Systematic evaluation of Stereotatic Radiosurgery effects in metastasis and acoustic neurinomas using MRI

INTRODUCTION: Stereotactic radiosurgery (SRS) is a specific form of radiation therapy used to treat tumors and other abnormalities in the brain. This technique uses a few highly focused X-rays beams usually delivered in a single high-dose. To maximize the radiation dose in the tumor, and to minimize the damage in collateral healthy areas, the treatment uses a frame-like device that keeps the head still in a stereotactic system, and a three-dimensional Treatment Planning System (TPS) that gives the location and strength for each Xray beam, using as input a tomographic image of the brain. Normally, treatments computed by the TPS are validated using physical models, phantoms, radiation detectors or radiosensitive gels [1]; which are used to verify prescribed doses and not the actual biological effects suffered by patients that undergo a SRS. We hypothesize that with MRI it could be possible build a map of the actual biological damage caused by the SRS treatment, so that to provide feedback in vivo to the TPS. With such a map, SRS treatments could be computed or modified taking into account the effective tissue damage in previous treatments, instead of only estimated dose distributions. In a previous work [2] we showed that T2w MR images could provide information about changes associated to radiated tissue, and those changes could be correlated with the induced tissue damage. In the present work we perform a systematic MRI follow up study to verify biological damage in three patients before and after a SRS, which now includes proton density (PD), T2w, T2w-Flair and ADC images.