A 60-year-old woman presented to the emergency department with a 2-month history of blurry vision in her left eye. Her medical history was notable for breast cancer treated 3 years earlier with lumpectomy, axillary-node dissection, radiation, and hormonal therapy.
Physical exam on upward gaze is demonstrated. At the time of the current presentation, examination revealed proptosis and an inability to look up with the left eye.
What is the diagnosis?