[A 32-year-old man who developed a posterior fossa mass 12 years after the radiation therapy for cerebellar arteriovenous malformation].

We report a 32-year-old man who developed cerebellar ataxia and a posterior fossa mass 12 years after the radiation therapy for a cerebellar arteriovenous malformation (AVM). The patient was well until 19 years of the age when he had an acute onset of vertigo and vomiting. A spinal tap was performed and the CSF was bloody. He was admitted to another hospital where an arteriovenous malformation was found in the cerebellum by angiography. Four years after the onset, he developed tingling sensation in the distribution of the second division of the right trigeminal nerve. He was admitted to the neurosurgery service of our hospital where the cerebellar AVM was confirmed. He was transferred to University of California where Bragg peak stereotaxic radiotherapy was successfully performed; this utilizes high energy alpha-ray produced by a cyclotron. Three years after the radiotherapy, marked reduction in the size of the AVM was confirmed by angiography. Twelve years after the onset of his initial symptom, he noted unsteadiness of gait. He was readmitted to our neurosurgery service where obstructive hydrocephalus was found. He was treated by ventriculoperitoneal shunting and placement of a Ommaya reservoir. After these therapy, he noted marked improvement in his gait and ataxia. However, in 1993, his unsteadiness of gait recurred, and he was again admitted to our neurosurgery service on June 20, 1993. On admission, T1-weighted MRI revealed a slightly low signal intensity mass lesion in the right cerebellar hemisphere compressing the brain stem; a spotty high signal intensity lesion and another small low intensity lesion were seen within the mass. Vertebro-basilar angiograms revealed upward displacement of the superior cerebellar arteries. No arteriovenous nidus was visualized. On July, 3rd, the cyst was surgically drained and the Ommaya reservoir was removed. Post-operative course was uneventful, however, he developed head tremor after the surgery. Neurologic examination on July 20, 1993 revealed an alert and well oriented man in no acute distress. General physical examination was unremarkable. Neurologic examination revealed no dementia; higher cerebral functions appeared intact. The optic discs were flat, and visual fields were intact. Ocular movements were full but convergence was restricted. Horizontal gaze nystagmus was noted more in the right lateral gaze. Pupils were intact. Facial sensation and facial muscles were intact. Hearing was normal. His voice was of nasal quality. Pharyngeal reflex was diminished. The tongue showed deviation to the left without atrophy. Head tremor at 5 c/s was noted. He was able to stand with support but was unable to walk. No muscle atrophy or weakness was noted. The finger-to-nose and the heel-to-knee tests showed dysmetria and decomposition more on the right. Rapid alternating movements were ataxic on the right. Muscle tone was diminished on the right. Muscle stretch reflexes were normally elicited and were symmetric. The plantar response was flexor bilaterally. Sensation was intact. On July 21, a posterior fossa exploration was performed. After the surgery, he was treated with 30 mg/day of alotinolol which showed no effect on his head tremor. He was then treated with gradually increasing doses of clonazepam; when he received 8 mg/day of clonazepam, his tremor showed marked improvement. He was discussed in a neurologic CPC on the nature of the posterior fossa lesion and his tremor. Opinions were divided between delayed radiation necrosis and a radiation-induced brain tumor. The chief discussant arrived at the conclusion that the patient had delayed radiation necrosis compressing the brain stem and cerebellar hemispheres. Regarding the nature of his tremor, he thought that his head tremor was of cerebellar type of postural tremor. Histologic examination of the biopsied specimen revealed accumulation of relatively fresh blood constituents in the deep area of the cerebellum forming a mass. Most of the