Apparatus for performing a medical treatment, comprising: an invasive probe, which is configured to apply energy to ablate myocardial tissue at a plurality of sites in a heart in a body of a patient to cut off an arrhythmic current path, by creating lesions in the tissue including at least first and second lesions at respective first and second, mutually-adjacent sites; and a processor, which is coupled to the probe and is configured to: record location coordinates and respective treatment parameters at each of the sites with respect to the applied energy, compute, based on the recorded treatment parameters, respective measures of size of the lesions, including at least respective first and second measures of the first and second lesions, generate an indication of extent of overlap between at least the first and second lesions responsively to the first and second measures and to a distance between the location coordinates of the first and second sites, compute the indication while the probe applies the energy, and characterised in that the processor is further configured to control application of the energy responsively to the indication by, after having created the first lesion, continuing to apply the energy at the second site until the computed indication is within a predefined target range. an invasive probe, which is configured to apply energy to ablate myocardial tissue at a plurality of sites in a heart in a body of a patient to cut off an arrhythmic current path, by creating lesions in the tissue including at least first and second lesions at respective first and second, mutually-adjacent sites; and a processor, which is coupled to the probe and is configured to: record location coordinates and respective treatment parameters at each of the sites with respect to the applied energy, compute, based on the recorded treatment parameters, respective measures of size of the lesions, including at least respective first and second measures of the first and second lesions, generate an indication of extent of overlap between at least the first and second lesions responsively to the first and second measures and to a distance between the location coordinates of the first and second sites, compute the indication while the probe applies the energy, and characterised in that the processor is further configured to control application of the energy responsively to the indication by, after having created the first lesion, continuing to apply the energy at the second site until the computed indication is within a predefined target range. record location coordinates and respective treatment parameters at each of the sites with respect to the applied energy, compute, based on the recorded treatment parameters, respective measures of size of the lesions, including at least respective first and second measures of the first and second lesions, generate an indication of extent of overlap between at least the first and second lesions responsively to the first and second measures and to a distance between the location coordinates of the first and second sites, compute the indication while the probe applies the energy, and characterised in that the processor is further configured to control application of the energy responsively to the indication by, after having created the first lesion, continuing to apply the energy at the second site until the computed indication is within a predefined target range.