Accurate 3D Dosimetry for Internal Radiotherapy By Considering the Effect of Nonuniform Activity Distribution

Introduction: Many methods like Dose Point Kernels and MIRD calculations, have been evolved for improving the accuracy of dosimetry in the context of 131I radionuclide therapy. However studies have not shown a strong relationship between tumor absorbed dose and response, due to inaccuracies in activity and dose estimation. Methods: GATE Monte Carlo code was used in this study, because it simply takes into account the accurate inhomogeneities in organs and activity distribution and distant energy deposition in a voxel based dosimetry approach. The activity of all organs and tumors were derived, using data from thyroid cancer patients treated with radioiodine. The realistic human phantom of Zubal with 56 segmented tissues was used. Results: The tumor activity was considered constant but with uniform and nonuniform distributions in different simulation setups and its calculated mean absorbed dose was respectively 0.65E-5 and 0.61E-5 (mGy/Mq.s) which does not vary considerably. However the Dose volume histograms shown that the tumor nonuniform activity distribution decreases the effective dose to most parts of the tumor volume by a factor of 3. Conclusion: In this case it can be misleading to quote the mean or maximum absorbed dose, because overall response is likely to be dependent on the extent of the volume that receives a low absorbed dose. So the more comprehensive approach is to consider the activity and dose distribution throughout the tumor to improve the response assessment and treatment planning for radionuclide therapy.