Targeted Hippocampal Irradiation In A Small Rodent Using Imrs And Rapidarc Srs: Preliminary Data

To use a rodent model to develop and optimize the procedures for the precise irradiation of the hippocampal region with minimum radiation dose to the remainder of the brain. For this purpose, IMRS and RapidArc VMAT SRS were used to irradiate one and two hippocampi, respectively, of athymic nude (ATN) rats. Prescribed dose was verified using Mapcheck while irradiated and spared brain region(s) were confirmed through brain cell staining postmortem. Seven ATN rats, 6 - 8 weeks old underwent human-like radiation treatment planning followed by image-guided SRS. MRI and CT axial images 0.8-mm thickness of the rat's skull were acquired and transferred to ECLIPSE treatment planning software to aid in contouring the brain, right and left hippocampi. A 6-field non-coplanar IMRS plan was generated to target the left hippocampus to a dose of 10 Gy while a coplanar two-arc RapidArc SRS plan targeted both hippocampi also to 10 Gy. Treatment was delivered using a 6 MV photon beam from a Varian Trilogy Linac equipped with on-board imaging (OBI). Prior to treatment, the dose was verified through a QA plan generated from the treatment plan and delivered to a Mapcheck detector array imbedded in a virtual water phantom. Orthogonal x-ray images taken with the OBI and co-registered to DRR images from the planning CT were used to precisely adjust the rat's treatment position. One month post-irradiation, rats were euthanized and 30-micron thick brain sections were evaluated using an immunocytochemical technique to verify the radiation effects in the targeted and non-targeted regions. Table 1 presents dosimetric results for one and two hippocampi irradiation using IMRS and RapidArc SRS, respectively. Percent differences between calculated and measured dose using Mapcheck for one hippocampus and two hippocampi were 5.0% and 2.0%, respectively. Cell staining performed in the single hippocampus irradiation cases revealed a significant reduction in cell population in the ipsilateral hippocampus while populations comparable to those in a non-irradiated subject were observed in the contralateral hippocampal region. Present results demonstrate that precise irradiation of small volumes within a rat's brain can be achieved with human-like image-guided IMRS and RapidArc SRS treatments. Postmortem clinical analysis of the rat brain provides evidence of high-precision targeted radiation damage and dose sparing.Table 1One Hippocampus (n = 2)Two Hippocampi (n = 5)Min (Gy)Max (Gy)Mean (Gy)Min (Gy)Max (Gy)Mean (Gy)Left Hippocampus7.013.611.79.410.710.2Right Hippocampus0.12.80.79.710.910.4Brain w/ Hippocampi0.113.62.60.611.04.4Brain w/o Hippocampi0.18.51.90.68.03.2 Open table in a new tab