New onset of myasthenia gravis 10 years after proton beam therapy for thymoma.

A 36-year-old woman underwent proton beam therapy for encapsulated type B1 thymoma for curative intent at 66 GyE. Radiographically partial response was achieved. Although the tumor size had been stable since that time, she developed systemic myasthenia gravis 10 years after the proton therapy. Extended thymectomy was performed. There were no adhesions between the tumor and the pericardium, and there were no adhesions also between the tumor and the sternum, probably due to the favor of Bragg peak effect. Extensive hyalinization with small foci of viable tumor cells showing degenerated type A-like morphology was observed in the resected tumor. Whether the viable cells represented recurrence with degenerative changes or de novo tumor formation was unable to be determined, and whether the viable cells were responsible for the onset of myasthenia gravis remained unclear.