Letter: Re-evaluation of published neutron rbe values for mouse intestine

The effect of neutron dose rate on jejunal crypt survival

The effect of dose rate of fast neutrons over a range from 0.02 Gy/min to 22.5 Gy/min on the survival of mouse jejunal crypts was investigated. A small but significant decrease in sensitivity with decrease in dose rate was observed. A 10-fold decrease in dose rate gave a decrease in effectiveness equivalent to 0.39 Gy. The sensitivity to changes in neutron dose rate is much smaller than the sensitivity to changes in dose rate of photons.

A randomised comparison of photons and 15 MeV neutrons for the treatment of carcinoma of the bladder

108 patients with T2 or T3 carcinoma of the bladder were randomised to receive either photon therapy or low- or high-dose 15 MeV neutrons. Fifty-nine patients received photon treatment, 20 low-dose neutrons (LDN) and 20 high-dose neutrons (HDN). Eight patients received a combination of photon and neutron therapy, due to machine breakdown, and one patient received only one exposure, due to intercurrent illness. The three-year survival for all groups was 43% but there was no difference in survival between the treatment groups. The complication rate in the photon group was low--minor 3%, major 5%. The complication rates in the neutron groups were higher--LDN 30% and 15% respectively, HDN 15% and 10% and combined 25% and 37%. Only six patients died of complications related to therapy--photons 2, LDN 1, HDN 1, combined 2. No therapeutic gain for neutron therapy has been found in this study.

Response of mouse intestine to 14 MeV neutrons

At the Hamburg-Eppendorf Hospital neutron facilities the relative biological effectiveness (r.b.e.) of d,T-neutrons was determined with respect to survival of mouse intestinal crypts. (CBA/Rij x C57BL/Rij)F1 mice were irradiated to the whole body at different depths inside a tissue-equivalent phantom. Irradiations were carried out with a collimated neutron beam at about 6 rad/min given in single doses ranging from 450 to 1000 rad. For reference, gamma-rays from a 60Co therapy unit were used. The number of surviving intestinal crypts per circumference of the jejunum was determined 3 1/2 days after irradiation according to the method of Withers and Elkind. The number of surviving stem cells was calculated on the basis of Poisson statistics. The doses necessary to reduce survival to ten crypt stem cells per circumference amounted to 689 +/- 19 rad for neutrons and 1449 +/- 29 rad for 60Co gamma-rays. From these figures an r.b.e. of 2 . 1 +/- 0 . 1 is obtained. Measurements at different depths in the phantom did not show any variation of r.b.e. with depth along the axis of the neutron beam.

Tolerance of rodent tails to necrosis after "daily" fractionated X rays or D-T neutrons

The rat tail has been used as a model system to study the necrosis of an organized tissue following fractionated doses of collimated D-T neutrons or 290 kV X rays. RBE values for tail tolerance - 10 per cent of tails necrosing after the early skin reactions - rise from about 1-7 (single doses) to about 3-1 (16 fractions in 22 days). Neutron tolerance doses are almost independent of fractionation from 2 to 16 fractions. The tissues at risk are shown to be rather hypoxic. Early skin reaction levels can be used to predict the fraction of tails that will necrose. Early peak reactions for a given fraction of necrotic tails were slightly higher for neutrons than for X rays, and this difference was consistent for all the dose fractionation schedules employed.