The quality of modern patient radiotherapy treatment is strongly dependent on reliable and accurate dosimetry techniques. Although desirable, in situ and in vivo dosimetry is usually hard to implement in routine radiotherapy procedures. The potential use of nanoparticles has been recently considered for biomedical applications achieving promising performance for diagnosis as well as therapeutic practices. The present work reports about a novel proposal based on the use of high atomic number nanoparticles for online estimation of absorbed dose during conventional radiotherapy treatment. In this first phase, the investigation handles with experimental and theoretical tasks regarding the potential use of the variations in the annihilation peak and its correlation with the presence of high atomic number nanoparticles acting as signal enhancers. Dedicated Monte Carlo simulations are also presented in order to complement experimental results and theoretical models. Although the correlation between annihilation peak with the presence of nanoparticles was successfully confirmed, further investigations are still necessary in order to estimate feasible absorbed dose from determinations of annihilation peaks.