Generation of dose-volume histograms using Monte Carlo simulations on a multicellular model in radionuclide therapy.

An accurate calculation of the absorbed dose at the cellular level can lead to the optimization of the administered activity and the best clinical response in radionuclide therapy. This paper describes the implementation of dose-volume histograms (DVHs) for dosimetry at the cellular level in radionuclide therapy. The FOTELP code, based on Monte Carlo simulations of photon and electron transport, was used on a three-dimensional multicellular tumor model, which includes tumor morphometry and cell-labeling parameters. Differential and cumulated DVHs were generated for different radionuclides (Cu-67, I-131, Sm-153, Y-90, and Re-188) and labeled cell densities (10, 20, 40, 80, and 100%). DVHs were generated as a percentage of tumor cells in the function of a relative absorbed dose, defined as a cell-absorbed dose divided by an average tumor-absorbed dose. DVHs for high-energy beta emitters, such as Re-188 and Y- 90, were very close to the average tumor-absorbed dose. For low-energy beta emitters, such as Cu-67 and I-131, spectra showed that many cells absorbed a much lower dose than the average tumor-absorbed dose. Nonhomogeneity of the radionuclide distribution in tumor, presented by labeled cell density, had a greater influence on DVHs for low-energy beta emitters. Radionuclide therapy plans can be optimized using DVHs.