Top-Antitop Pair Production close to Threshold: Synopsis of recent NNLO   Results
  Using non-relativistic effective theories, new next-to-next-to-leading order (NNLO) QCD corrections to the total $t\bar t$ production cross section at the Linear Collider have been calculated recently. In this article the NNLO calculations of several groups are compared and the remaining uncertainties are discussed. The theoretical prospects for an accurate determination of top quark mass parameters are discussed in detail. An outlook on possible future improvements is given. 
A.H. Hoang, M. Beneke, K. Melnikov, T. Nagano, A. Ota, A.A. Penin,   A.A. Pivovarov, A. Signer, V.A. Smirnov, Y. Sumino, T. ...