BSBR breast imaging classification

Last revised by Derek Smith on 15 Dec 2020

The British Society of Breast Radiologists (BSBR) publish with the Royal College of Radiologists a standardized classification for breast imaging in the United Kingdom. The first edition in 2009 was based on findings from the RCR Breast Group (RCRBG) 1 with the current fourth edition published in November 2019 2. This 5-point scale is used to classify the suspicion of malignant lesions, for both symptomatic and screening populations.

Each breast is scored separately, and according to the most suspicious lesion:

  • 1: normal
  • 2: benign
  • 3: intermediate / probably benign
  • 4: suspicious for malignancy
  • 5: highly suspicious of malignancy

The classification system is common to the major forms of breast imaging, as well as clinical examination and pathology:

Examples of normal findings (U1/M1) include normal involutional changes, and other benign findings commonly seen on screening mammograms and non-symptomatic. These include bilateral powdery microcalcifications and small (<5 mm) well-defined nodules.

U2/M2 findings include clearly benign lesions e.g. simple cysts, lipomas, normal intramammary lymph node or fat necrosis (with an appropriate history).

The BSBR recommend inclusion of the score within both the report and the radiological summary/opinion, e.g.:

  • right breast: no abnormality; U1
  • left breast: irregular right upper outer mass with indistinct margin; U4

The recommendation for any atypical or suspicious features resulting in a higher lesion score is for "further investigation". In most centers this is by using US-guided core biopsy.

Patient age is taken into consideration for forms of imaging, and particularly for the requirement of fibroadenoma biopsy. The current Royal College of Radiologists guidance 2 is if a patient is under 25 years old with a typical presumed fibroadenoma (ellipsoid, wider than tall, well-defined, <4 gentle lobulations, no calcification/shadowing, thin echogenic pseudocapsule) then no further investigation is required. Even if the above features are present and the patient is >25 years old, then diagnostic core biopsy is recommended.

The 2019 update also includes a scoring system for axillary nodal lesions, which are commonly included in lesion workup.

  • A1: normal
  • A3: indeterminate; nodal biopsy recommended
  • A4: suspicious of malignancy; nodal biopsy recommended
  • A5: highly suspicious of malignancy; nodal biopsy recommended

No agreed national threshold for nodal cortical thickness, although some units practice between 2-4 mm 2.

The BSBR 5-point score was developed after the ACR BI-RADS which is in common usage across North America and Europe. A 2011 UK study 3, early in the use of the BSBR system, found malignancy rates as follows:

  • M1 - 1.8%, M2 - 1.3%, M3 - 40.8%, M4 - 94.6%, M5 - 97.8%
  • U1 - 0.4%, U2 - 1.8%, U3 - 17.7%, U4 - 88.2%, U5 - 97.1%

While the benign and highly suspicious lesions compare well (0.4% versus 0%, 97.8% versus >95%), there is some discrepancy comparing RCR with BI-RADS, as BI-RADS 4 can be attributed to indeterminate U3/M3 or suspicious U4/M4 lesions. In practical terms however, all lesions U3/M3 or above should be biopsied.

The BI-RADS nomenclature is recommended for breast MRI reporting, although the BSBR score is to be included in the opinion.

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