ESTRO PT 2018

The 1970s rise and fall of neutron beam therapy

The rise: tumor control

• Relatively small installations - spread of neutron therapy facilities • Demonstration of tumor control in radio-resistant tumors • Salivary gland • Prostate • Pancreas •

Neutron beams produced by protons or deuterons with energies greater than about 50 MeV could produce tumor control with side effects no worse than low LET radiation. For this reason facilities which had performed clinical trials using relatively low energy beams either stopped treating patients or upgraded their accelerators to a higher energy.

The fall: toxicity

• Computations of absorbed dose did not include additional neutron capture in hydrogen-rich tissues, which results in higher energy release in hydrogen-rich tissues. Such tissues include white matter in the brain and the fat that surrounds most important organs, which is closely associated with their blood supply • Neutron therapy using the 2-D techniques of the 1970s irradiating large volumes normal tissue • The well-established finding that RBE varies in different tissues was dismissed, along with the important fact that RBE increases with falling dose/fraction, which mitigates the effect of a reduction in physical dose beyond the region of cancer • The fact that RBE also varies with cell proliferation rate, so that slow-growing cells have higher values, was not appreciated. It is the slow-growing cells that make up the majority of normal tissue and which contribute to severe tissue damage at extended time periods after irradiation

medicalphysicsweb.org/cws/article/opinion/32466 and other sources