EVALUATION OF AGD IN DIGITAL BREAST TOMOSYNTHESIS RELATIVE TO THOSE IN TWO- VIEW-FULL-FIELD DIGITAL MAMMOGRAPHY

H. S. Niroshani 1* , H. D. N. S. Hathurusinghe 2 and R. Tudugala 1 . 1. Department of Radiography & Radiotherapy, Faculty of Allied Health Sciences, Kotelawala Defence University, Sri Lanka 2. Department of Radiology, Nawaloka Hospitals Plc, Sri Lanka. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

Digital Breast Tomosynthesis (DBT) is performing in high resolution limited angle tomography at radiographic dose levels. Advanced system facilitates the DBT along with 2 view Full Field Digital mammography (FFDM). The objectives of this study were to compare the Average Glandular Dose (AGD) ofDBT and FFDM and tocalculate the percentage of radiation dose reduction when using DBT with compare toFFDM. The study was carried out using data base of DBTsystem in a private hospital. In the investigation we analyzed dose of 251 patients who underwent mammographic examinations of both FFDM and DBT. All data analyses were done using IBM SPSS statistical software version 20.0.P-value 0.05 was considered as statistically significant. The explanatory variables were age, breast thickness, kVp, mAs, target/filter combination, and AGD values of DBT and FFDM. Mean values for the patient age and compressed breast thickness were 50 years and 49 mm(±11.9SD) respectively. The majority of the images were acquired using W/Rh target/filter combination and 51% patients came for the diagnostic mammograms and 49% for screening mammograms. A wide kVp range was observed for DBT than FFDM while mAs range was lower in DBT.According to the results total average glandular dose (TAGD) from FFDM and DBT for diagnostic was 4.21mGy (±1.46SD), for screening 4.04(±1.31SD) andthere was a statistically significant difference between mean values of TAGD. MeanAGD forMediolataral Oblique (MLO) view in DBT was 2.05mGy(±0.60SD), in FFDM 2.73mGy(±1.02SD).AGD for Craniocadal (CC) view in DBT was 1.63mGy(±0.36SD) and for FFDM it was 1.83mGy(±0.66SD).AGDto the breast from DBT was significantly lower than that for FFDM while range was lower in FFDM than DBT. There was a significant difference between mean values of CC and MLO views in DBT and FFDM (P<0.05). It was evident that AGD from DBT was lower than that for FFDMfurther AGD was reduced by 55.3% by using DBT with compared to TAGD, 198 using both FFDM and DBT together for same patient and AGD was reduced 19.19% by using DBT with compared to AGD from FFDM.

…………………………………………………………………………………………………….... Introduction:-
Mammography is considered to be the most effective method for the early detection of breast cancer [1] .Although mammographic process is using low energy x-rays to examine the human breast the use of ionizing radiation implies the risk of inducing fatal breast cancer [2] . Minimizing radiation risk is important in general as manifested by the as low as reasonably achievable (ALARA) principle. [3] [4] As glandular tissue is radiosensitive organ, the average glandular dose (AGD) to the breast is the most important quantity to estimate the risk of radiation-induced carcinogenesis from mammography. AGD is currently accepted as an estimation of the patient dose in mammography. [4] Digital Breast Tomosynthesis (DBT) is a FDA approved radiological imaging method for breast imaging [1] . It performs high resolution limited angle tomography at radiographic dose levels [5] [6] . DBT allows the detection of a greater number of expansion lesions and a better morphological analysis of masses and architectural distortions [7] . Advanced system facilitates the DBT along with two view Full Field Digital mammography (FFDM) [8] [9] . Therefore the purpose of the study was to compare the Average Glandular Dose (AGD) of DBT and FFDM and to calculate the percentage of radiation dose reduction when using DBT with compare to FFDM .

Methodology:-
The study was carried out using data base of Digital Breast Tomosynthesis system at a private hospital,Colombo. This was a quantitative, retrospective research project. The study sample was included 251 patients exposure parameters. Data was collected through machine storage in the period of 01.01.2016 to 01.04.2016.KV, mAs, thickness, age,force, view, AGD, ESD, Exposure Index (EI) was recorded for each individual subject in to excel sheet. The data was analyzed by SPSS 20.0 software with P<0.05 indicating level of significance and using descriptive statistics.

Results:-
Below  66SD). AGD to the breast from DBT was significantly lower than that for FFDM while range was lower in FFDM than DBT. There was a significant difference between mean values of CC and MLO views in DBT and FFDM (P<0.05).   The above graphs show negative linear regression between AGD and age for both DBT and FFDM in both views. Ionizing radiation such as x-rays widely used in radiological examination. Hence there should be a special concern on radiosensitive organs which are exposed to direct x-ray beam. In mammography average glandular dose should be kept minimum due to high radiosensitivity of mammary glands.Chevalier M etal states that TAGD was 3.8mGy and it slightly lower than TAGD from current study.Further mean age and thickness from both study were similar. [9] In both study AGD form CC view was lower than MLO due to thickness of the CC view lower than MLO.

Relationship of average glandular dose with breast thickness for FFDM and DBT
Total average glandular dose (TAGD) from FFDM and DBT for diagnostic was significantly higher than that for screening owing to dose is directly propotional to mAs and screening category has much lower mAs value than diagnostic [1] .And it was ensure that negative linear regression between AGD and age for both DBT and FFDM of screening and diagnostic categories due to the fatty tissue of the breasts may become more prominent than the glandular tissue as women ages.
According to the results mean average glandular dose to the breast from DBT was significantly lower than that for FFDM. It was evident that AGD from DBT was lower than that for FFDM. Which is similar to the study of Svahn TM etal but controversial to the study of Olgar T etal [3] . Further AGD was reduced by 55.3% by using DBT with compared to TAGD, using both FFDM and DBT together for same patient and AGD was reduced 19.19% by using DBT with compared to AGD from FFDM.