A biocompatible curable composition and a method of detecting a border of a tumor, a tissue of interest, or both including injecting the biocompatible curable composition and contacting the border of a tumor or a tissue, the biocompatible curable composition crosslinks to form a three-dimensional cured nanocomposite, and imaging the three-dimensional (3D) cured nanocomposite, and imaging the 3D cured nanocomposite by at least one of MRI, CT, ultrasound, and X-ray, to detect the border of the tumor or the tissue of interest or track tumor motion during radiotherapy treatment. The biocompatible curable composition comprising an organic polymer having a hydrolysable functional group, a metallic nanoparticle, and a polar or a non-polar solvent. A brachytherapy strand consisting of a biocompatible curable composition and a radio-isotope seed. The biocompatible curable composition is shaped into an elongated cylinder and forms a 3D cured nanocomposite with a radio-isotope seed embedded.