The method of the invention for preparing a treatment product is characterized by using a starting plasmid based on a virus belonging to the Togaviridae stock from which the genes encoding capsid proteins of the virus have been deleted. An RNA encoding virus-like particles (VLP-RNA) is prepared by manipulating the starting plasmid by connecting to it a spreading enabling gene and a treatment gene. The invention is furthermore concerned with such a treatment product and a plasmid construct encoding virus-like particles, which is prepared from the Sindbis virus in which the capsid protein of the virus has been substituted by a spreading enabling gene and a treatment gene. Example of spreading enabling gene is a gene encoding vesicular stomatitis virus glycoprotein (VSV-G). A particular example of treatment product is Herpes simplex virus thymidine kinase linked to GFP, said product being a Suicude/Reporter construct