The dose, D, absorbed in cavity exposed to neutron radiation depends on cavity size, d. D(d) may not lie in the region given by the dose in an infinitesimally small cavity, D(0), and that in an infinite one, D(∞). The dose may vary between 0 and D(0)+D(∞). Local extrema in D(d) may appear. The effect depends on the difference between initial energy distributions of the charged particles originating in the wall and those in the gas, and between their stopping powers in gas and wall material. The effect is explained by means of a simple model. A condition for extremum is also given.