In evaluating the Spencer-Attix cavity integral careful consideration must be given to the dissipation of energy by the track ends. The authors have shown that the expressions for the Spencer-Attix integral given by Nahum (1978) and ICRU (1984) are entirely consistent with the original Bragg-Gray formulation for the extreme case of Delta =E₀/2. They have also shown that an alternative definition of delta -rays results in a simplified way of writing the cavity integral.