Absence of a demonstrable gain factor for neutron beam therapy of epidermoid carcinoma of the head and neck.

A comparison of normal tissue and tumor responses in patients treated with the high energy Fermilab neutron beam and conventional photons (Cobalt and 4 MeV X rays), yielded the following parameters. For neutrons the median dose for significant radiation injury in the irradiated tissues was 31 (+2) Gy and the median dose for local control of the tumor was 26 (+2) Gy. The corresponding doses for photons were 90 (+4) Gy for normal tissue injury and 74 (+3) Gy for local control of the tumor. These figures show that the therapeutic ratio is roughly 1.2 for both neutrons and photons. Similarly, the RBE of neutrons relative to photons is about the same for normal tissue tolerance and for tumor control. Under these conditions, there is no demonstrable therapeutic gain factor for neutrons relative to photons. The overall local control rate was the same for both modalities (44%).