We present an approximated analytical model for calculating the attenuation features of the wavefront power density, or SAR, for linear radiators inside a coaxial cylindrical cavity in both non-lossy and lossy tissues. The results are evidencing the determinant role of the cavity radius in affecting the SAR radial decay and the associated penetration depth. A further explicit finding is that the upper limit for the penetration depth in endocavitary radiative heating is equal to the cavity radius, a limit of general validity which holds in both lossy and non-lossy media for any radius value, and is not affected by the approximated nature of the model. Thus, a simple exponential equation allows a straightforward predictive evaluation of both the penetration depth intrinsic upper limit and the approximate penetration depth values, with only the knowledge of the cavity radius and the operating frequency required, without the need to refer to time-consuming electromagnetic field calculations.