Keywords
Aneurysmal bone cyst - MRI - solid Variant
Introduction
As first described by Sankerin et al. (1983),[[1]] the solid variant of aneurysmal bone cyst (sABC) is an uncommon non-tumorous bone
lesion.[[1],[2],[3]] The predominant histological component in these tumours resemble the solid components
found in a classical ABC.[[4]] Musculoskeletal pathologists are well acquainted with this entity. sABC has been
referred to variously as giant cell reparative granuloma (GCRG) and extra gnathic
GCRG due to their histological similarities.[[5]] sABC can masquerade as a malignancy with its clinical presentation, imaging and
pathology confounding radiologists, pathologists and clinicians alike. While pathologists
are usually the first to suggest this diagnosis, radiologists must be familiar with
the imaging features of this otherwise innocuous condition, allaying clinical concerns,
thus avoiding unnecessary ancillary invasive diagnostic interventions and potentially
radical therapies. We discuss the distinctive radiographic and MRI features of four
histologically diagnosed cases of sABC of long bones.
Case Presentations
We encountered four cases of histologically proven solid variant of ABC: the demographic
profile and radiologic features of which are summarised in [[Table 1]].
Table 1
Summary of demographic profile and radiological features of sABC cases
|
Demographic and Radiological features
|
Case number
|
|
1
|
2
|
3
|
4
|
|
Age (years)
|
50
|
27
|
30
|
16
|
|
Sex
|
Female
|
Female
|
Female
|
Female
|
|
Bone involved
|
Humerus
|
Ulna
|
Humerus
|
Radius
|
|
Location
|
Diaphysis
|
Diaphysis
|
Diaphysis
|
Diaphysis
|
|
Radiological features
|
Eccentric, cortical based, lytic expansile
|
Eccentric, cortical based, lytic expansile
|
Eccentric, cortical based, lytic expansile
|
Eccentric, cortical based, lytic expansile
|
|
MRI findings
|
|
|
|
|
|
Soft tissue component
|
+
|
+
|
+
|
+
|
|
Bone marrow oedema
|
+
|
+
|
+
|
+
|
|
Soft tissue oedema
|
+
|
+
|
+
|
+
|
|
Blood-fluid levels
|
+
|
+
|
+
|
+
|
Case 1
A 50-year-old female presented at the orthopedic out-patient department with progressively
increasing painless swelling in the right arm for a year. Clinically, the swelling
was bony hard and non-mobile. Radiograph of the right arm showed an eccentric, lytic
and expansile lesion with thin shell in the mid-diaphyseal region of right humerus
[[Figure 1]]. Subsequently the MRI showed cortical-based expansile lesion with heterogeneously
hyperintense soft tissue component on fat-suppressed sequences with blood-fluid levels
and extensive surrounding soft tissue and bone marrow oedema.
Figure 1 (A-E): A 50-year-old female with swelling in the right forearm (Case 1). (A) Lateral radiograph
of arm including elbow showing expansile, eccentric, cortical-based lytic lesion with
thin shell of cortex in lower diaphysis of right humerus (arrow) B-D: Sagittal T1W
(B), T2W fat saturated (C), Axial T2W fat saturated (D and E) MR images is showing
lesion in lower end of humerus which is hypointense on T1W (arrow, B), hyperintense
on T2W (arrow, C) associated with a soft tissue component (D) having blood-fluid levels
(arrow, E) with extensive marrow (curved arrow, C) and soft tissue edema (block arrow,
C)
Case 2
A 27-year-old female presented with gradually progressive painless swelling in the
left forearm of 6 months’ duration. Radiograph of the forearm showed a cortical-based
expansile lytic lesion with a thin shell located in the diaphysis of the ulna [[Figure 2]]. On MRI, the lesion showed heterogeneously hyperintense soft tissue component on
T2WI and STIR sequences with a single large blood-fluid level. Extensive marrow and
soft tissue oedema were present around the tumour.
Figure 2 (A-E): A 27-year-old female with painless progressive swelling in the left forearm. (A and
B) AP and lateral radiographs of the forearm including elbow joint showing expansile,
eccentric lytic lesion with a thin shell of the cortex (arrow) arising from the proximal
diaphysis of the ulna. (C-E) Coronal and axial T2W fat-saturated MR images are showing
brightly hyperintense lesion having single blood-fluid level (arrow, D and E) associated
with bone marrow (arrow, C) and adjacent soft tissue oedema (block arrow, C)
Case 3
A 30-year-old female presented with swelling in the right arm for 8 months. Though
painless, the patient complained of an increase in size and associated cosmetic deformity.
The radiograph showed an expansile, lytic lesion, eccentrically located in the mid-humeral
diaphysis. The MRI showed heterogeneously hyperintense soft tissue on T2WI and STIR
sequences with the predominant solid soft tissue component admixed with cystic areas
having small blood-fluid levels. Akin to the above two cases, soft tissue and marrow
oedema were characteristically present [[Figure 3]] and histopathology in [[Figure 4]].
Figure 3 (A-E): A 30-year-old female with swelling in the left arm. AP radiograph of the left arm
shows expansile, eccentric lytic lesion in mid-diaphysis of the left humerus (arrow,
A). (B-E) Axial T2W, Sagittal T2W fat-saturated, and Coronal T2W fat saturated MR
images showing large exophytic soft tissue component with blood-fluid levels (arrow,
B and C) with extensive marrow (arrow, D) and soft tissue oedema (arrow, E)
Figure 4 (A-D): (A) Microscopic sections show cystic spaces filled with blood (black arrow) (H and
E, ×40). (B) The fibrocollagenous wall containing giant cells (red arrow) forms intervening
septa devoid of the endothelium (H and E, ×100). (C and D) Fibrous septae contain
bland spindle cells, scattered as well as the focal collection of osteoclastic giant
cells (red arrow) (H and E, ×200)
Case 4
A 16-year-old female presented with progressive insidious onset swelling in the right
forearm for 1-year duration. Clinically the lesion was painless and bony hard. Radiograph
of the right forearm showed an expansile lytic lesion with a thin cortical shell in
the radial diaphysis. The MRI for further characterization revealed a large soft tissue
component with few blood-fluid levels as well as extensive bone marrow. Soft tissue
oedema was also present [[Figure 5]].
Figure 5 (A-E): A 16-year-old young female with swelling in the right forearm. (A and B) AP and lateral
radiographs of the forearm showing expansile, eccentric cortical based lytic lesion
with a thin-shelled cortex arising from the proximal diaphysis of radius (arrow, A
and B). (C-E) Axial T2W, Axial T2W fat-saturated and Sagittal T2W fat-saturated MR
images showing large exophytic soft tissue component with blood-fluid levels (arrow,
C) with extensive soft tissue (arrow, D) and marrow oedema (arrow, E)
Discussion
sABC is a rare non-neoplastic bone lesion accounting for 3.4%-7.5% of all ABC with
a slight female predominance (1:1.5).[[3],[6]] In all of the four cases, the lesions were located in the diaphysis of the upper
extremity long bones [[Table 1]]. On radiography, the lytic and expansile lesions were eccentrically located, with
a cortical-based thin shell. However, the diaphyseal location and the demographic
profile was inconsistent with classical ABC and other differentials were considered.
Since none of the patients had any history of renal disease, clinical features of
hyperparathyroidism or a known malignancy – these possibilities were discounted. The
MRI carried out for further characterization showed distinctive features of sABC with
a predominantly solid soft tissue component and interspersed cystic areas showing
few blood-fluid levels, which differentiated it from classic ABC. Furthermore, all
the four cases showed bone marrow and soft tissue oedema further ruling out classical
ABC.
On radiography, sABC is usually eccentric, lytic and expansile with a variable degree
of mineralization. A shell may also be present, mimicking a classic ABC. However,
as opposed to classic ABC (which is epi-metaphyseal), sABC is usually meta-diaphyseal
and can be cortical, periosteal or juxta-articular in location. While the textbook
description of classic ABC is a “soap bubble lytic lesion” sABC in comparison may
be non-cystic and up to a third of sABC may be non-expansile. Also, as opposed to
classic ABC, multiple blood-fluid levels are not a consistent occurrence in sABC.[[1]] Other radiographical differentials for such a lesion would include brown tumours,
lytic metastasis from renal cell carcinoma, follicular thyroid carcinoma, telangiectatic
osteosarcoma and fibrosarcoma.[[2],[4],[7],[8]] An aggressive extra-osseous tumour causing saucerisation of the bone may also be
a differential in non-expansile non-cystic sABC on radiography.
MRI is quite instructive in differentiating sABC from conventional ABC. On T1WI, sABC
is iso to slightly hyperintense to muscles with a predominant solid soft tissue component
appearing heterogeneously hyperintense on T2WI or STIR images. Hypointensity on T2-weighted
sequences may be caused by the mineralisation of the solid component. Cystic areas
may also be variably present, more commonly seen in larger lesions which may also
show blood-fluid levels. Presence of solid areas with blood-fluid level makes differentiating
sABC from secondary ABCs a diagnostic dilemma. Presence of blood-fluid level has been
postulated as an indicator of progression towards classical ABC.[[1],[3]] While conventional ABC shows septal enhancement, sABC shows solid enhancement of
the constituent soft tissue.[[2]]
Interestingly a characteristic feature of sABC is the extensive surrounding bone marrow
and soft tissue oedema, attributed to the inflammatory response incited by the release
of prostaglandins and COX-2 isozymes. While this feature is not found in classical-ABC,
it can be found in up to 50% of sABC. The extensive surrounding oedema should not
be mistaken for aggressiveness of this classically indolent lesion (osteoid osteoma,
chondroblastoma, osteoblastoma and eosinophilic granuloma are a few other osseous
lesions showing surrounding marrow oedema on MRI).
It is essential to differentiate telangiectatic osteosarcoma (TO) from sABC because
of widely different prognosis, management and long-term therapeutic outcomes. sABC
though aggressive on radiographs has less bone destruction compared to telangiectatic
osteosarcoma (TO). Classically Telangiectatic osteosarcoma (TO) has geographic bone
destruction, wide zones of transition and endosteal scalloping. Matrix mineralisation
though seen in only 50% of radiographs would suggest a diagnosis of TO over sABC.
Demonstration of a calcified matrix on CT or radiographs in the soft tissue component
of the tumour would preclude a diagnosis of sABC. Extensive soft tissue involvement
and cortical erosion demonstrated on MRI would weigh the diagnosis of TO over sABC.[[9]]
Histopathologically [[Figure 4]], sABC show fibroblast proliferation without significant cellular or nuclear pleomorphism,
osteoclast-like giant cells, with foci of calcifying matrix and a variable amount
of blood-filled cystic spaces with a predominant fibrocollagenous wall. Though initially
considered to be a reactive phenomenon occurring as the result of intraosseous haemorrhage
inciting an inflammatory response, identifiable molecular genetics in the form of
USP6/CDH11 and USP6/TRE7 translocations has established it as a neoplastic pathology.[[10]] While GCRG (whose histology resembles sABC) involves the craniofacial skeleton
and short tubular bones, sABC specifically refers to extra-gnathic GCRG which occurs
in the long bones and the spine. While sABC of long tubular bones are rare, the bones
around knee joint are the favoured sites.[[4]]
There is a paucity of literature describing MRI findings of sABC. We recommend performing
MRI in any lytic lesion on X-ray which does not fit into the demographic or anatomic
profile of classic ABC. Radiologists must be well versed with the imaging appearance
and histological appearance of sABC since sABC can be treated conservatively and a
majority of sABC undergo spontaneous regression. Surgical resection and curettage
with bone grafting are reserved for symptomatic lesions.
Conclusion
Though an eccentric diaphyseal cortical-based thin shelled expansile and lytic osseous
lesion of long bones has several aggressive differentials, radiologists must be aware
of sABC as a possible benign differential diagnosis. Presence of soft tissue component,
marrow and soft tissue oedema (though alarming) with a paucity of blood-fluid levels
can suggest sABC as a differential. While a primary biopsy would be essential to rule
out a malignant aetiology: a diagnosis of sABC by the pathologist for such a lesion
should not catch the radiologist unawares and should direct the clinical team towards
less aggressive management of this benign lesion.
Teaching points about sABC
Female predominance.
MC site: Upper extremity.
Usual location within the bone: Diaphysis, eccentric, cortical based.
Characteristic MRI findings: Large soft tissue component with few blood-fluid levels
(c.f. conventional ABC), extensive bone marrow and soft tissue oedema.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms.
In the form the patient(s) has/have given his/her/their consent for his/her/their
images and other clinical information to be reported in the journal. The patients
understand that their names and initials will not be published and due efforts will
be made to conceal their identity, but anonymity cannot be guaranteed.
