The present invention relates to compositions and methods for use in medical diagnosis 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 nanoparticle aggregates, wherein: i) the density of each nanoparticle and of each nanoparticle aggregate is 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 nanoparticle aggregate being covered with a biocompatible coating； ii) the nanoparticles' and/or nanoparticle aggregates' concentration is of at least about 1% (w/w)； and iii) the apparent viscosity at 2 s−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.
