Dedifferentiated chordoma arising in irradiated sacral chordoma.

: A patient who received radiation therapy for a sacral lesion 10 years ago and eventually developed dedifferentiated chordoma at the primary site is reported. Initially, the patient received 54 Gy with Co-60 to the sacral tumor, and 6 years later an additional 50 Gy with 10 MV X-rays to the local recurrence. Four years later, an evident sacral tumor was noticed. When readmitted for re-confirmation of histology, a large rapidly growing lung metastasis was observed. The patient eventually died of progressive disease with incidental aspiration pneumonia. At autopsy, 10 years after the initial treatment, the recurrent sacral tumor proved to be dedifferentiated chordoma accompanied by typical chordoma. Although this case suggests the possible malignant transformation of chordoma rather than collision, the factors inducing this change remain unknown.