Crack pinning by tougher heterogeneities is in principle an interesting way to toughen brittle materials. To study the impact of highly heterogeneous toughness field, we investigate numerically the propagation of a tensile penny-shape planar crack within an axisymmetric heterogeneous toughness field. In particular, we take into account the large crack front deformations induced by high toughness contrasts. To compute the variations of stress intensity factor along the crack front arising from its progressive deformation, a perturbation approach based on Bueckner-Rice weight function theory is used iteratively. For low enough toughness contrasts, the crack front deforms until reaching an equilibrium shape for which the local stress intensity factor equals the local toughness value at each point of the front. For larger contrasts, however, this equilibrium shape is never reached. Instead, some points of the crack front remained pinned by strong impurities, while some other part of the front advances continuously. The mechanism at the origin of this fingering instability is finally discussed.