In this talk I will present how the idea of energy-peak invariance, already applied at the jet-energy level, can be applied on the decay-length spectrum of B-hadrons produced at the LHC from top quarks to measure the top-quark mass without uncertainties from jet energy scale calibration and without any significant sensitivity to the precise dynamics governing top production at the LHC. These two uncertainties have played a major role in top-quark mass extraction so far, thus this method opens the way to new measurements with reduced uncertainty. I will also discuss in detail the possible theory uncertainties that affect this strategy and the necessary level of control on hadronization dynamics that is going to be needed to carry out this measurement.