In a turbid medium illuminated by a collimated light beam the effect of the attenuation of the coherent field is usually described by assuming a single scattering model. This approximation is justified if we limit to consider weak diffusive media in which the individual scatterers does not differ substantially from the surrounding medium. However in order to replicate the scattering properties of turbid biological media it is important to explore the range corresponding to high volume concentrations of random particles where multiple scattering effects must be taken into account. In this paper we extend the Keller theory for the calculation of the attenuation constant by including third order terms in the perturbation expansion of the stochastic equation governing wave propagation in dense random media. We have derived a new analytical expression for the quadratic and cubic coefficient in concentration as a function of the scatterers diameter and of the light wavelength in the medium. The paper presents numerical calculations of these coefficients for various particles diameters and concentrations.