Graphic representation of the relation between the SD^rel of the specific energy distribution and the target volume.

The relations were plotted with Eq 3, using k₁ and k₂ calculated by [15] for ⁶⁰Co. The relations for doses of 0.5, 1 and 2 Gy are represented by light grey, grey and dark grey curves respectively. A target volume of around 1.6 μm³ was calculated to explain the totality of the SD^rel of the RIF distribution by the spread of energy specific distribution, regardless of the dose considered.