Skull base chondroblastoma presenting as a deep lobe parotid tumour

Abstract

Chondroblastomas are rare, benign cartilage-producing primary bone tumours that account for 1% of all primary bone tumours. They are usually seen in young adult males and affect long tubulous bones such as the femur or humerus. Occurrences in non-tubular flat bones such as the craniofacial skeleton do occur but are seen in older adults. With only around 100 cases reported in the English literature, ‘Temporal Bone Chondroblastomas’ can present a diagnostic challenge for both surgeon and histopathologist. Clinical presentation can be subtle and patients may have longstanding symptoms due to compression of surrounding structures. Imaging in the form of contrast CT and/or MRI is recommended to assess size, proximity to neurovascular structures and plan operative approach. Definitive treatment is surgical excision, with radiotherapy reserved for recurrence or unfit surgical patients. Long-term follow-up is recommended for surveillance due to high recurrence rates. We present our experience managing this rare entity.

Background

First described by Ewing,¹ classified by Codman² and coined ‘chondroblastomas’ by Jaffe and Lichtenstein³; chondroblastomas are rare cartilage producing, benign primary bone tumours that occur more often in skeletal immature individuals and account for 1% of all bone tumours.⁴ Site of origin of these tumours in younger patients includes the proximal tibia or femur, proximal humerus and distal femur. Tumours arising in short tubular bones (hands and feet) as well as in non-tubular flat bones (craniofacial skeleton) are seen in older adults.^{5 6}

The occurrences of temporal bone chondroblastomas are extremely rare, with only around 100 cases reported in the English literature since they were first classified by Codman.^{2 7} They arise most commonly from the squamous portion of the temporal bone possibly due to its cartilaginous origins.^{8 9} Clinical presentation varies depending on the location, size and growth rate of the tumour within the temporal bone and may include: otological symptoms (hearing loss, tinnitus, otorrhoea), temporomandibular joint (TMJ) symptoms (trismus, dental pain) and neurological manifestations (facial nerve palsy, vertigo, headache). The mainstay of treatment is surgical excision, as though benign, these tumours are locally destructive.^{2 4}

In the following case report, we describe a unique presentation of a temporal bone chondroblastoma masquerading as a deep lobe parotid tumour and the surgical approach used for excision.

Case presentation

A 52-year-old woman presented to the head and neck clinic with a 1-year history of right sided hearing loss, aural fullness, facial paresthesia and more recently, a fullness to the right parotid region. Physical examination showed a right otitis media with effusion and a vaguely palpable mass in the right parotid region. Flexible nasoendoscopy showed normal postnasal space tissue. There were no cranial nerve deficits or palpable cervical lymphadenopathy.

Pure tone audiogram confirmed a right-sided 30 db conductive hearing loss. Neck ultrasound revealed a deep lobe parotid mass. Fine-needle aspiration was attempted but was inconclusive. Contrast MRI scan revealed a right-sided 5.0×3.2×2.8 cm tumour centred in the parapharyngeal space, abutting and displacing the deep lobe of the parotid gland and distorting the TMJ capsule with compression of the eustachian tube. It had a low signal on short T₁ recovery and T₂. No cervical lymphadenopathy was detected (see figure 1). A contrast CT scan of the neck and mandible was requested to assess the extent of bony involvement. This scan showed the lesion to be within the deep lobe of the parotid gland, displacing the masticator space structures anteriorly and parapharyngeal space medially. There was evidence of bony remodelling at the right neck of the mandible and the lateral pterygoid plate (see figure 2).

Figure 1.

Postcontrast MRI neck T₁ image showing a large right-sided parapharyngeal space lesion (blue arrow). The lesion is well defined with areas of heterogeneous enhancement. Note the extension anteriorly and superiorly displacing surrounding structures. (A) axial view. (B) coronal view.

Figure 2.

Postcontrast CT neck image highlighting bony remodelling at the right neck of the mandible (blue arrow) and the lateral pterygoid plate (red arrow). (A) axial view. (B) coronal view.

After seeking advice from our local multidisciplinary head and neck meeting (MDT), a core biopsy was taken in clinic under local anaesthesia. The histology results of this biopsy showed a hypocellular lesion with dense eosinophilic cytoplasm. Sparse cells were seen with small spindled to round nuclei. Focal osseous metaplasia with focal giant cells were also identified. There was a weak positivity for actin on immunohistochemistry. No evidence of malignancy was seen. The features were not suggestive of a salivary gland neoplasm but more in keeping with a fibroblastic lesion with focal osseous metaplasia. Definitive diagnosis could not be made from the core biopsy.

Treatment

After discussing the results of the core biopsy with the patient, the option of surgical excision was discussed for definitive treatment of her symptoms. This would be via a transcervical approach incorporating a lip-split mandibulotomy if necessary for surgical access. The patient was also consoled on the possible need for a temporary tracheostomy and nasogastric tube (NGT) feeding postoperative.

Complications were explained to the patient with emphasis on possible injury to the lower cranial nerves (facial, hypoglossal and vagus). The patient opted for surgical excision and gave written consent.

The procedure was then carried out on an elective basis. Preoperative blood investigations were normal. At operation, a cervical-parotid incision was made, the main trunk of the facial nerve was identified and the superficial lobe of the parotid gland was reflected. The submandibular gland was then excised to facilitate access followed by division of the stylomandibular ligament, thereby exposing the tumour. The lesion was firm to palpation and appeared to be involving the deep lobe of the parotid gland with significant superior extension within the parapharyngeal space, posterior to the pterygoid muscles and adherent to surrounding structures. Due to its size and extension within the parapharyngeal space, a stepped mandibulotomy was performed for further surgical access. The medial and lateral pterygoids had to be divided to fully gain access to the tumour. It was adherent to the facial nerve (laterally), the inferior alveolar nerve, the inner periosteal layer of the mandible as well as to the capsule of the TMJ. The facial nerve was dissected off of the tumour, however, the inferior alveolar nerve was sacrificed to facilitate complete excision of the lesion (see figure 3). A surgical drain was placed and the wound was closed in layers. Direct laryngoscopy was performed to assess for laryngeal oedema and determine need for a temporary tracheostomy. With a grade 1 view seen, tracheostomy was not performed. An NGT was inserted to facilitate enteral feeding.

Figure 3.

Lip-Split mandibulotomy showing right skull base chondroblastoma. (A) facial nerve retracted (B) skull base chondroblastoma (C) lingual nerve.

Outcome and follow-up

Postoperatively, the patient had a right facial nerve palsy with a House-Brackmann score of 3. She made an uneventful recovery and was discharged from hospital on day 12 postoperative. Histological analysis required external review to confirm the diagnosis of a temporal bone chondroblastoma (see figure 4). Completeness of excision could not be commented on the histological analysis given the fragmented and friable nature of the lesion. Her case was discussed at our local sarcoma who advised conservative observation with interval imaging. A repeat MRI neck 3 months’ postoperative showed no evidence of recurrence. The patient continues to be followed up in the head and neck clinic. At her last follow-up appointment (6 months postoperative), she had recovery of her facial tone but complained of numbness around the incision site and mild pain on chewing in keeping with first bite syndrome.

Figure 4.

H&E, original magnification ×10. Tumour cells appear spindled and stellate within a dense eosinophilic matrix showing focal chondroid tissue.

Discussion

Chrondroblastomas are seen more commonly in younger adults with the average age at diagnosis between 19 and 23 years. There is also a male predominance of 2:1.¹⁰ Long bones are commonly affected in younger patients with the femur, tibia and humerus being common sites of origin. Tumours arising in short tubular bones (hands and feet) as well as in non-tubular flat bones (craniofacial skeleton) are seen in older adults.^{5 6} Pain is usually the most common symptom however site-specific complaints can occur with tumour growth.⁴

The occurrence of temporal bone chondroblastomas are extremely rare. No more than 100 cases have been described in the English-speaking literature.⁷ They arise commonly from the squamous portion of the temporal bone, most likely due to its cartilaginous origins.^{8 9} In their systematic review on temporal bone chondroblastomas, Omar et al⁷ found the average age of diagnosis for these specific tumours to be 42.3 years with a slight male predominance of 1.3:1. Clinical presentation usually results from compression of surrounding structures such as the eustachian tube, TMJ and neurovascular structures. Reported symptoms include: hearing loss, tinnitus, aural fullness, disequilibrium, otalgia, facial paralysis, paresthesia, trismus, headaches and seizures.⁷ Symptoms can be long standing and subtle.⁹ In this report, the patient suffered with her symptoms for more than a year before diagnosis. The duration of her symptoms appears to be in keeping with other case reports/series on this topic.^{7 9}

Due to the rarity of these tumours, diagnosis can be quite difficult without histological confirmation. Initially, the patient’s presentation and radiological findings on CT and MRI were more in keeping with a possible deep lobe parotid tumour or a tumour related to the TMJ. However, the core biopsy result, though not diagnostic, quickly ruled out salivary gland tumours as focal osseous metaplasia and giant cells are not seen in such pathologies.

Temporal bone chondroblastomas do have characteristic radiological features and have been described as being: well-demarcated lobulated lesions with osteolytic and calcific foci.^{9 11 12} On CT-imaging, they may appear as a high-density mass with mild homogeneous enhancement and central, small unenhanced areas. Calcifications and osteolytic foci may also be present.^{11 13} On MRI, these tumours have variable patterns of enhancement on T₁ and T₂-weighted images. Partial enhancement can be seen with gadolinium contrast.¹³ However, these features are not distinctive to these tumours alone and can be seen in other pathologies including cholesteatomas, giant cell tumours and aneurysmal bone cysts.¹¹

Histologically, these tumours are characterised by compact polyhedral chondroblast with abundant eosinophilic cytoplasm and small hyperlobulated nucleoli. Spindle-shaped cells as well as multinucleated giant cells are also seen embedded in eosinophilic stroma. Tumour cells may have variable mitotic activity. Calcifications may be present in the cytoplasm or stroma, appearing in a pericellular lacelike or ‘chicken-wire’ pattern.^{4 9 14} Immunohistochemical staining shows at least focal positivity for S-100, vimentin, neuron-specific enolase and muscle specific actin. Recently, a new marker ‘discovered on gastrointestinal stromal tumour 1’ may be used to differentiate chondroblastomas from other giant cell containing tumours such as chrondromyxoid fibroma.¹⁵ In this case report, the final histopathology report showed features in keeping with a chondroblastoma such as positive immunohistochemistry and areas of calcification with osteoclast like giant cells. However, given the fragmented and friable nature of the lesion, an external review was required to confirm the final diagnosis.

The mainstay of treatment for temporal bone chondroblastomas is surgical excision. Radiotherapy is recommended for recurrence and patients who are unfit for surgery.⁹ The chosen surgical approach for these tumours depend on their size and location. Infratemporal, middle cranial fossa and transzygomatic approaches have been described in the literature.7–9 13 16 Our case highlights the use of a lip-split mandibulotomy approach to the parapharyngeal space (PPS). Other approaches to the PSS include: transoral, transcervical, transparotid, transmandibular and infratemporal.¹⁷ Transmandibular approach gives the best exposure to the PPS, allowing for en bloc surgical resection of the tumour with distal and proximal control of vital neurovascular structures. Patients should be counselled on the risk of malocclusion, first bite syndrome, lip paresthesia, orocutanoeus fistula and possible requirement for temporary tracheostomy in the event of postoperative oedema

The recurrence rate of these tumours after removal have been reported to be as high as 20%,^{11 18} therefore, long-term follow-up is recommended. In the current report, as completeness of excision could not be ruled out from the final histopathology report, long-term follow-up will be required with serial interval surveillance scans.

Learning points.

• Temporal bone chondroblastomas should be considered a differential for a parapharyngeal space tumour arising from the skull base.

• Symptoms can be subtle and longstanding and are usually the result of compression to surrounding structures and include: hearing loss, tinnitus, otorrhoea, cranial palsies and trismus.

• Radiological imaging may reveal a well demarcated lobulated mass with areas of osteolytic and calcific foci, however, these features are not diagnostic and can be seen in a range of similar bone tumours.

• Definitive treatment is surgical with radiotherapy reserved for recurrence or patients who are unfit for surgery.

• Long-term follow-up is recommended as recurrent rates are high.