Evaluation of the effect of high-energy proton irradiation treatment on meningiomas by means of C-11-L-methionine PET

A remnant meningioma of WHO grade I that is located at the base of the skul l and is treated with radiotherapy has to be followed up for at least 5-10 years to evaluate the treatment effect and detect recurrence. The tumour ha s to grow considerably to show detectable volume increase on computed tomog raphy (CT) or magnetic resonance imaging (MRI). Owing to the location at th e base of the skull, a small increase in tumour volume may be hazardous. It is thus important to find a method to evaluate treatment effects earlier a nd potentially detect those tumours that have a tendency to grow. Nineteen patients with intracranial meningiomas were given irradiation with the 180- MeV proton beam at the Svedberg Laboratory, Uppsala, Sweden. The fractionat ion schedule used was in general a total dose of 24 Gy in four consecutive daily 6-Gy fractions. Serial C-11-L-methionine PET examinations were used t o evaluate the effect of stereotactic proton beam treatment. The radioactiv ity uptake in the tumour was evaluated as the ratio to the uptake in normal brain tissue. The follow-up period thus far is 36 months. In 15 of the 19 patients, C-11-L-methionine uptake was reduced 36 months after irradiation compared with the pre-treatment uptake of the tracer. In the total patient group the average reduction was 19.4%. Our results reveal that proton beam irradiation of meningiomas had an inhibitory effect on the methionine uptak e in the meningiomas, although tumour size remained unchanged. The combinat ion of unchanged tumour morphology and a reduction in methionine uptake aft er irradiation suggests that C-11-L-methionine PET might enable earlier eva luation of the treatment effect than is possible with CT or MRI.