The RBE of the leakage radiation from the Hiletron neutron therapy unit

Residual haemopoietic damage in the mouse after fractionated gamma-irradiation, down to 0.1 Gy per fraction

Residual damage in haemopoietic progenitor cell populations, spleen and granulocyte-macrophage colony-forming cells (CFU-S and GM-CFC) was detected in mice after 15 daily fractions where the dose per fraction was as low as 0.1 Gy. The injury was dose-dependent and after higher total fractionated doses of 7.5-10 Gy the CFU-S population recovered to about 50% of control between 2 and 12 months after irradiation. Residual damage was also detected in the stroma, in the form of reduced numbers of fibroblastoid colony-forming cells and of CFU-S in ossicles under the kidney capsule. The response to a second course of 15 fractions, given 3 weeks after the end of the first course, was similar and additive to the response to the first course in the short term. However, in the long term, recovery levels were similar after either one or two courses.

Survival of cells in mammalian tissues after low doses of irradiation: a short review

A survey of data in the literature indicates that the radiosensitivity of cells to doses less than 1 Gy varies widely within cell lineages and less so between lineages. This is due in large part to the differentiation status and division capacities of the cells, and possibly also to the grouping of cells into 'viable units'. In addition, the mode of cell death is important, and cells susceptible to natural apoptosis are particularly radiosensitive. There are also quite marked differences in cell sensitivity between species.

Dose levels outside radiotherapy beams

The specification of a radiotherapy generator calls for the leakage radiation through the housing of the source to be 0.1% (the ICRP has recently (1982) changed this figure to 0.2% of the dose rate inside the useful beam). When a patient is irradiated, the dose level outside the useful beam is not only determined by this leakage radiation, but also by scattered radiation from the beam-defining system and from the treatment volume of the patient. Measurements have been made on a 300 kV X-ray unit, and on two different 4 MV and one 8 MV linear accelerators to determine the levels of leakage radiation and scattered radiation in air, and in a phantom comparable in size to the trunk of the patient. Similar measurements have also been made for a 15 MeV neutron generator. It is shown that for all these generators the dose delivered outside the radiation beam is determined mainly by the two scattering processes mentioned, and it is argued that for linear accelerators it would be feasible to increase the permitted level of leakage radiation by a factor of 2 or 3 without a significant increase in the stray radiation delivered to the patient.