The Bone Reporting and Data System (Bone-RADS) is an algorithm developed and proposed by the Practice Guidelines and Technical Standards Committee of the Society of Skeletal Radiology for the diagnostic workup of incidentally encountered solitary bone lesions in adults on MRI and/or CT 1. It has yet (c. 2022) to be validated in clinical testing 8.
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Classification
Based on two algorithms for CT and MRI each for solitary lucent/solitary sclerotic or mixed density lesions on CT and solitary high T1/low T1 bone lesions on MRI the following scheme for management recommendations has been proposed for use 1:
Bone-RADS 1: likely benign: leave alone
Bone-RADS 2: incompletely assessed on imaging: additional imaging modality required
Bone-RADS 3: indeterminate entity: follow-up imaging (at 6, 12 and 24 months after initial detection)
Bone-RADS 4: suspicious for malignancy: oncologic referral and/or biopsy necessary
The recommendations have been made for the following incidentally discovered solitary lesion categories based on different algorithms with respective flowcharts 1:
solitary lucent bone lesion on CT in adults
solitary sclerotic or mixed density bone lesion on CT in adults
solitary high T1 and solitary low T1 bone lesions on MRI
Clinical features
Clinical features that play a role in the classification are the following 1:
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pain attributable to the lesion (aggressive feature)
makes a lesion suspicious and will lead to Bone-RADS 4 category in most cases
has not been specified for low T1/low T2 or high T1 lesions
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history of malignancy with a propensity for metastasis
will lead to further workup and at least to a Bone-RADS 2 or Bone-RADS 3 category depending on whether the results of other imaging modalities are available or not
might lead to Bone-RADS 4 if there are aggressive imaging features or associated pain
CT
On CT solitary bone lesions can primarily be classified based on their density into bone lesions of lucent, sclerotic or mixed density. Within the bone reporting and data system (Bone-RADS) two different algorithms with respective flowcharts have been developed for the following 1:
solitary lucent bone lesions
solitary sclerotic or mixed density bone lesions
Both types of lesions are assessed for attributable pain and aggressive imaging features, that make a lesion suspicious for malignancy if present and thus will lead to a Bone-RADS 4 category.
The following features are considered aggressive 1:
cortical involvement
soft tissue extension
aggressive periosteal reaction
pain attributable to the lesion
Lesions without any aggressive features on CT are evaluated for a history of malignancy with a propensity to bone metastasis which makes those lesions indeterminate and requires different imaging and/or follow-up.
Lesions without any aggressive features and no history of malignancy are further evaluated for their density, their location and whether they are entirely consistent with specific benign or suspicious entities.
Solitary lucent bone lesion
Solitary lucent bone lesions with no aggressive features and no history of malignancy are treated as follows 1:
a mean density of -30 HU to -120 HU or visible fat suggests a lipomatous lesion such as a lipomatous lipoma and has been categorized as likely benign 1
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non-lipomatous solitary lucent lesions that are entirely consistent with the following entities can be also considered benign
non-lipomatous solitary lucent lesions without aggressive features that are not consistent with the above-mentioned lesions require further workup either with different imaging modalities, follow-up imaging or oncologic referral
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suspicious solitary lucent lesions that might present without aggressive features include the following entities:
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cortically based lucent lesions
cortical bone metastases
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medullary based lucent lesions
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Solitary sclerotic or mixed density bone lesion
Solitary sclerotic or mixed density bone lesions with no aggressive features and no history of malignancy are treated as follows 1:
mixed density lesions with a mean density of -30 HU to -120 HU or visible fat suggest a lipomatous origin such as an intraosseous lipoma, hemangioma, Paget disease, bone infarction, fibrous dysplasia or non-ossifying fibroma and are likely benign
a ground-glass appearance in the absence of a history of malignancy suggests fibrous dysplasia and is likely benign
a cartilaginous matrix might be benign, indeterminate or suspicious depending on associated concerning features (see below*) and the size of the lesion
solitary sclerotic or mixed density bone lesions that are not lipomatous, do not display a ground-glass appearance or a cartilaginous matrix and are not entirely consistent with bone island (enostosis) 2, sclerosing bone dysplasia or non-ossifying fibroma are considered indeterminate
*Concerning imaging features in a cartilaginous matrix include 1,3,4:
expansile remodeling
cortical breakthrough
periosteal reaction
soft tissue mass
incomplete mineralization
epiphyseal location
location in the axial skeleton
Assessment categories
Bone-RADS 1
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no aggressive imaging features or pain attributable to the lesion and no history of malignancy:
lipomatous lucent or mixed density bone lesions (visible fat or mean density -30 to -120 HU)
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lucent solitary bone lesions entirely consistent with one of the following:
-
sclerotic or mixed density solitary bone lesions with one of the following:
ground glass appearance suggesting fibrous dysplasia
cartilaginous matrix without additional concerning features and a lesion size <5 cm suggesting enchondroma
-
sclerotic or mixed density solitary bone lesions entirely consistent with:
Bone-RADS 2 or Bone-RADS 3
no aggressive features but a history of malignancy
no aggressive features or history of malignancy but not entirely consistent with specific solitary bone lesions that can be categorized as entirely benign or suspicious
in addition, the following lesions are considered indeterminate (Bone-RADS 3):
solitary bone lesions with a cartilaginous matrix and a lesion size >5 cm but no concerning features
Bone-RADS 4
aggressive imaging features or pain attributable to the lesion
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lucent solitary cortical bone lesions consistent with:
cortical bone metastasis
-
lucent medullary bone lesions consistent with:
solitary sclerotic or mixed density bone lesions with a cartilaginous matrix and additional concerning features
MRI
Solitary bone lesions are categorized based on their signal intensity compared to skeletal muscle or the adjacent intervertebral disc primarily on T1 weighted imaging 1,5 and in the second place on T2 weighted images where it is compared to the cortical bone, skeletal muscle or fat if fat suppression has been used 1.
Within the bone reporting and data system (Bone-RADS) two algorithms with respective flowcharts have been developed for the following 1:
high T1 solitary bone lesions
low T1 solitary bone lesions
Radiographic correlation has been recommended for all incidental bone lesions 1.
Some high T1 bone lesions are treated like low T1 bone lesions, where the signal is only slightly higher than skeletal muscle and not associated with a signal drop in chemical shift imaging or lesions where the high T1 signal is seen in a fluid-fluid level or can be associated with hemorrhage 1.
High T1 solitary bone lesion
High T1 bone lesions are categorized further depending on whether their signal is much or slightly higher than skeletal muscle or the adjacent intervertebral disc and whether the T1 signal is seen in a fluid-fluid level or hemorrhage.
Lesions that present with intralesional macroscopic fat that is with a much higher signal than skeletal muscle or the intervertebral disc similar to subcutaneous are likely benign 6 and categorized (Bone-RADS 1) 1.
Lesions with a much higher signal than skeletal muscle or the intervertebral disc different than subcutaneous fat have to be assessed further with contrast or chemical shift imaging and categorized accordingly 1.
Solitary bone lesions that are isointense to skeletal muscle or intervertebral disc on T1 weighted images or with a slightly higher signal and no signal drop on chemical shift imaging should be managed like solitary low T1 bone lesions 1.
A high T1 signal within a fluid-fluid level or hemorrhage has been recommended to be categorized as low T1/high T2 lesions 1.
Low T1 solitary bone lesion
Solitary bone lesions that are hypointense or isointense to skeletal muscle or the adjacent intervertebral disc on T1 weighted images are further categorized according to their signal on T2 weighted images compared to cortical bone, skeletal muscle or suppressed fat 1.
High T2 solitary bone lesion
Low T1/high T2 solitary bone lesions are evaluated for the following 1:
-
specific features indicating an aggressive nature of the lesion such as 1:
a history of malignancy prone to bone metastases
pain attributable to the lesion
cortical involvement
soft tissue extension
surrounding bone marrow edema
solid mass like enhancement
a lesion in the sternum in the setting of breast cancer
elevated prostate-specific antigen
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and if there are no such features whether they are entirely consistent with the following benign lesions 1:
Low T2 solitary bone lesion
Low T1/low T2 solitary bone lesions are evaluated for the following 1:
-
specific features indicating an aggressive nature of the lesion such as 1,7:
presence of a high T2 halo
solid mass like enhancement
a lesion in the sternum in the setting of breast cancer
elevated prostate-specific antigen
and if there are no such features whether there is a history of malignancy or not
Assessment categories
Bone-RADS 1
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high T1 solitary bone lesions with the following features:
much higher signal than skeletal muscle and similar signal as fat on all sequences
much higher signal than skeletal muscle different than fat and with no or a thin peripheral enhancement pattern
slightly higher signal than skeletal muscle or intervertebral disc and a signal drop >20% on chemical shift imaging suggesting a focus of red marrow
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low T1/high T2 lesions in the absence of aggressive features and entirely consistent with:
-
low T1/T2 lesions in the absence of aggressive features and entirely consistent with:
Bone-RADS 2 or Bone-RADS 3
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high T1 solitary bone lesions with the following features:
slightly higher signal than skeletal muscle or intervertebral disc and no chemical shift imaging available
much higher signal than skeletal muscle different than fat and no postcontrast or chemical shift imaging available
-
low T1 solitary bone lesions with the following features:
low T2 lesions with a history of malignancy but no features pointing to an aggressive nature
high T2 lesions with no aggressive features that cannot be categorized as likely being (Bone-RADS 1)
Bone-RADS 4
high T1 solitary bone lesions with a signal different than macroscopic fat and a nodular or central intralesional enhancement pattern
low T1 solitary bone lesions with any aggressive features (criteria differ slightly based on T2 signal)
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