Comparison of the Effect of Absorbed Dose on Recurrence of Tumors in External Beam Radiation and Targeted Therapy

Radiotherapy is the treatment of cancer using ionizing radiation. When a tumor is large in comparison to the range of the ionizing particles, most of the energy is absorbed within the tumor. In cases when the tumor dimensions are smaller than the range of ionizing particles, a large proportion of the energy can escape. Therefore, the tumor size and the range of the ionizing particles are important to be realized in the recurrence probability of tumors. The purpose of this study is to investigate the relationship of recurrence probability to tumor size and paricles energy in external radiotherapy and targeted radionuclide therapy. The absorbed fractions for spheres that ranged in radii from 20 µm to 4.5 cm were calculated using the MCNPX code. A mathematical model based on Poisson distribution was used to investigate the relationship of recurrence probability to tumor size for tumors treated with external beam radiotherapy and targeted 131I and 90Y. The results show that for targeted radionuclide therapy, the relationship between the recurrence probability and tumor size is different from that for external beam radiotherapy. The analysis shows that there is a minimum value of the recurrence probability that occures at a diameter of approximately 3.5 cm for 90Y. For 131I, the minimum recurrence occurs at a tumor diameter of approximately 3.5 mm. The results show that there is an optimal tumor size for the tumor curability. The recurrence probability has a minimum value for tumors whose diameters are close to the optimum value which depends on the particles energy. Smaller tumors are more recurrence because of the incompetent absorption of radiation energy, i.e., it dose not occure with the external beam iiradiation, and larger tumors are more recurrence because of the greater cell number. The results are shown that single agent targeted radiotherapy should not be used for treatment of disseminated cancers when multiple tumors of differing size may be present. The use of several radionuclides, including long-range and short-range beta emitters, concurrently or from combining targeted radiotherapy with external beam irradiation would be more effective than the reliance on a single radionuclide.