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Multicellular tumor spheroids are scaffold-free spherical self-assembled aggregates of cancer cells. It is a 3 dimensional culture model which closely models oxygen gradients in small avascular tumors. They are cellular model used in cancer research to assess drug response.
Kelm, Jens M.; Timmins, Nicholas E.; Brown, Catherine J.; Fussenegger, Martin; Nielsen, Lars K. (20 July 2003). "Method for generation of homogeneous multicellular tumor spheroids applicable to a wide variety of cell types". Biotechnology and Bioengineering. 83 (2): 173–180. doi:10.1002/bit.10655. PMID12768623.