absorber is 1 HVL. The HVL can beeasily calculated from the linear atten-uation coefficient, and vice versa. themathematically form of evaluation itis show in the equation 2 [3]. Thecondition for obtain the HVL in mam-mography during the measurementsis take into account compression plateSelect the target, filter and the valuefor tube voltage and typical tube load-ing that would be used for the routineclinical examination of the breast be-ing simulated (see figure 1 Scheme ofmeasurement HVL in reference condi-tion).

N0

2= N0 × e−µ(HVL) (1)

HVL =ln2µ

(2)

Figure 1: Measurements scheme HVL [4]

II. Mean glandular dose

The Mean Glandular Dose (MGD)is the special dose quantity used in

mammography. It is defined as themean, or average, dose to the glan-dular tissue within the breast. The

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assumption is that the glandular tis-sue, and not the fat, is the tissue at riskfrom radiation exposure. Obviously, itis just about impossible to determinethe actual dose to the glandular tissueduring a specific mammographic pro-cedure because of variations in breastsize and distribution of glandular tis-sue within the breast. The MGD isbased on some standard breast param-eters. MGD values are determined byfollowing a standard two-step proto-col.

• The first step is to determine theentrance surface exposure, or airkerma, to the breast. This canbe measured directly with smalldosimeters placed on the breastor calculated from the know cali-bration factors for the mammog-raphy equipment.

• Then, the MGD is determinedby multiplying the surface ex-posure value by published dosefactors. The dose factor valuesare tabulated according to breastsize and composition and thepenetrating characteristics of thex-ray beam as determined by theanode material, filtration, andKV.

For comparison of imaging tech-niques, evaluation of equipment per-formance, general dose management,and regulatory and accreditation pur-poses, the MGD to a "standard" breastis used. The standard is a 4.2cm thickcompressed breast consisting of 50%glandular tissue and 50% fat. Thiscorresponds to the standard phantomthat is used for image quality evalua-

tion and comparative dose determina-tions.

Conversion coefficient to estimatemean glandular dose from incident airkerma are dependent on the glandu-larity of each breast. Two approchesare possible: Assume glandularity of50% estimate the effective glandular-ity.

DG = CDG50,ki,PMMA × S × Ki (3)

Where CDG50,ki,PMMA is the Conver-

sion factor, in mGy/mGy, used to cal-culate the mean glandular dose tobreast of 50% gladularity from inci-dent air kerma, S is the factor fordifferent mammographic target/filtercombinations, Ki is the incident airkerma. [4]

Estimation of PMMA equivalence:for the dosimetry protocols underconsideration, MGD may be deter-mined using exposures of slabs ofPMMA which are equivalent to spec-ified model breasts. The equivalentthickness of PMMA is the thickness ofPMMA that gives the same incidentair kerma at its upper surface as thatfor a model breast of specified thick-ness and composition. [4]

Figure 2: mounting for MGD

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III. Incident Air Kerma

The incident air kerma, Ki, is thekerma to air from an incident X raybeam measured on the central beamaxis at the position of the patient orphantom surface (Fig. 1). Only theradiation incident on the patient orphantom and not the backscattered ra-diation is included. Unit: J/kg. Thename for the unit of kerma is gray(Gy). [2]

Figure 3: mounting for Ki

—————————————————————————————-

III. Methods

I. Evaluation of differentpoints in quality control ofmammography

In this practical exercise we use themammograph giotto, chamber paral-lel plate PTW, phantom PMMA andmeter measure, for measurement thedistance focus chamber.

I.1 Evaluation of MGD use themanufacturer value

The standard method of estimatingthe MGD dose on patients undergoingmammography X-ray examinationsis based on the incident air kermameasurements without backscatterand the conversion to glandular doseusing appropriate conversion factorsdepending on the type of phantomused. The air kerma value may bedetermined either for patients or for astandard breast phantom; polymethylmethacrylate (PMMA) is normallyused as a breast substitute phantom

(see figure 2).

The first measurement was aboutthe phantom exposure, we positioningthe PMMA phantom with two differ-ent: thickness, filters, KV and mAs atthe same distance in the equipment ofmammography giotto, these parame-ter it is show in table 1.

I.2 Determination of incident airKerma

In this case was evaluate X-ray tubeoutput values of the free in air inci-dent air Kerma to the chamber for amammography system’s target and fil-ter combination, kV, mAs, and sourceto chamber distance (see figure 3). As-sume that a mammography systemwith a tungsten target and silver fil-ter, uses 28 kV and 74 mAs for a Fo-cus Chamber Distance of 56.5cm. Thechamber was positioning down thecompressor plate the follow parame-ter use are expressed in the table 2.

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I.3 Determination of Half ValueLayer

The this part was determined of theHVL added different thickness ofmmAl betwen the radiation beam andionization chamber, we went collected

each incident air kerma obtained fordifferent thickness, After these mea-sures were plotted incident air kermavs thickness and interpolate to findthe value of HVL the value was ex-pressed in graph 4

IV. Results and discussion

Focus table distance 61.7cm 61.7cmPhantom thickness 4.5cm 2cm

Anode Tungsten TungstenFilter Ag RhKV 28 25

mAs 74 40

Table 1: Determine the mean glandular dose for the manufacturer value

The data shown in this table wereused to determine mean glandulardose using the manufacturer values,in this case we evaluate the mean glan-dular dose for two different thicknessof PMMA using two different com-bination of anode filter, for the com-

bination tungsten anode with silverfilter for standard thickness to 45mmPPM the mean glandular dose was1.6mGy, for the case when we com-bine tungsten anode with Rh filter,20 mm PMMA thick we obtained 0.8mGy of mean glandular dose.

Focus table distance 56.5cmAnode TungstenFilter AgKV 28

mAs 74Polarization of electromiter 200volt

RQR M2Factor of chamber calibration 4.385 × 1008cGy/C

reading I 5.025mGyreading II 5.015mGyreading III 5.035mGyreading IV 5.030mGy

mean of the reading 5.026mGy

Table 2: Determination of incident Air Kerma

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The graph 4 shows the half-valuelayer, in which the relation of the in-tensity of the kerma in air with thick

of aluminium, the result of HVL is0.64mmAl was obtained by interpola-tion.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

3.00

4.00

5.00

Thickness (mmAl)

Ki(m

Gy)

Fit HVL(mmAl) = 5.0002e−1.075x

Figure 4: Determination of half value layer; adding mm aluminum filter

The result of the Mean Glandular Dose was find using these follow value:HVL=0.64 mm,Ki = 5.026mGy and CDG50,ki,PMMA = 0.349mGy/mGy using theequation 3 MGD=1.83 mGy.

V. Conclusion

• The difference of MGD betweenmanufacturer and MGD ob-tained for measure value duringthe practice was 14.2% this out-come is high because the condi-

tion take into account during themeasure was not the adequate,according to the specific condi-tions for determining HVL.

References

[1] AAPM report Number 49, Equipment requirements and Quality Control

for Mammography,

[2] IAEA TRS 457Dosimetry in Diagnostic Radiology: An international

of code of practice, IAEA , 2007

[3] Jerrold T Bushberg the Essential Physics of Medical Imaging,second edition,Lippincott Williams-Wilkins, 2012

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[4] Paola Bregant , Lecture Physics of Diagnostic with x-ray 2, ICTP Trieste Italy,2015

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Health & Medicine

Session 5 mammography