Chondroblastoma of squamous part of the temporo-parietal region of skull vault: a case report and review of literature.

Chondroblastoma of squamous part of temporal bone is a very rare bone tumor. Although most of them are benign, the prognosis is not predictable. A 14-year-old girl presented with recurrent slowly growing mass over the right side of the temporo-parietal region of skull vault. Initial curettage material showed extensive chondroid areas and diagnosed histologically as "enchondroma." Histology of completely excised recurrent mass showed identifiable chondroblastic foci. She was followed up for 3 years and was free from recurrence. Chondroblastomas at very rare sites such as squamous part of temporal bone have variable histology, confusing with other giant cell lesions, variable prognosis, and require prompt diagnosis and complete excision.

Introduction

Chondroblastoma of the temporal bone is a rare benign tumor. Most of the cases occur in petrous part and involvement of squamous part (skull vault) is very uncommon. It is very important to recognize this lesion for many reasons.

At this unusual site the lesion may mimic other giant cell containing lesions such as giant cell tumor, giant cell reparative granuloma, etc. and the diagnosis may be missed. They have unusual and variable histology when compared to chondroblastomas at usual locations (ends of long bones).[1] Because of variable histology whether benign or aggressive, chondroblastomas at this location have variable prognosis and have tendency of recurrence, sarcomatous change, intracranial extension, and pulmonary metastasis.[2] Because of lack of data and rarity of tumor, the prognosis is not predictable and hence the preferred management is complete excision rather than curettage.[2]

Here is a case report of such similar case in a young adolescent girl.

Case Report

A 14-year-old girl presented with slowly growing recurrent mass over right side of skull of 2 years’ duration. Initially she underwent curettage of the lesion and histologically diagnosed as “enchondroma”. A year later she developed recurrence. Local examination revealed a firm to hard, nontender swelling measuring 7.5 × 7 × 4 cm over the right temporo-parietal region of the skull vault. The overlying skin was freely mobile. There were no neurological findings. Her routine investigations were within normal limits. Plain and contrast axial CT showed [Figure 1a] heterogeneously enhancing soft tissue density growth measuring 7.4 × 4 cm with calcified cartilaginous tumor matrix with thin sclerotic margins and speculated or sunray type of periosteal reaction. There was no intracranial extension and intracranial parenchymal abnormality. Radiological diagnosis was “tumor of cartilaginous origin with possible malignant transformation.”

Figure 1

(a) CT scan showing heterogeneously enhancing mass with foci of calcification. (b) Gross appearance of cut section of growth

The patient underwent gross total excision of the recurrent mass followed by curettage and chemical (phenol) cauterization of underneath “outer table” of skull vault as well as a margin of about 0.5 cm of uninvolved zone, leaving “inner table” of skull vault intact. The postoperative radiation was not planned as there was no intracranial extension. The postoperative period was uneventful.

Grossly, the mass was globular, whitish firm to hard resilient, measuring 7.5 × 7 × 4 cm, and gritty sensation felt on cutting. The cut surface was whitish glistening, with linear gray brown hemorrhagic streaks [Figure 1b].

Histopathologically, a major portion of the lesion showed chondroid areas with adjacent foci of cellular areas showing proliferating mononuclear cells and microscopic aneurysmal bone cyst like areas [Figure 2a and b]. The chondroid areas showed foci of osteoid metaplasia and calcification [Figure 2b]. The mononuclear cells are oval to spindle to polyhedral having eosinophilic to vacuolated cytoplasm with distinct cell border [Figure 2c]. The nuclei are oval to spindled showed irregularity of nuclear contour in the form of indentation, grooving with fine dispersed chromatin and mitotic activity of less than 1 per high power field. Amidst there were scattered osteoclast-type giant cells [Figure 2b] and microscopic foci of aneurysmal bone cyst like areas [Figure 2a] filled with blood, which corresponds to the gray brown hemorrhagic streaks observed in gross examination of the lesion [Figure 1b]. Immunohistochemistry for S-100 protein were positive [Figure 2d]. Reticulin stain preparation showed characteristic “honeycombing” with “chicken wire” calcification around each cell [Figure 2e].

Figure 2

(a) Chondroid areas (arrow head), mononuclear cellular areas (thick arrow), and aneurysmal bone cyst like areas (black stars) (H and E, ×10). (b) Mononuclear cellular areas merging with chondroid areas, with multinucleated giant cells (thin arrows) and foci of calcification and ossification (arrow head) (H and E, ×40). (c) Chondroblastic foci showing mononuclear cells with distinct cell margins, vacuolated cytoplasm (H and E, ×40). (d) S-100 immunostain positivity (×100). (e) Characteristic “chicken wire” calcification around each cell in reticulin stain preparation (×40)

The patient was followed up for 3 years and was free from recurrences.

Discussion

Chondroblastoma of bone is an uncommon chondroid neoplasm accounts to about 1% of all bone tumors. The incidence in skull and facial bones ranges from 6.6 to 7.1% of all chondroblastomas.[13-5] The common sites in skull include petrous part of temporal bone; mandible, temporo-mandibular joint, parotid region, skull base, and least common sites are skull vault regions (squamous part of temporal bone, parietal bone, etc). So far there are a total of 46 cases of temporal bone involvement with majority involving petrous part. There are only three case reports (from Mayo clinic consultation files)[4] of skull vault involvement (squamous part of temporal bone and parietal bone) [Table 1].

Table 1

Published case reports of chondroblastoma of the squamous part of the temporo-parietal region (skull vault)

Chondroblastoma when it occurs at unusual locations most likely affects elderly individuals and tends to have unusual histological findings.[134] But the present case is in younger patient. It was observed from most studies that most striking difference in age was found in chondroblastomas occurring in skull and facial bones.[134]

Recognizable areas of chondroid differentiation is a relatively less common finding in chondroblastomas of skull and facial bones than those occurring in usual sites.[1] In the present case there were extensive areas of chondroid differentiation masking the chondroblastic foci and hence the initial diagnosis was “enchondroma.” However, the excision of the whole tumor showed identifiable chondroblastic foci, a key feature to diagnosis.

The mitotic activity and cellular atypia are common findings in chondroblastomas of unusual locations and does not have prognostic significance. The present case showed low mitotic activity (less than 1 per high power field) and no cellular atypia even though it is of uncommon location but showed recurrence.

Aneurysmal bone cyst like areas are uncommon findings in chondroblastomas of skull and facial bones.[1] When it is present, there is likelihood of recurrence of the lesion. Huvos and Marcove found a positive relationship between the presence of aneurysmal bone cyst like areas and the recurrence rate of the lesion.[6] The present case showed numerous microscopic aneurysmal bone cyst like areas and recurrence as well.

Multinucleated osteoclast-like giant cells are common finding in chondroblastomas and hence mimic other giant cell lesions such as giant cell tumor, giant cell reparative granuloma, and aneurysmal bone cyst. But the presence of chondroblastic foci and focal calcification rules out other giant cell lesions.[7] Hence every attempt is made in thorough sampling and search for chondroblastic foci in all giant cell containing lesions at these unusual sites.

Majority of ossification of temporal bone occurs by intramembranous ossification. Temporal bone develops from two embryologically distinct bony precursors the petrous part which develops from endochondral ossification and the squamous part which develops from intramembranous ossification of primitive mesenchymal connective tissue.

Chondroblastomas of petrous part thought to arise from the remnants of fetal chondroskeleton in the form of surviving islands of primary endochondral cartilage in temporal bone. Chondroblastomas of squamous part of temporal bone thought to arise from residual mesenchyme in temporal bone or from the pluripotential embryonal rests lying along the fusion lines of primitive bones. It is postulated that chondroblastoma was the response of normal chondrocytes to an altered local environment, rather than an intrinsic alteration in chondrocytes. With appropriate initiating event the lesion could develop from the rests of chondrocytes within the endochondral portion of temporal bone.[78] The initiating event could be the entrapment of either aberrant mesenchyme or primitive cartilage islands in the petrosquamous suture lines of developing temporal bone.

Chondroblastomas of temporal bone is widely accepted as a benign tumor and is best treated by gross total removal and the regrowth is very rare. Commonly employed surgical procedures include curettage; curettage followed by chemical or thermal cauterization with packing of bone chips; curettage followed by radiation and complete resection.[39] The data on treatment for chondroblastomas of unusual sites such as squamous part temporo-parietal region of skull vault are limited. The data obtained from Mayo clinic consultation files showed curettage as the commonly employed procedure [Table 1].[4] The present case showed recurrence following curettage but following complete excision there was no recurrence.

The postoperative radiation is reserved only for those chondroblastomas at unusual locations where there is complex anatomical structure, tumor show encroachment on vital structures such as intracranial extension and complete excision is difficult. Hence the preferred management should be curettage followed by radiation.

Conclusion

The diagnosis of chondroblastoma of temporo-parietal region of skull vault is usually missed because of its rarity, unusual site and variable histology. At this unusual site, the lesion is confused for other giant cell containing lesions. Since the lesion has variable prognosis, a prompt diagnosis and gross total excision are essential to prevent recurrences and sarcomatous change.