The present invention relates to compositions and methods for use in medical diagnostic and patient monitoring, typically in the context of therapy, in particular in the context of oncology to optimize tumor bed local irradiation. It more particularly relates to a biocompatible gel comprising nanoparticle and/or nanoparticles aggregates, wherein i) the density of each nanoparticle and of each nanoparticle aggregate is of at least 7 g/cm3, the nanoparticle or nanoparticles of the aggregate comprising an inorganic material comprising at least one metal element having an atomic number Z of at least 25, more preferably of at least 40, each of said nanoparticle and of said nanoparticle aggregate being covered with a biocompatible coating； ii) the nanoparticles and/or nanoparticle aggregate concentration is of at least about 1% (w/w)； and iii) the apparent viscosity at 2 s&minus；1 of the gel comprising nanoparticles and/or nanoparticle aggregates, is between about 0.1 Pa·s and about 1000 Pa·s when measured between 20° C. and 37° C.
