Purpose: The purpose of the present study was to investigate if there were differences between U715 spheroids and single cells in the radiotoxic effect of 67Ga on cell growth and clonogenic capacity in vitro and to generate dosimetric approaches for the multicellular tumor model.
Methods and materials: Human lymphoma U715 cells were cultured in vitro as single cells and multicellular spheroids, grown with the use of a combination of fibrin clot technique, spinner flasks, and liquid-overlay culture. Cells were incubated with 2.96-8.88 MBq/ml 67Gallium for 4 days. Spheroids were dispersed to single cells by treatment with plasmin. Residual proliferative and clonogenic capacity after 67Ga incubation were assayed using the MTT-test and clonogenic test, respectively. Autoradiography was performed with 1 microm sections and Ilford L4 liquid photographic emulsion. Dosimetric approaches were made, based on the MIRD-approach.
Results: During 67Ga incubation proliferation was inhibited. The residual proliferative or clonogenic capacity was inhibited by 8.88 MBq/ml for 39 and 88%, respectively. For single cells with 6.66 MBq/ml these inhibitions were 64 and 96%, respectively. Autoradiography showed an homogeneous distribution of 67Ga in spheroids and single cells. In single cells a 2.1-3.5 times higher 67Ga uptake/cell than in spheroids produced an equitoxic effect. The uptake parameters were implemented in new dosimetric approaches and showed that the efficacy of intracellular 67Ga was two times higher in spheroid clusters than in single cells due to energy deposition of internal conversion electrons within the cell clusters with a mean diameter size of nine cells. Both for proliferative and clonogenic capacity the exponential survival curves were superimposed.
Conclusions: With the new approaches made in our dosimetric model the discrepancy found between 67Ga accumulation and radiotoxic effect in spheroids as compared to single cells can be explained by additional effects of the crossfire of internal conversion electrons within clusters of about nine cells in diameter in spheroids. Only twice as much 67Ga was needed to reach equitoxic absorbed doses in spheroids than was needed in single cells. Such might be important for the use of 67Ga treatment of small metastasis of malignant lymphoma.