The assessment of the results of neutron therapy

The control of tumors and the incidence of complications are major criteria for assessing the effectiveness of neutrons in the treatment of cancer. However, both of these are seriously affected by the neutron machines available at present. These have beams with poor penetration, wide penumbras, fixed field sizes and fixed positions. Undesirably large volumes of skin and normal tissues are therefore irradiated through the multiplicity of beams used and side scatter of radiation. Even machines with high energy neutrons are fixed in position and restricted in the sizes of fields available. Complications are therefore caused by inferior machines and compromised treatment plans independently of the biological effects of neutrons. The incidence of complications has caused some centers to reduce the dose of neutrons to a level where tumor control is not maintained and the recurrence rate is fairly high. The Hammersmith experience shows, however, that by improving treatment techniques and the care of early and late reactions, complications have been reduced from 18% to 5%, while maintaining the standard dose. There is close correlation between the clinical and laboratory data which show biological and physical advantages for neutron therapy. Higher energy machines with isocentric heads, mechanically adjustable collimators and the ability to use shields and wedges concurrently are now available and should be used to extend the investigation of neutrons, especially to tumors of the abdomen and pelvis.