Researchers: P. Välimäki^{*}, J. Lampinen^{*},
S. Ilvonen^{+}, A. Kuronen, S. Savolainen^{*} and
J. Stepanek

^{*}Helsinki University Central Hospital, Finland

^{+}Electromagnetics Laboratory, HUT

University of Zurich and Paul Scherrer Institute, Switzerland

New promising methods in cancer treatment like radioimmuno and neutron capture therapies provide new challenges to dosimetry. Contrary to the therapies exploiting external beam, radiation distributions in these new methods are generally highly non-uniform. Since there exists at the moment no means to measure the doses absorbed by the patients from internally deposited radionuclides, the only way to proceed is to develop theoretical methods to estimate these doses. One remarkable, but mostly disregarded factor in analytical microdosimetry is the cell cluster model to which the dosimetric calculation itself is applied.

We have developed a method based on mechanical hard-sphere collisions to build densely packed cell cluster models consisting of a tumor surrounded by healthy cells. The results show that the interface between the tumor cells and healthy cells is more densely packed by the optimization method as compared to the conventional model of cubic-close packed structure. Assuming that the activity is concentrated in the tumor cells and that the radiation components of the activity have short ranges - as is often the case in radioimmuno and neutron capture therapies - the calculated absorbed dose of the healthy tissue depends strongly on the cell model used in the calculations.

Figure 39:
Cell models used in cell-level dosimetry. On the left: Cross section
of a cell cluster based on the optimization model. Red spheres are
the smaller tumor cells and green spheres denote the larger healthy
cell surrounding the tumor. On the right: The number of cells N
inside a radius R are plotted for different cell cluster models.
The tumour radius was set to 75 m.
Results of the optimization model are compared with the simple
close-packed cubic geometry and
with the results of the CellPacker program where cells are piled in
a systematic way by stepping incrementally away from the origin. By
using the optimization model the interface between the tumor and the
healthy tissue ( m) is more densely packed. |