Understanding Unexpected Bone Lesions: A Patient's Guide to the Bone-RADS Classification System

Table of Contents

• Introduction: Why These Guidelines Matter
• What These Guidelines Cover
• How Bone Lesions Are Classified
• The Four Management Categories (Bone-RADS)
• Evaluating CT-Detected Bone Lesions
• Evaluating MRI-Detected Bone Lesions
• Important Clinical Factors
• What These Guidelines Cannot Determine
• Patient Recommendations and Next Steps
• Source Information

Introduction: Why These Guidelines Matter

Unexpected solitary bone lesions are frequently discovered during computed tomography (CT) and magnetic resonance imaging (MRI) scans performed for unrelated medical reasons. Despite being common findings, clear and consistent guidelines for managing these incidental discoveries have been lacking until now. The Society of Skeletal Radiology recognized this gap and assembled a team of 12 bone imaging specialists and one orthopedic cancer surgeon to develop evidence-based management algorithms.

These experts reviewed current medical literature and combined their clinical experience to create the Bone Reporting and Data System (Bone-RADS). This system provides radiologists, particularly those who aren't bone specialists, with clear pathways for determining when a bone lesion can be safely ignored, requires additional imaging, needs monitoring over time, or warrants immediate biopsy and specialist referral. The guidelines underwent extensive review and revision based on feedback from the broader radiology community before publication.

The primary goal of these guidelines is to provide consistent, evidence-based recommendations for managing incidental bone findings while minimizing unnecessary patient anxiety, additional testing, and procedures. The system is weighted toward correctly identifying truly benign lesions that require no further action, ensuring patients aren't subjected to needless follow-up for harmless findings.

What These Guidelines Cover

These guidelines specifically address solitary bone lesions discovered unexpectedly on CT and MRI scans in adults. The committee focused on single lesions because patients with multiple lesions often have metastatic cancer or systemic conditions that typically require biopsy and oncology referral regardless of imaging characteristics. The guidelines do not apply to children, as pediatric patients have different types of bone lesions and unique bone marrow appearances that require separate consideration.

The algorithms are designed specifically for lesions unrelated to the reason the imaging was performed. For example, if a shoulder MRI was performed to evaluate rotator cuff pain and reveals a bone lesion, that lesion would be considered incidental if it's not causing the shoulder symptoms. The guidelines help determine whether that lesion requires attention or can be safely ignored.

It's important to understand that these guidelines provide a framework for evaluation but cannot replace expert clinical judgment or account for individual patient circumstances. They standardize the approach to these common findings but don't address advanced management decisions like surgery, radiation, chemotherapy, or ablation therapy, which remain beyond their scope.

How Bone Lesions Are Classified

Bone lesions are classified differently depending on whether they're detected on CT or MRI scans. On CT scans, lesions are categorized by their density compared to normal bone, while MRI classification focuses on signal characteristics visible on different imaging sequences.

For CT scans, lesions are classified as either lucent (darker than normal bone), sclerotic (denser than normal bone), or mixed density (combination of light and dark areas). Lucent lesions are defined as having lower attenuation than normal trabecular bone in more than 90% of their volume, typically measuring between 0 and 200 Hounsfield units (HU), which is the standard measurement of density on CT scans. Fatty lesions containing macroscopic fat measure between -120 and -30 HU.

Sclerotic lesions have higher density than surrounding bone, while mixed density lesions contain approximately equal amounts of sclerotic and lucent components. Common sclerotic lesions include bone islands (enostoses) and osteoblastic metastases, while mixed density lesions often include benign fibro-osseous lesions, cartilage tumors, bone death (osteonecrosis), and degenerative joint cysts.

For MRI detection, lesions are first classified by their appearance on T1-weighted images as either hyperintense (brighter than muscle) or isointense/hypointense (same brightness or darker than muscle). T1 hyperintense lesions might contain fat, like bone hemangiomas or red marrow, while most tumors and metastases fall into the isointense/hypointense category. These lesions are further characterized by their appearance on T2-weighted images as hypointense (dark), intermediate intensity, or hyperintense (very bright, similar to fluid).

The Four Management Categories (Bone-RADS)

The Bone-RADS system provides four clear management recommendations that guide what should happen next when an incidental bone lesion is discovered:

Bone-RADS 1: Leave Alone - The lesion is clearly benign and requires no additional workup, monitoring, or treatment. Examples include bone islands (enostoses), non-ossifying fibromas, and typical red marrow deposits. These lesions have characteristic features that allow radiologists to identify them with high confidence.

Bone-RADS 2: Perform Different Imaging Modality - The current imaging doesn't fully characterize the lesion, so additional imaging with a different technique is needed. This might involve getting an MRI if the lesion was found on CT, or a CT if found on MRI, or potentially specialized scans like PET scans or bone scans for further evaluation.

Bone-RADS 3: Perform Follow-up Imaging - The lesion is indeterminate but doesn't show concerning features. The recommendation is to repeat the same imaging after specific time intervals to check for changes. The standard follow-up schedule is imaging at 6 months, then another 6 months later (12 months from discovery), and then 12 months after that (24 months from discovery) for a total of 2 years of monitoring.

Bone-RADS 4: Biopsy and/or Oncologic Referral - The lesion shows concerning features that suggest possible malignancy or need for treatment. This recommendation triggers immediate further action, typically starting with referral to an orthopedic oncologist who will determine whether biopsy, surgical treatment, or other management is appropriate.

Evaluating CT-Detected Bone Lesions

The evaluation process differs for lucent versus sclerotic/mixed density lesions found on CT scans. For all lesions, the first step is assessing for concerning features that would immediately warrant Bone-RADS 4 classification.

Concerning features include pain specifically attributable to the lesion, cortical involvement (including bone destruction, tunneling, scalloping, expansion, or thickening), soft tissue extension beyond the bone, pathologic fracture (break through abnormal bone), or aggressive periosteal reaction (new bone formation patterns suggesting rapid growth). Any of these features automatically classify the lesion as Bone-RADS 4, requiring biopsy and/or oncology referral.

For lucent lesions without concerning features, the next step is determining if the patient has a known malignancy that commonly spreads to bones (such as kidney, prostate, breast, lung, or thyroid cancer). If yes, the lesion is classified as Bone-RADS 2 (different imaging needed) or Bone-RADS 3 (follow-up imaging) to ensure it's not a new metastasis.

If no cancer history exists, the radiologist checks for fat within the lesion (less than -30 HU), which indicates benign conditions like intraosseous lipoma, hemangioma, or red marrow (Bone-RADS 1). The radiologist also looks for characteristic features of five specific benign entities: fibrous dysplasia, non-ossifying fibroma, enchondroma, subchondral cyst, and hemangioma. If the lesion matches any of these patterns perfectly, it's classified as Bone-RADS 1.

Lesions that don't fit these benign categories but lack concerning features may represent various conditions including aneurysmal bone cysts, unicameral bone cysts, giant cell tumors, chondroblastomas, clear cell chondrosarcomas, myeloma, or metastasis. These typically receive Bone-RADS 4 classification, though some indeterminate cases might be designated Bone-RADS 2 or 3 based on specific imaging characteristics.

The evaluation process for sclerotic/mixed density lesions follows a similar pathway, beginning with assessment for concerning features that would warrant immediate referral (Bone-RADS 4). Without concerning features, the evaluation considers cancer history and specific imaging characteristics that might indicate benign conditions like bone islands, healing non-ossifying fibromas, bone infarcts, or Paget's disease.

Evaluating MRI-Detected Bone Lesions

MRI evaluation begins with assessing the lesion's appearance on T1-weighted images. If no T1 sequence is available, the guidelines recommend the patient return for additional imaging including T1-weighted sequences, as this information is essential for proper classification.

T1 hyperintense lesions (brighter than muscle) are evaluated for fat content using chemical shift imaging, which can detect microscopic fat. Lesions containing fat are typically benign (Bone-RADS 1), including intraosseous lipomas, hemangiomas, and red marrow deposits. Lesions without fat but with characteristic benign features might also be classified as Bone-RADS 1.

T1 isointense or hypointense lesions (same brightness or darker than muscle) require evaluation on T2-weighted images. T2 hypointense lesions (dark) might represent bone islands, osteoblastic metastases, or other dense lesions, while T2 hyperintense lesions (very bright, fluid-like) could include enchondromas, simple bone cysts, or other fluid-containing lesions.

Post-contrast imaging (after IV contrast injection) helps distinguish cyst-like lesions from solid tumors. Enhancing solid components raise concern for malignancy, while non-enhancing fluid-filled spaces are more characteristic of benign cysts. As with CT evaluation, concerning features like pain attributable to the lesion, cortical destruction, soft tissue extension, or aggressive appearance automatically classify the lesion as Bone-RADS 4 regardless of other characteristics.

Important Clinical Factors

Several clinical factors significantly influence how bone lesions should be managed. The most important is whether the patient has a known history of cancer that commonly spreads to bones, particularly kidney, prostate, breast, lung, or thyroid cancers. In these patients, even benign-appearing lesions might require more aggressive evaluation to exclude metastasis.

Pain specifically attributable to the lesion is another critical factor. However, determining whether pain comes from the lesion or from other conditions can be challenging. For example, if an incidental bone lesion is found on a shoulder MRI performed for rotator cuff pain, orthopedic consultation might be needed to determine whether the lesion is causing pain or if the rotator cuff is the true pain source.

The guidelines emphasize that they apply only to solitary lesions discovered incidentally. Patients with multiple lesions typically require different management approaches, as multiple bone lesions often indicate metastatic disease or systemic conditions that warrant comprehensive evaluation regardless of how benign individual lesions might appear.

These clinical factors combined with imaging characteristics help determine the appropriate Bone-RADS category and subsequent management recommendations. The system is designed to balance thorough evaluation with avoidance of unnecessary procedures, particularly for lesions that are clearly benign.

What These Guidelines Cannot Determine

While these guidelines provide valuable framework for managing incidental bone lesions, they have important limitations. They cannot provide specific diagnoses for every lesion—instead, they offer management pathways based on imaging characteristics and clinical context. Some lesions with atypical or overlapping features might not fit neatly into the algorithms.

The guidelines don't address advanced management decisions like specific surgical approaches, radiation protocols, chemotherapy regimens, or ablation techniques. These treatment decisions remain the domain of orthopedic oncologists and other specialists based on comprehensive patient evaluation.

There's also inherent subjectivity in assessing some imaging features, particularly for mixed-density lesions or lesions with borderline characteristics. Different radiologists might interpret the same lesion slightly differently, though the algorithms are designed to minimize this variability by providing clear decision pathways.

The guidelines assume appropriate imaging technique and quality. Suboptimal imaging (due to patient motion, technical factors, or incomplete protocols) might limit accurate classification and necessitate repeating studies or obtaining additional views for proper evaluation.

Finally, these guidelines cannot account for every clinical scenario or patient preference. They provide evidence-based recommendations but should be applied thoughtfully within the context of individual patient circumstances and in consultation with treating physicians.

Patient Recommendations and Next Steps

If your radiology report mentions an incidental bone lesion and provides a Bone-RADS classification, here's what the different categories typically mean for you:

Bone-RADS 1: Your lesion appears definitely benign and requires no further action. You can resume normal activities without concern or need for additional imaging related to this finding.

Bone-RADS 2: Your doctor will likely recommend additional imaging with a different modality (such as MRI if the lesion was found on CT, or CT if found on MRI) to better characterize the lesion. This is a precautionary measure to ensure nothing concerning is missed.

Bone-RADS 3: Your doctor will recommend follow-up imaging at specific intervals (typically 6 months, 12 months, and 24 months from discovery) to monitor for any changes. Most Bone-RADS 3 lesions remain stable or resolve, but monitoring ensures early detection of any changes that might require different management.

Bone-RADS 4: Your doctor will refer you to an orthopedic oncologist or other specialist for further evaluation, which may include biopsy to determine the nature of the lesion. This classification doesn't mean you definitely have cancer—many Bone-RADS 4 lesions turn out to be benign—but it indicates that thorough evaluation is necessary.

Regardless of the classification, discuss the findings with your doctor, who can explain what they mean in your specific situation and address any concerns you might have. Bring copies of your imaging studies and reports to consultations with specialists to ensure they have all necessary information for your care.

Source Information

Original Article Title: 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)

Authors: Connie Y. Chang, Hillary W. Garner, Shivani Ahlawat, Behrang Amini, Matthew D. Bucknor, Jonathan A. Flug, Iman Khodarahmi, Michael E. Mulligan, Jeffrey J. Peterson, Geoffrey M. Riley, Mohammad Samim, Santiago A. Lozano-Calderon, Jim S. Wu

Publication: Skeletal Radiology (2022) 51:1743–1764

Note: This patient-friendly article is based on peer-reviewed research published in a major medical journal. It aims to translate complex medical information into accessible language while preserving all essential scientific content from the original publication.