Malignant transformation of a recurrent vestibular schwannoma

Malignant nerve sheath tumours are uncommon tumours, particularly those that affect cranial nerves. They are most frequently seen within the context of neurofibromatosis type II. Malignant transformation of benign cranial nerve sheath tumours has been reported on very few occasions.^1–4 We report a case of malignant nerve sheath tumour arising as a recurrent tumour at the site of a previous benign schwannoma.

Tissue was received from the resection of a mass surrounding and involving the right acoustic nerve of a 53 year old man who had previously undergone resection of a tumour involving the same site. Histology from the previous resection, seven years earlier, had shown the tumour to be a benign vestibular schwannoma. Several years before the final resection, our patient had undergone a short course of radiotherapy in an attempt to reduce tumour size. He had no stigmata of neurofibromatosis type II and reported no family history of the condition.

The specimen was received as fragments of haemorrhagic tissue measuring 5 ml in aggregate. The specimen was processed routinely for histological examination, including immunohistochemistry.

Microscopic examination of the tissue showed adjacent foci characteristic of both benign and malignant nerve sheath tumours. There were areas composed of irregularly arranged spindle cells with elongated wavy contours characteristic of Antoni A type cells. These areas showed mild nuclear pleomorphism; no mitoses or necrosis were seen. The features were typical of a benign schwannoma. Other areas of tissue were very hypercellular, showing highly pleomorphic spindle cells with bizarre, hyperchromatic nuclei. Scattered giant cell forms were seen, as were zones of necrosis. There was a high mitotic count of, on average, 12 mitoses/10 high power fields in the malignant areas. The appearances were those of a malignant nerve sheath tumour. Both the benign and malignant tissue showed positive staining for S100 and vimentin. The malignant tissue showed positive staining for Ki67, with a staining index of 20%. The benign tissue showed a Ki67 index of less than 1%.

Sections from the original tumour were reviewed. All of the sections showed typical features of a benign schwannoma with cellular and poorly cellular areas (Antoni A and B). Sections were stained for Ki67 and all were negative. The histological features were similar to the benign areas of the later tumour.

Malignant transformation of benign nerve sheath tumours is extremely unusual and reported cases are few.^1–4 We believe that our case represents such an example.

A previous resection from the same site showed a schwannoma with no evidence of malignancy, and similar benign areas are present in the current biopsy. This supports our assumption that this malignant tumour has arisen by transformation from the previous lesion. An issue with this case is the history of previous radiation. It has been reported that irradiation may induce neurofibrosarcoma.⁵ These cases report malignancy arising within previously normal nerves and do not describe the induction of malignancy within a previously benign tumour. Regardless of this possible aetiology, we believe that this case represents malignant transformation within a previously benign vestibular schwannoma, and therefore presents a rare case.

Figure 1.

Malignant area within tumour; original magnification, ×20.

Figure 2.

Benign area within tumour; original magnification, ×10.