LAUSR

Background and objectives There is significant interest and potential in the treatment of neuroendocrine tumors via peptide receptor radionuclide therapy (PRRT) using one or both of 90Y and 177Lu-labeled peptides. Given the presence of different tumor sizes in patients and differing radionuclide dose delivery properties, the present study aims to use Monte Carlo simulations to estimate S-values to spherical tumors of various sizes with 90Y and 177Lu separately and in combination. The goal is to determine ratios of 90Y to 177Lu that result in the largest absorbed doses per decay of the radionuclides and the most suitable dose profiles to treat tumors of specific sizes. Material and methods Particle transfer calculations and simulations were performed using the Monte Carlo GATE simulation software. Spherical tumors of different sizes, ranging from 0.5 to 20 mm in radius, were designed. Activities of 177Lu and 90Y, individually and in combination, were homogeneously placed within the total volume of the tumors. We determined the S-values to the tumors, and to the external volume outside of the tumors (cross-dose) which was used to approximate background tissue. The dose profiles were obtained for each of the different tumor sizes, and the uniformity of dose within each tumor was calculated. Results For all tumor sizes, the self-dose and cross-dose per decay from 90Y were higher than that from 177Lu. We observed that 177Lu had the most uniform dose distribution within tumors with radii less than 5 mm. For tumors greater than 5 mm in radius, a ratio of 25% 90Y to 75% 177Lu resulted in the most uniform doses. When the ratio of 177Lu to 90Y was smaller, the uniformity improved more with increasing tumor size. The cross-dose stayed approximately constant for tumors larger than 15 mm for all ratios of 177Lu to 90Y. Finally, as the size of the tumor increased, differences in the S-values between different ratios of 177Lu to 90Y decreased. Conclusion Our work showed that to achieve a more uniform dose distribution within the tumor, 177Lu alone is more effective for small tumors. For medium and large tumors, a ratio of 90Y to 177Lu with more or less 177Lu, respectively, is recommended.