Fig. 1. Tumour cell irradiation: radiotherapy versus radiopharmaceutical therapy.

a | An external beam delivers the same absorbed dose per cell regardless of the number of cells. b | In radiopharmaceutical therapy, the absorbed dose delivered per cell by emissions originating from cells is influenced by the range of the emissions, the number of cells that are clustered together and the number of cells that have been targeted. A single cell is very difficult to sterilize with radiopharmaceutical therapy. If the range of the emitted particle is much longer than the dimension of the cell nucleus, a smaller fraction of the total emitted energy will be absorbed in the nucleus.