Null cell PitNET/adenoma of the pituitary gland with post-radiation sarcoma and dedifferentiation

Abstract A male patient at the age of 30 was operated due to an inactive pituitary adenoma of null cel cell type. The tumor relapsed twice and received sterotactic radiotherapy in 1992 and 2006. Re-examination of the tumor of 2006 in 2021 showed SF-1 expression leading to diagnosis of gonadotrophic PitNET/adenoma. Additionally a second spindle cell tumor was identified in 2021. This mesenchymal tumor expressed vimentin, smooth muscle actin and nuclear p53. FISH revealed a polysomy for chromosome 12 but no amplification of MDM2. This tumor was classified as spindle- and pleomorphic sarcoma. In contrast to the tumor in 2006 the PitNET/adenoma was negative for SF-1 and had to be classified as null cell PitNET. This dedifferentiation is likely to be due to a loss of SF-1 transcription factor formation.