Guidelines for the diagnostic management of incidental solitary bone lesions on CT and MRI in adults

Key points

• Bone-RADS sorts incidental solitary bone lesions on CT and MRI in adults into four categories: 1, leave alone; 2, perform a different imaging modality; 3, perform follow-up imaging; 4, biopsy and/or oncologic referral.
• Pain attributable to the lesion, cortical involvement, soft tissue extension, pathologic fracture or aggressive periosteal reaction lead to Bone-RADS4.
• Internal fat (mean density below -10 HU on CT) or features typical of certain benign lesions allow a Bone-RADS1 designation.
• Bone-RADS3 lesions are followed up at 6, 6 and 12 month intervals, for a total of 2 years.
• The algorithms provide a framework and are not meant to override expert opinion or clinical context.

Overview

This white paper from the Practice Guidelines and Technical Standards Committee of the Society of Skeletal Radiology (SSR) proposes a bone reporting and data system (Bone-RADS) for solitary bone lesions found incidentally on CT and MRI in adults. Algorithms for CT (lucent lesions, and sclerotic or mixed density lesions) and for MRI lead the radiologist to one of four management recommendations. The approach focuses on management rather than on a specific diagnosis, and is weighted towards safely identifying lesions that need no further workup.

Development and scope

Twelve SSR members and one orthopedic oncologist formed the committee, tested the algorithms on sample cases and presented them at the SSR 2021 annual meeting for comments. Children are not addressed because they have a unique set of lesions and a different appearance of bone and marrow. The scope is limited to solitary lesions, because patients with multiple lesions often have metastatic or systemic disease that typically requires biopsy and/or oncologic referral. An incidental lesion is defined as one detected on an imaging study performed for an unrelated reason.

CT assessment

A lesion is lucent if more than 90% of its volume is lucent; any other lesion is assessed with the sclerotic/mixed density chart. The first step is to look for concerning clinical or imaging features, which lead to Bone-RADS4. If the patient has a known malignancy with a propensity for bone metastases, such as kidney, prostate, breast, lung or thyroid cancer, further characterization with FDG-PET, bone scan or MRI (Bone-RADS2) or follow-up imaging (Bone-RADS3) is recommended. Lesions containing fat, and lesions entirely consistent with fibrous dysplasia, non-ossifying fibroma, enchondroma, subchondral cyst or hemangioma, are Bone-RADS1. In a purely sclerotic lesion, a mean density above 885 HU is sensitive and specific for enostosis. In cartilage lesions, endosteal scalloping of more than two-thirds of the cortical thickness, an axial location or an epiphyseal location raises concern for malignancy.

MRI assessment

Assessment starts with T1 signal compared with skeletal muscle or intervertebral disc; if no T1-weighted sequence is available, the patient should return for one. Lesions that match subcutaneous fat on all sequences are almost always benign. On chemical shift imaging, a signal drop of more than 20% supports a benign diagnosis; this cutoff has a reported sensitivity of 95-100% and specificity of 61-95%. Low T1 lesions are then assessed by T2 signal and for concerning features such as surrounding marrow edema, solid mass-like enhancement or elevated PSA. The halo sign has a reported sensitivity of 75%, specificity of 99% and accuracy of 88% for metastasis. A solitary sternal lesion in a patient with breast cancer has a 76% likelihood of being metastatic.

Expert opinion and controversies

At the SSR meeting, one concern was that lesions with a classic appearance to musculoskeletal radiologists might be designated Bone-RADS4 by non-specialists. The authors stress that the flowcharts should never override expert opinion. They chose to err on the side of not ignoring possibly malignant lesions, since dismissing a malignancy has greater consequences than over-imaging a benign lesion.

Frequently asked questions

How reassuring is fat inside a bone lesion?

Very reassuring: identifying fat within a bone lesion has a negative predictive value for malignancy of 99.5%.

Can radiographs help?

Yes. Radiographs can help in lesion assessment, especially for lesions discovered incidentally on MRI, and should be taken into account if available.

Should lesion density be measured on contrast-enhanced CT?

Values should be assessed on non-contrast images when possible, because intravenous iodinated contrast can elevate lesion density.

Source

Chang CY, Garner HW, Ahlawat S, Amini B, Bucknor MD, et al. Society of Skeletal Radiology - white paper. Guidelines for the diagnostic management of incidental solitary bone lesions on CT and MRI in adults: bone reporting and data system (Bone-RADS). Skeletal Radiology 2022;51(9):1743-1764. DOI: 10.1007/s00256-022-04022-8. Open access article. This page is a summary prepared by Medpresso from the original publication and is not a substitute for the full text or for medical advice.