ORIGINAL ARTICLE Long-term evaluation of the effect of hypofractionated high-energy proton treatment of benign meningiomas by means of 11 C-L-methionine positron emission tomography Mats Ryttlefors 1 & Torsten Danfors 2 & Francesco Latini 1 & Anders Montelius 3 & Erik Blomquist 4 & Olafur Gudjonsson 1 Received: 24 September 2015 /Accepted: 5 January 2016 # Springer-Verlag Berlin Heidelberg 2016 Abstract Purpose To determine if 11 C-L-methionine PET is a useful tool in the evaluation of the long-term effect of proton beam treatment in patients with meningioma remnant. Methods Included in the study were 19 patients (4 men, 15 women) with intracranial meningioma remnants who received hypofractionated high-energy proton beam treatment. Patients were examined with 11 C-L-methionine PET and MRI prior to treatment and after 6 months, and 1, 2, 3, 5, 7 and 10 years. Temporal changes in methionine uptake ratio, meningioma volume, meningioma regrowth and clinical symptoms throughout the follow-up period were evaluated. Results In 17 patients the tumour volume was unchanged throughout the follow-up. The methionine uptake ratio on PET decreased over the years in most patients. In two patients the tumour remnant showed progression on MRI. In these patients, prior to the volume increase on MRI, the methionine uptake ratio increased. One patient experienced transient clin- ical symptoms and showed radiological evidence of a radiation-induced reaction close to the irradiated field. Conclusion Proton beam treatment is a safe and effective treatment for achieving long-term growth arrest in meningio- ma remnants. Follow-up with 11 C-L-methionine PET may be a valuable adjunct to, but not a replacement for, standard radio- logical follow-up. Keywords Meningioma . Proton beam treatment . Stereotactic irradiation . 11 C-L-Methionine . Positron emission tomography Introduction Most intracranial meningiomas are benign extraaxial tumours and the preferred treatment is total surgical excision [1–4]. However, the location of the tumour may in some instances prohibit total resection due to interference with neural and/or vascular structures. Such is often the case when the tumour is located at the skull base [5]. In these cases the safest treatment option may be to partially resect the tumour leaving inacces- sible parts of the tumour to avoid the unnecessary risk of debilitating cranial nerve deficits and vascular injury [6–11]. The tumour remnant can then either be watched for recurrent growth or be treated with adjuvant therapy [3, 12, 13], such as irradiation [14], to achieve at least temporary growth arrest. Different types of radiotherapy have been used for postopera- tive radiation of meningioma remnants: conventionally frac- tionated conformal external photon beams [15–19], stereotac- tic photon beam radiosurgery using multiple cobalt sources with Gamma Knife surgery [17, 19–23] or linear accelerators [17, 24–26], fractionated radiotherapy with high-energy pro- tons [17, 27–30], or a combination of photons and protons [17, 31–35]. At our institution, the treatment strategy for skull base me- ningiomas has been to resect the portion of the tumour * Mats Ryttlefors Mats.Ryttlefors@Neuro.uu.se 1 Department of Neuroscience, Neurosurgery, Uppsala University, Uppsala University Hospital, 751 85 Uppsala, Sweden 2 Department of Surgical Sciences, Radiology, Uppsala University, Uppsala University Hospital, 751 85 Uppsala, Sweden 3 Department of Immunology, Genetics and Pathology, Medical Radiation Science, Uppsala University Hospital, 751 85 Uppsala, Sweden 4 Department of Immunology, Genetics and Pathology, Experimental and Clinical Oncology, Clinical oncology, Uppsala University, Uppsala University Hospital, 751 85 Uppsala, Sweden Eur J Nucl Med Mol Imaging DOI 10.1007/s00259-016-3310-z