58 The Neuroradiology Journal 22: 58-71, 2009 www. centauro. it Dynamics of Metabolic Changes in Intracranial Metastases and Distant Normal-Appearing Brain Tissue after Stereotactic Radiosurgery: a Serial Proton Magnetic Resonance Spectroscopy Study SUMMARY – The present study evaluated the dynamics of metabolic changes in intracranial me- tastases and distant normal-appearing brain after stereotactic radiosurgery (SRS). Forty neoplasms were evaluated with single-voxel proton magnetic resonance spectroscopy ( 1 H-MRS) both before and after treatment. From one to six examinations (median, 3) were done in each individual case dur- ing follow-up. At the time of each investigation additional 1 H-MRS was obtained from the nor- mal-appearing brain distant from the radiosurgical target. Investigated metabolites included N- acetylaspartate (NAA), choline-containing compounds (Cho), creatine (Cr), and mobile lipids (Lip). Within the first month after SRS responded tumors showed a statistically significant increase in NAA/Cho ratio, and decrease of Cho content and Lip-to-normal brain Cr (nCr) ratio. By contrast, statistically significant metabolic alterations were not detected in stabilized tumors. Statistically significant volumetric and metabolic changes were not marked between three and 12 months after treatment in non-progressing lesions. Alternatively, decrease of NAA/Cho ratio, NAA content and Cr content, and increase in Lip/nCr ratio and Cho content were evident in progressive neoplasms, and subtle metabolic alterations could be revealed even before the increase in the lesion volume. Meta- bolic characteristics of normal-appearing brain distant from the radiosurgical target did not show statistically significant changes within the first year after treatment. In conclusion, additional use of serial 1 H-MRS during follow-up after SRS for intracranial metastases permits detailed evaluation of the metabolic tumor response and may be potentially helpful for early prediction of recurrence. M.F. CHERNOV* , ** , ***, M. HAYASHI** , ***, M. IZAWA***, K. NAKAYA***, N. TAMURA***, Y. ONO****, K. ABE****, M. USUKURA****, S. YOSHIDA****, R. NAKAMURA* , **, T. SUZUKI**, Y. MURAGAKI** , ***, H. ISEKI* , ** , ***, O. KUBO***, T. HORI***, K. TAKAKURA* , ** , *** * International Research and Educational Institute for Integrated Medical Sciences (IREIIMS); ** Faculty of Advanced Techno-Surgery, Institute of Advanced Biomedical Engineering and Science; Departments of *** Neurosurgery and **** Neuroradiology, Neurological Institute; Tokyo Women’s Medical University, Tokyo, Japan Key words: metastatic brain tumor, stereotactic radiosurgery, metabolic effects, proton magnetic resonance spectroscopy initial cytotoxic effect of the high dose radia- tion, followed by a delayed vascular reaction, with possible induction of a local cellular im- mune response as well 4 . Nevertheless, while volumetric changes in metastatic brain tumors after SRS are well documented, the associated metabolic alterations within and at a distance from the target are not so clear. Proton magnetic resonance spectroscopy ( 1 H- Introduction The efficacy of stereotactic radiosurgery (SRS) for the management of both single and multiple intracranial metastases had been well proved. Tumor response rate varies from 47 to 79%, and its actuarial 12-months control rate constitutes more than 80% in most series 1-3 . Such effectiveness seemingly results from the