Histologically Proven Radiation‐Induced Brainstem Glioma 93 Months After External Beam Radiotherapy for Pituitary Macroadenoma: Radiation Treatment Dose and Volume Correlation

ABSTRACT Patient is a 29‐year‐old with a history of recurrent growth hormone‐secreting pituitary macroadenoma diagnosed 12 years prior to presentation. Eight years prior to current presentation, the patient underwent re‐resection and received 50.4 Gy external beam radiotherapy (EBRT) in 28 fractions of 1.8 Gy each. Serial postradiation MRIs demonstrated regression in pituitary tumor size. Patient presented with new headaches 7.5 years after completing EBRT. Brain MRI demonstrated new FLAIR hyperintensity and contrast enhancement within the pons and medulla, corresponding to the 36 Gy isodose line of each radiation dose fraction. Differential diagnosis included radiation necrosis and radiation‐induced glioma (RIG). The patient's neurologic exam worsened over the following 4 months. MRI showed progressive increase in mass effect, extent of FLAIR hyperintensity, and contrast enhancement in the brainstem. Stereotactic‐assisted biopsy showed infiltrating astrocytoma with moderate atypia. A PubMed search showed this is the first case of histologically verified brainstem RIG correlated with 3‐dimensional conformational radiation therapy dose and volume planning following EBRT for a pituitary adenoma. The rare occurrence of brainstem RIG after radiation therapy for pituitary tumor supports the need for long‐term imaging monitoring of such patients.