A method for permanently applying via the urethra a reference marker in the prostate gland in order by means of the marker to allow precision aiming of a treatment beam at a cancer tumor therein, whereby a catheter comprising a duct and a reference marker disposed for movement in the duct is introduced into the urethra and inserted through it to the prostate gland, a device, with guidance by the reference marker, is caused to, from the duct, penetrate the catheter and/or a membrane totally or partly covering the catheter and is inserted in glandular tissue surrounding the catheter and the urethra, and the marker is thereafter moved by a means along the penetrating and guiding means and is driven by the movement means, with guidance by the penetrating and guiding means, into said glandular tissue for permanent application therein. An instrument for this purpose comprises a catheter (9) with a duct (11), a reference marker (7) disposed in the duct, a means (12) for indicated penetration and guidance, and a means (14) for moving the reference marker. Said instrument may also be used for permanent application of a reference marker in other tissue.