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The inherent brittleness of the refractory metal tungsten represents a major challenge for its application as divertor material in future nuclear fusion reactors. Grain refinement to the ultrafine-grained regime is a promising strategy to increase the fracture toughness of W, but it also promotes intercrystalline crack growth. Therefore, the strengthening of grain boundary cohesion in W is of great importance. In this work, grain boundary doping with B and Hf, two elements that were identified in previous work to increase bending strength and ductility, is applied to ultrafine-grained W. The fracture toughness is measured utilizing small-scale testing techniques. Fracture mechanical experiments on the microscale provide a plethora of challenges to correctly assess size-independent toughness values, which are presented and discussed within this work. It was found that the toughness of W can be under- and overestimated …