Spontaneous fibrosarcoma in an experimental aged Lewis rat.

This case report describes the results of the gross pathological, histological and immunohistological examinations of a fibrosarcoma that spontaneously developed in the left neck region of an aged male Lewis rat that participated in a long-term facial nerve regeneration study. A 10 mm long polylactic acid (PLA) tube was implanted into a 7 mm critical defect gap of the buccal branch of the left facial nerve of a male eight-week-old Lewis rat. Forty-two weeks after implantation, an approximately 8 cm × 9 cm × 2.5 cm rapidly growing mass with ulceration of the overlying skin was found in the left neck region. Gross examination of the tumour and the surrounding tissues before tumour excision revealed that the tumour had clear boundaries, and had not invaded any facial tissues, the facial nerve and the PLA nerve guide. Gross examination of the tumour's cut surface revealed that the tumour comprised numerous smooth nodules with a homogeneous white tan colour. Examination of the haematoxylin and eosin-stained sections of the tumour revealed that the tumour was predominantly composed of bundles of spindle-shaped atypical proliferating cells that were mixed with bundles of collagen fibres and arranged in a storiform pattern. Tumour emboli and skin invasion were also observed. Immunohistological examination revealed that the tumour cells were weakly positive for vimentin, but negative for keratin, α-smooth muscle actin, S-100 protein and CD34. From the results of these analyses, the final pathological diagnosis of this tumour was a fibrosarcoma.