The reported energy densities at the tube surface in the canine experiments (Figs. 9 and 10) exceeded the safety limit for skin (i.e., with a 1064 nm wavelength46), but the main safety concern should be the energy density at the first laser-tissue contact interface (i.e., the urethra), which is likely lower than the reported values. Assumptions to achieve more relevant estimates include negligible energy loss between the quartz tube and catheter, negligible spacing between the fiber and catheter (and, therefore, negligible light diffraction in the urine), no deformation of the cylindrical catheter, and similar light refraction in the catheter and quartz tube given the similar refractive indices of their constituents (i.e., silicone and silicon dioxide, respectively48,49). Based on these assumptions and Eq. (4), the energy density incident on the urethra with a 0.49 NA, 1 mm core diameter fiber, and 5.3 mm tube diameter (i.e., the outer diameter of the catheter) is 88 to , which is within the limit for skin exposure. In addition, Fig. 4(b) shows that the energies used in the canine experiments would have been of the safety limit if the fiber diameter were 1.5 mm instead of 1 mm. Moreover, the prostate and urethra could potentially withstand higher energy densities than the existing limit for skin.11 The derived equations enable nonempirical surface area estimates that can be used to further explore safety limits and photon transport with Monte Carlo simulations.