In our novel technique of tumor vessels treatment, High Z (HZ) contrast atoms are injected into the blood vessel and the tumor region is irradiated with "narrowband" fluorescence photon (FP) beam tuned to the "resonance energies". Theoretically, this technique guarantees a dose 102 - 103 higher than that achieved in conventional radiation therapy (RT). Meanwhile, this high dose is confined to a region of tens of micrometers. This will minimize the side effects caused by the high dose to the surrounding tissues. The FPs are generated by electrons impinging onto target made of the same material as the HZ contrast. In order to support the experiment, an estimation model has been developed based on Geant4 Monte Carlo (MC) simulation. This model takes into account physical and biological factors, which can be determined separately. In this work, the derivation of the model was described in detail, and four HZ atoms, gadolinium (Gd), platinum (Pt), gold (Au) and uranium (U) were evaluated using the model. The scaling law for the capability to yield FPs from IEs had been deduced for these HZ atoms. The results also showed that the minimum molar concentration required for apoptosis of tumor endothelial cells (ECs) for Gd, Pt, Au and U in normal experimental condition were 220.44 nmol/ml, 55.57 nmol/ml, 49.78 nmol/ml and 9.05 nmol/ml, respectively.