Diffuse-type giant cell tumor/pigmented villonodular synovitis arising in the sacrum: malignant form.

Diffuse-type giant cell tumor (GCT)/pigmented villonodular synovitis (PVNS) in the axial skeleton or spine is rare. Herein is reported a case of diffuse-type GCT/PVNS involving the sacrum and the fifth lumbar vertebra, in which the patient developed regional lymph node swelling after recurrence. The recurrent tumor was found to have atypical histological features such as spindle cell morphology, cytological atypia and high mitotic rate, which are compatible with the diagnostic criteria of secondary malignant diffuse-type GCT/PVNS. Although the nodal lesions were not sampled histologically, the clinical and histological features indicate that the current case is an example of malignant diffuse-type GCT/PVNS. This case is considered to be the first case of malignant diffuse-type GCT/PVNS in the spine, because no such lesions have been previously reported in the axial skeleton or spine. Careful surveillance should be required for diffuse-type GCT/PVNS arising at unusual site.