Background Radiotherapy is used to treat recurrent oligodendrogliomas, WHO grade 2 tumours. Potential morbitities include steroid–responsive radiation necrosis and radiation leucoencephalopathy, characterised pathologically by reactive astrogliosis, focal necrosis, demyelination, axonal loss, and clinically by progressive subcortical deficits (ataxia, amnesia, incontinence, cognitive decline), with relative sparing of cortical function.

Although subcortical features may overlap with neurodegenerative conditions (eg frontotemporal dementia), focal cortical atrophy of FTD causes loss of language function in addition to memory, and specific histopathological features characterise FTD subtypes (eg Pick disease). Association between mitotic disease and tauopathy has not been reported widely, but co–existence is possible. Diagnostic accuracy may guide management.

Case description A 54 year old man presented with increased seizure frequency, despite anti–epileptic therapy. Over 6 months, he had developed progressive ataxia, aphasia, personality changes, memory loss, incontinence, somnolence, and binge–eating. Left frontal oligodendroglioma had been resected 7 years before, with radiotherapy to treat local recurrence 2 years earlier. MRI brain showed no further recurrence. Radiation leucoencephalopathy with possible necrosis was diagnosed, although clinical progression was steroid–resistant and disproportionate to imaging. Chemotherapy was considered, despite absent proof of recurrence, due to recurrent seizures and overall deterioration.

Because of marked aphasia and mild rigidity, re–examination of initial surgical tissue was performed.

Results of investigations: MRI brain: left frontal ‘post–op’ gliosis, high signal T2, non–enhancing. Neuropathology: neuronal loss in cortical tissue surrounding oligodendroglioma, gliosis, Pick cells, argentophilic inclusions (Pick bodies).

Discussion Progressive aphasia is uncharacteristic of radiation leucoencephalopathy. Despite absence of recurrence on MRI, chemotherapy, with potential morbidity, was considered due to progressive deterioration. Re–examination of pathology specimen, looking for additional (Tau) pathology clarified diagnosis as Pick disease, a frontotemporal dementia. Although no specific therapy could modify disease, diagnostic clarification helped family to accept symptoms. This highlights the importance of reconsidering the possibility of dual pathology when clinical signs do not correlate with working pathological diagnosis.