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History and physical examination findings

  • Presented for a progressive lytic, expansile lesion within the right ilium
  • One year prior to referral presentation, the patient sustained an unknown trauma at doggie-daycare resulting in a right pelvic limb, toe-touching lameness. At that time pelvic radiographs revealed an ovoid, smoothly margined, expansile and well-defined lucency in the crest of the right ilium. The patient was treated for a soft tissue injury and the reported lameness resolved with rest and NSAID treatment. A radiograph taken 3 months prior (following foreign body ingestion) was normal.
  • One year after the initial episode of right coxal lameness, the patient presented to their referring DVM with a one-week history of a non-painful, firm swelling over the right coxal joint. External rotation and offloading of the right pelvic limb were noted. Referring DVM radiographs revealed progression of the expansile lesion within the crest of the right ilium which was now surrounded by a thin rim of cortical bone with numerous septations.
  • Biopsies were obtained 3 weeks prior to MR.
  • Thoracic radiographs were normal.

Study performed:

  • Pelvic MR 3.0T system (MAGNETOM Skyra, Siemens Medical Technologies, Malvern, PA)
  • Pre-contrast Sequences
    • Sagittal T2W (TR 3700ms, TE 93ms, 4mm slice thickness)
    • Transverse T2W (TR 4900 ms, TE 100ms, 4mm slice thickness)
    • Transverse T1W (TR 800ms, TE 12ms, 4mm slice thickness)
    • Dorsal T2W (TR 4500ms, TE 101ms, 3mm slice thickness)
    • Post-contrast Sequences obtained after intravenous injection of 0.2 ml/kg (0.1 mmol/kg) gadopentetate
      dimeglumine (Magnevist®)
    • Transverse T1W Dixon (TR 700ms, TE 15ms, 4mm slice thickness, FA 120◦)
    • Dorsal T1W Dixon (TR 600ms, TE 12ms, 3mm slice thickness, FA 120◦)

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Dorsal T2W (TR 4500ms, TE 101ms, 3mm slice thickness)

Transverse T1-weighted images, pre-contrast

Transverse T1-weighted images, pre-contrast

Dorsal T1-weighted images, post-contrast

  • Centered in the right ilium there is a 7.0 cm W x 11.0 cm L x 6.6 cm H expansile, cavitary, fluid-filled mass. The fluid within the lesion is completely suppressed on the T2 Dixon fat sequence but is of intermediate signal intensity in T1W. Fluid-fluid levels are observed as T2W and T1W hypointensity dorsally (dependent), with the remainder being hyperintense in T2W. The mass contains T2W and T1W hypointense internal septations and is, for the most part, surrounded by a thin rim of similar intensities. Conforming to the internal septations, there is strong peripheral enhancement on postcontrast images. Caudally the mass extends to the right coxal joint with lysis of the articular margin of the acetabulum, but without frank invasion of the joint space. The mass extends through the right sacroiliac joint, continues into the lamina, body, and right cranial articular process of the sacrum, and breaks into the sacral canal, obliterating sacral foramina and causing severe compression and leftward displacement of the caudal spinal nerves. The L5-6 and L6-7 spinal nerves are normal. The right seventh lumbar spinal nerve exits the intervertebral foramen and travels in close apposition to the mass. The right lumbosacral trunk is moderately larger than the left, T2W hyperintense and illdefined due to regional fat stranding. The right sciatic nerve remains mildly T2W hyperintense and courses along the caudolateral aspect of the mass and over the greater ischiatic notch. The right erector spinae muscles are moderately atrophied and possibly invaded by the mass.
  • The right gluteal muscles are moderately atrophied with mild diffuse T2W hyperintensity. A linear tract extends from the partially imaged skin surface into the mass, consistent with the recent biopsy approach. Between the rectum and mass, a fascial plane is retained at all levels. The right sacrocaudalis, coccygeus, and iliopsoas muscles are displaced ventrally and to the left. Lymph nodes of the iliosacral center are mildly enlarged and irregular in shape, with normal contrast enhancement and hilar fat definition.
  • Expansile mass involving the right ilium and sacrum
    • These findings are suggestive of an aneurysmal bone cyst
    • Signal intensities of the fluid-fluid layers are suggestive of heme products.
  • Right seventh lumbar spinal neuropathy
    • The primary differential diagnosis is secondary neuritis
  • The appearance of the medial iliac lymph nodes may represent reactive hyperplasia (e.g. recent surgical biopsies, young age) and less likely metastasis.
  • The changes in the right gluteal muscles and adjacent subcutaneous tissues are attributed
    to fatty degeneration related to muscle atrophy and cellulitis/ myositis following the recent
    surgical intervention
  • Three weeks prior to the magnetic resonance study, a contrast enhanced computed tomography study of the pelvis, ultrasound-guided fine needle aspirate of the right ilium, and surgically obtained biopsy samples were obtained.
  • The ultrasound-guided fine needle aspirate of the mass revealed increased cellularity with a uniform population of osteoblasts with several osteoclasts. The cells present did not exhibit enough criteria of malignancy to suggest neoplasia.
  • At surgery, a thin shell of discolored purple bone 1-2 mm thick was encountered. The interior of the mass was hollow and contained serosanguineous fluid in addition to several fragments of mucosa and fibrous tissue. The lesion was sampled, and histopathology suggested a mesenchymal neoplastic process. Decalcification of the histopathology samples revealed a profound, aggressive, and reactive fibrous tissue response.
  • Given the patients age, it was proposed that a congenital, primitive mesenchymal cyst grew over time causing pressure and modelling of the bone. An exuberant fibrous response ensued.
  • The mass was assessed as nonresectable and radiation therapy is being pursued.
  • In both dogs and humans, aneurysmal bone cysts (ABCs) have been described as benign expansile osteolytic lesions with invasive local growth.1-3 They are of variable size and contain internal fluid accumulations consistent with free flowing blood with thin connective tissue septa composed of trabecular bone or osteoid tissue.1,2,4 A unique feature of ABCs is the content: unclotted blood in constant motion.2
  • In people, predilection sites include the pelvis, spine and long bones.2 ABCs have been reported frequently in people and infrequently in several animal species including dogs, cats, horses, cattle, and llama.5,6
  • The pathogenesis of ABCs is speculative, but several mechanisms have been proposed. One includes an insult to the bone marrow vasculature that causes disruption leading to arteriovenous shunting and cavernous formation within the bone-containing blood filled cavities.7 Other mechanisms include trauma causing subperiosteal hemorrhage, fibrous dysplasia, and benign or malignant neoplasia.2,5 In our case, the history of an unwitnessed prior trauma may have been an inciting event. However, the significance of this is uncertain. A high percentage (32%) of ABCs in a case series of 66 humans revealed concurrent benign primary bone lesions.7
  • Radiographs taken immediately prior to referral revealed a characteristic bone cyst lesion that was expansile, lytic, and contained numerous areas of lucency resembling “soap bubbles” divided by bony septa and surrounded by a thin rim of cortical bone.2,5
  • Differential diagnoses typically listed for such findings include primary bone tumors (e.g. chondrosarcoma, hemangiosarcoma, fibrosarcoma, telangiectatic osteosarcoma, giant cell tumor, hemangioma), secondary bone tumors (e.g. lymphoma, multiple myeloma), osteomyelitis, and unicameral bone cysts.6 Hemangiomas typically occur in the vertebral bodies with less expansion and are usually multifocal.8 A unicameral bone cyst has a more uniform, less markedly expansile chamber.8
  • In canine patients, bone cysts have typically been reported in young, large breed dogs in metaphyseal areas of long bones.6 Malignant transformation has been reported in humans and in a dog whereby transformation to a chondrosarcoma was described.5 Two cases have been associated with underlying osteosarcoma.9,10
  • In people, ABCs have a predilection for younger patients, with one study reporting 70% of 66 cases under the age of 20.7 Most lesions are found in the metaphyses of long bones or the axial skeleton.7 In two larger case series, a total of 5 cases had lesions that arose from the ilium, similar to the current case.7,11
  • In veterinary patients, ABCs similarly tend to occur in the metaphyseal area of long bones, but reported locations have included a metacarpal bone, os penis, scapula, vertebra, rib, and pelvis.3,10,12-16
  • The MR imaging features of the lesion characteristically revealed a lobular, expansile structure compartmentalized by thin hypointense septations, with each compartment containing various fluid levels indicated by homogeneous but differing signal intensities. The fluid levels represent sedimentation of red blood cells and serum, reportedly enhanced in humans when the patient is motionless for > 10minutes.4 The signal intensities of the fluid represents different relaxation times of the contents; early subacute hemorrhage has hyperintense signal (reduced T1) and chronic hemorrhage is characterized by low signal on both T1W and T2W images due to macrophage removal of iron from hemoglobin and methemoglobin.4,17 Both of these layers were detected in the present case.
  • Treatment in humans has consisted of curettage, en bloc removal, amputation, radiation therapy and cryosurgery.7 ABCs are not true cysts as they lack an epithelial lining, suggested as a cause for the lack of success with curettage.6 In three large case series in humans, recurrence rates ranged from approximately one third to one half of the cases depending on the treatment technique used. 7,11,18 In one study 59% of patients undergoing curettage experienced recurrence.7 In a review of 105 cases, incomplete surgical removal was an important factor in recurrence.11 In veterinary patients treatment has involved amputation of the affected limb, en bloc resection, curettage and cancellous bone grafting with varying rates of post-operative morbidity and recurrence reported.2 Successful treatment with curettage and cancellous bone grafting6 and en bloc resection has been reported.2,15 In our case, the lesion was deemed unresectable due to the extensive sacral involvement.
  • Like prior reports in people, the lesion is rarely to minimally painful when slowly growing. Pain is encountered when pathological fracture occurs,8 or when compression of the adjacent soft tissue structures, periosteum, or intraarticular involvement is present.5
  • In this case both MR and CT were performed, allowing further characterization of sacral involvement. MRI provided superior contrast resolution to allow closer assessment of nerve involvement for potential surgical planning, however arterial supply to the mass by branches of the right internal iliac artery (L7 lumbar and iliolumbar a.) necessitated an arterial phase only performed in CT. Although infrequent, ABCs should be considered when the above imaging features are identified.


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