Characterisation of microcalcification clusters on 2D digital mammography (FFDM) and digital breast tomosynthesis (DBT): does DBT underestimate microcalcification clusters? Results of a multicentre study.

Introduction

Digital breast tomosynthesis (DBT) is a novel mammography technique capable of studying the breast using 3D reconstructions of the tissue from multiple low-dose digital mammographic images, acquired along a 15 to 50 degree arc.

This process increases lesion visibility because of the reduction of overlapping breast tissue.

Recently, digital breast tomosynthesis alone or combined with digital mammography has been shown to detect cancers occult to 2D mammography.

Its potential role as a screening test has also been recently demonstrated in large scale prospective screening trials.

In addition, a recent meta-analysis showed that DBT may have a higher sensitivity and specificity in breast diagnosis than digital mammography.

Regarding radiation dose, an increase in radiation dose due to the 3D acquisition has been detected with an increase of entrance skin air kerma and average glandular dose.

Very few reports in the literature have compared FFDM and DBT for classification of microcalcification clusters.

Some authors have outlined that DBT may underestimate BI-RADS cluster classification compared to FFDM. In theory, using DBT, the radiologist may be more accurate in characterising some breast lesions to reduce the rate of false positive breast biopsies but it is unknown whether this applies to microcalcifications.

The purpose of this study was to assess differences between DBT and FFDM in the characterisation of microcalcification clusters using the standard BI-RADS classification.

Methods

Institutional Review Board approval and written informed consent were received.

The study was undertaken in three different centres: IRCCS San Martino Hospital-University of Genoa, IRCCS Molinette Hospital Torino and Bolzano Central Hospital.

Procedures

A total of 120 mammographic examinations showing microcalcification clusters (MCs) were identified based on searching the electronic databases of the radiological reports between January 2010 and June 2012.

All women had digital mammography using a Selenia Dimensions unit with integrated 2D and 3D mammography done in the COMBO mode (acquisition of 2D plus 3D images in the same session).

The X-ray tube moves over a 15° arc.

The 2D and 3D images are acquired at the same examination with a single breast position and compression.

Each 2D and 3D image consisted of a bilateral two-view (mediolateral oblique and craniocaudal) mammogram.

Only MCs with both DBT and FFDM images available and with a follow-up of at least 12 months were included in this study.

Clearly benign calcified lesions such as calcified fibroadenomas were not included in the analysis.

Final histology was used to confirm malignant cases.

Bilateral mediolateral oblique and craniocaudal views were considered an image set for FFDM, whereas the 3D mediolateral oblique and craniocaudal view reconstructed images and the video clip were considered the DBT image set.

Six breast radiologists, each with at least 7 years of experience in digital mammography and 2 years of experience in the clinical use of DBT, read the FFDM and DBT image sets prospectively and assigned a BI-RADS score to each cluster.

The two image sets (FFDM and DBT) were read independently in sessions separated by at least 4 weeks to minimise recall bias.

Data analysis

The overall sensitivity and specificity with a 95 % confidence interval were calculated for FFDM and DBT using final histology and clinical follow-up as the reference standard.

Comparative sensitivity of FFDM and DBT was calculated classifying results as positive or negative as follows:

benign (R1-R2) vs. malignant (R3-R4-R5).

Results

The calcification clusters with complete data comprised 107 of 120, with 13 clusters excluded because of incomplete follow-up (n=10 patients lost to follow-up) or to missingDBT data in the hospital PACS (n=3).

All clusters were visible on both DBT and FFDM.

The mean age was 51.7 years.

The benign/malignant ratio of MC was 66/41.

There were 11/107 discordant results where DBT classified calcification clusters as R2, whereas FFDM classified these as R3 in 9 and R4 in 2.

Three of these (3/107=2.8 %) were malignant and incorrectly underclassified as R2 on DBT.

Eight (8/107=7.5 %) were non malignant and were correctly classified as R2 on DBT but were incorrectly classified as R3 on FFDM.

Left craniocaudal (CC) view in FFDM and DBT shows a microcalcification cluster that was classified with the BI-RADS classification as R4 in FFDM and R2 in DBT (ductal carcinoma at histology)

DBT correctly classified 8/107 discordant clusters that were benign.

The patients with these clusters may have avoided unnecessary biopsies based on DBT.

On large numbers these data may be relevant since they imply that DBT could potentially reduce the burden of recall for benign calcifications and overtreatment in a screening setting.

Discussion

This study focused on evaluation of MC because clinical experience with DBT for the assessment of breast lesions has revealed some potential pitfalls with the imaging of microcalcifications.

Possible underlying reasons for this include that the morphology of some calcifications in DBT appear to be different when compared to the 2D FFDM images or that some calcifications seem less visible on DBT.

This study stated that FFDM appears to be slightly more sensitive than DBT for the detection of calcification.

However, diagnostic performance using BI-RADS was not significantly different between the two modalities.

The global analysis of the data suggests that DBT has the potential disadvantage of missing a very small proportion of malignant MCs.

Therefore, if only 3D is used, there is a risk of lesion underestimation if the only mammographic sign is a microcalcification cluster, so it is possible that a small proportion of lesions such as DCISs could be missed using only 3D.

At present, trial evidence supports use of 2D plus 3D images (and not only 3D images) for the purpose of screening.

Conclusion

Conclusion

This study suggests that microcalcification clusters may be classified differently on FFDM and DBT using the BI-RADS classification, although this only occurred in a minority of cases.

In discordant cases, DBT-assigned lower BI-RADS classes compared to FFDM, and DBT may have missed some malignant and premalignant lesions.

On the other hand, DBT may have the advantage of avoiding unnecessary biopsies in patients with benign conditions manifest as microcalcifications.

Conclusion

Most MCs are similarly scored on BI-RADS on FFDM and DBT, but DBT has the potential disadvantage of underclassifying a very small proportion of malignant lesions that manifest as MC.

Authors suggest careful evaluation and further research on the use of DBT for assessment of microcalcifications.

Take home message

2D plus 3D images (and not only 3D images) should be used for the purpose of screening to avoid underestimation of MC clusters if 3D images are used alone.

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