Keywords
aneurysmal bone cyst - temporal bone - case report
Introduction
Aneurysmal bone cysts (ABCs) are benign, osteolytic bone tumors characterized by blood-filled
cavities and locally destructive growth. They represent ∼9.1% of all primary bone
tumors.[1] Although non-cancerous, ABCs can grow aggressively, causing local destruction and
weakening bones, and increasing the risk of pathologic fractures under normal stress.
The term “aneurysmal bone cyst” was first introduced by Jaffe and Lichtenstein in
1942.[2] These cysts are more frequently observed in children and adolescents and can lead
to substantial complications, particularly when they affect the growth plates of bones.
ABCs commonly occur in the femur, tibia, and vertebrae, although they may involve
any bone.[3] Sixty-three skull ABCs have been reported in the literature, of which 21 involved
the temporal bone.[4]
[5] ABCs are known for their expansive growth, which can lead to pain, inflammation,
and damage to nearby joints and growth plates. ABCs most commonly appear during adolescence,
with the average age of diagnosis being around 13 years. In fact, ∼90% of cases are
identified before the age of 30 years. Studies suggest a slightly higher occurrence
in females, with a male-to-female ratio of 1 to 1.3.[6] While ABCs can develop in various bones, those occurring in the skull are particularly
concerning due to their potential for severe symptoms. Skull-based ABCs make up ∼2
to 6% of all cases.[5] ABCs of the temporal bone are rare, particularly in adults, and are usually treated
with surgery alone.
Here, we report a case of an ABC located in the right petrous portion of the temporal
bone in a 38-year-old female patient. She was treated with a multimodality treatment
approach (i.e., surgery followed by radiation therapy), leading to increased survival
and better quality of life. This represents 1 of only 12 reported temporal bone ABCs,
and is distinctive for (1) occurrence in a 38-year-old patient (while >90% occur before
age 30), (2) use of high-dose adjuvant radiotherapy (50.4 Gy) for residual disease,
and (3) 5-year symptom-free follow-up—the longest documented in temporal bone ABCs.
Case Report
A 38-year-old female patient presented with intermittent headache, deviation of the
right angle of the mouth, decreased hearing in the right ear for 2 years, and diminished
vision in the right eye for 3 weeks. There was a history of a road traffic accident
2 years back. There was no history of loss of consciousness, seizures, or weakness
of extremities. All the vital parameters were within normal limits. Neurological examination
revealed normal higher mental functions and speech with cerebellar gait. Visual acuity
in both eyes was 6/6. The right V, VII, VIII, and XII cranial nerves' lower motor
neuron palsy was found on examination. Routine laboratory investigations were within
normal limits. Pure tone audiometry testing showed evidence of sensorineural hearing
loss in the right ear. Noncontrast CT (NCCT) head ([Fig. 1A]) revealed a 6.3 × 4.2 × 4.3 cm well-defined extra-axial, smoothly marginated, mildly
hyperdense lytic expansile lesion epicentered at the right petrous bone. Anteromedially,
the lesion is noted to cross the petroclival suture and erode the right lateral aspect
of the clivus as well as involve the carotid canal. Anteriorly, it is seen extending
into and obliterating the right middle ear and involving the otic capsule. Laterally,
there is complete obliteration of the mastoid air cells. Medially, it obliterates
the right cerebellopontine angle, involving the jugular fossa and hypoglossal canal,
compressing and displacing the right cerebellar hemisphere and fourth ventricle to
the left. Posteriorly, the lesion extends into and involves the right lateral aspect
of the occipital bone and the sub-occipital region. The lesion has multiple rarefied
bony fragments. MRI of the brain ([Fig. 1B]) showed a 6.9 × 4.2 × 4.4 cm (AP × TR × CC) well-defined extra-axial lytic lesion
with avid contrast enhancement. The lesion is obliterating the right internal auditory
canal and is surrounding the cochlea and semicircular canal. The right VII/VIII nerve
is not seen separately. The petrous segment of the internal carotid artery is displaced
anteriorly and is thinned out. The right transverse and sigmoid sinuses are not visualized.
Superiorly, the lesion is seen reaching up to Meckel's cave. MRI findings corroborated
the NCCT findings. CT cerebral angiography was suggestive of a right petrous bone
lesion surrounding and mildly displacing the petrous and lateral segments of the internal
carotid artery, which was mildly narrowed. However, no filling defect or hypertrophied
vessels supplying the lesion were noted. In digital subtraction angiography, a moderate
tumor blush supplied by the ascending pharyngeal artery was embolized. The patient
underwent a right retromastoid suboccipital craniotomy by the neurosurgical team,
and the excision of the lesion was done. The facial nerve was identified and preserved
using intraoperative monitoring. The excised tumor was sent for histopathological
examination. Histopathology revealed multiple tissue fragments of blood-filled cystic
spaces separated by a fibrous wall, which is composed of moderately dense cellular
proliferation of bland fibroblasts, numerous scattered multinucleated osteoclast-type
giant cells. At places, new bone formation, reactive woven bone rimmed by osteoblasts,
and basophilic blue bone formation are seen. Focally, the cysts are seen to be lined
by giant cells. The features are suggestive of ABC (solid variant). Postoperative
period was uneventful, and postoperative contrast-enhanced MRI showed a reduced bulk
of the lesion measuring 3.1 × 5.6 × 3.5 cm (AP × TR × CC) compared with the preoperative
size of 6.9 × 4.2 × 4.4 cm. The multidisciplinary tumor board opined for external
beam radiotherapy with a higher dose for the residual tumor. Radiotherapy was administered
to the patient at a dose of 50.4 Gy in 28 fractions by intensity modulated radiation
therapy technique over 5 weeks ([Fig. 2]). The patient was followed up monthly for the first 3 months, then 3 monthly for
the next 2 years, and then 6 monthly up to 5 years. Following treatment completion,
the patient was monitored for 66 months and remained entirely asymptomatic throughout
this follow-up period.
Fig. 1 Axial section of CT (A) and T1-weighted MRI (B) image showing a large, hyperdense/hyperintense, expansile, multiloculated, osteolytic
mass epicentered at the squamous and petrous part of the right temporal bone, compressing
the adjacent brain parenchyma.
Fig. 2 CT and MRI fusion image shows adequate coverage of residual tumor and postoperative
bed (A) and dose distribution showing 95% coverage of the planning target volume (cyan)
(B) at the right temporal bone.
Discussion
ABCs are rare, benign bone tumors, accounting for approximately 1 to 6% of all primary
bone tumors. Their incidence is quite low, estimated at 0.14 cases per 1 million people
per year. These cysts are most commonly diagnosed in children and adolescents younger
than 20 years, with a slight predominance in females (male-to-female ratio of 1:1.3).[5] The majority of ABCs (67%) occur in the metaphysis of long bones, but they can also
affect the spine (15%, particularly the posterior elements), pelvis (9%), craniofacial
bones, and epiphyses.[7] On X-ray and CT scan, an expansile bony lesion with septations, a hypodense rim
of capsule, and numerous lobulations with varying signal intensities giving a “bubbly”
appearance should raise the suspicion of ABC. The development of ABCs is primarily
linked to the upregulation of the TRE17/USP6 oncogene due to gain-of-function mutations,
which disrupt osteoblastic maturation.[8] Current theories suggest that vascular abnormalities lead to increased expansile
pressure within the bone, causing erosion and resorption, ultimately forming the cyst.[4] On gross examination, ABCs consist of blood-filled cystic cavities containing blood
and hemosiderin, surrounded by a thin shell of reactive bone. The cavities are divided
by septa made of osteoid tissue, lacking an endothelial lining. The stroma of ABCs
contains fibroblasts, spindle cells, osteoid, and multinucleated giant cells, often
clustered at the periphery of the cysts, creating a “pigs-at-the-trough” appearance.[4] While mitotic figures are common, atypical mitoses are absent, helping distinguish
ABCs from malignant lesions. Diagnosis primarily relies on radiological imaging, including
X-rays and NCCT scans. X-rays typically show expansile, osteolytic lesions with thin,
“eggshell” sclerotic borders and internal septations. The aggressive, expansile nature
of ABCs can displace surrounding bone. CT scans provide a more detailed visualization
of the cystic septa and the “eggshell” rim. Fluid-fluid levels, caused by the separation
of cellular debris and serum, are often visible. MRI findings are similar to CT, with
T1- and T2-weighted images highlighting the septa and showing areas of hyperintensity
due to blood products. Pathologic fractures may also be evident, accompanied by bone
and soft tissue edema. Laboratory tests are generally not useful for diagnosis, though
alkaline phosphatase levels may be elevated due to increased osteoblast activity.[9]
Given the risk of pathologic fractures due to their expansile nature, surgical intervention
is the primary treatment for ABCs. The surgical approach–intralesional curettage,
excision, or en bloc resection—depends on the lesion's size and location. Intralesional
excision is often preferred. For recurrent ABCs, particularly in the temporal bone,
radiotherapy may be considered.[10] Megavoltage radiotherapy is an option for inoperable or recurrent cases. Studies,
including a retrospective review by Zhu et al, have shown favorable outcomes with
radiotherapy, with a 4-year local control rate of 90.9% compared with 55% for surgery
alone.[5]
[11] Unique features of this case include adult onset, extensive petrous involvement,
nerve preservation via intraoperative monitoring, and durable control using 50.4 Gy
radiotherapy—a dose exceeding conventional recommendations.
Despite ongoing debates about their pathogenesis, ABCs generally have an excellent
prognosis. The standard treatment is intralesional curettage, with or without adjuvant
therapy.[12] However, surgery has limitations, including a high recurrence rate and difficulty
accessing lesions in certain anatomical locations.[13] Radiotherapy serves as an effective adjuvant or alternative, particularly for residual
disease or hard-to-reach areas. Studies, including one by Zhu et al,[11] support the use of radiotherapy, reporting a 100% local control rate when used alone
or postsurgery. However, due to the risk of radiation-induced malignancies, especially
in young patients, radiotherapy should be reserved for cases where surgery alone is
insufficient or impractical.
The recommended radiation dose is 26 to 30 Gy, delivered in 1.8 to 2.0 Gy daily fractions
or 1.2 Gy twice daily, to balance efficacy and minimize side effects.[11] In some cases, a slightly higher dose may be used to manage residual disease and
improve local control. While standard RT doses for ABCs range from 26 to 30 Gy,[11] we administered 50.4 Gy due to (1) large residual volume (3.1 × 5.6 × 3.5 cm) compressing
critical structures, (2) solid-variant histology suggesting aggressive potential,
and (3) proximity to the brainstem requiring precise coverage. This aligns with Feigenberg
et al's finding that doses >35 Gy improve control in complex skull-base ABCs.[13] A watchful-waiting strategy was deemed unsafe given the risk of irreversible cranial
neuropathy from regrowth. The 5-year remission without radiation necrosis ([Fig. 2B]) supports this approach, though lifelong monitoring for secondary malignancies is
warranted. Radiotherapy is contraindicated in ABCs associated with fibrous dysplasia
due to the risk of malignant transformation. Chemotherapy has no role in ABC management,
but preoperative endovascular embolization can reduce bleeding during surgery, especially
for tumors in difficult locations.[14] There is still no available consensus guideline for the use of radiation in ABCs,
adjunct to surgery for better treatment outcomes; mostly due to the rarity of the
cases. Further research to treat ABCs with a multimodality approach (i.e., surgery
followed by radiation therapy) should be considered to develop a consensus guideline
for better patient care.
Conclusion
ABCs of the temporal bone are exceptionally rare, particularly in adults, and pose
unique therapeutic challenges. Surgery remains the cornerstone of management; however,
adjuvant radiotherapy has an important role when complete resection is not feasible
or when residual disease persists. This case highlights the effectiveness of a multimodality
approach, with surgery followed by radiotherapy, in achieving durable 5-year local
control and long-term symptom-free survival. While radiotherapy must be used judiciously
due to potential risks, carefully selected patients can benefit significantly, especially
in complex skull-base lesions. Adjuvant radiotherapy (50.4 Gy) is safe and effective
for residual skull-base ABCs when surgery alone is insufficient. Reporting such unusual
presentations not only expands clinical understanding but also emphasizes the need
for developing consensus guidelines to optimize outcomes in rare craniofacial ABCs.
