Small-for-size graft: Not defined solely by being small for size

Abstract

The phrase small-for-size (SFS) graft came into use in clinical liver transplantation in the 1980s. There had already been discussions about graft size mismatching in liver transplantation in the era of whole or split liver transplantation, in which initially right trisegments were used for adults. Interest in the issue flourished when living donor liver transplantation (LDLT) was extended to adult patients. Controversies included the pathogenesis and clinical manifestations of SFS grafts and strategies for successful SFS graft transplantation. On the other hand, the definition of SFS graft has always been ambiguous, and it has been defined tentatively in respective studies by the graft weight relative to the body weight or to the estimated whole liver weight (a value derived from the body size). Furthermore, the phrase SFS syndrome, which is also ambiguous and is often defined arbitrarily, is widely used to define SFS graft (and vice versa). One of the early clinical studies attempting to elucidate the impact of graft size mismatching in LDLT has been frequently cited in later articles on the topic: it has been cited 350 times for 11 years according to the Web of Science. This study used 0.8%, 1.0%, 3.0%, and 5.0% of the recipient body weight as graft size category borders to show the prognostic impact. However, the study design had several pitfalls. Children younger than 15 years composed the vast majority of the study population; older patients represented only 10%. The graft sizes encountered in pediatric LDLT are highly variable, in contrast to adult LDLT, in which the range is relatively narrow (0.5%-1.5% of the body weight). Furthermore, the body weight of patients with end-stage liver disease fluctuates with the occurrence of ascites or edema or the use of diuretics. Even the actual graft weight is potentially influenced by the presence of blood or preservation solution with a high viscosity. Hence, it was uncertain whether the impact of small differences in the graft weight was not masked by the wide range of variables in the recipients, donors, and surgical techniques in adult LDLT. Actually, multiple successive clinical studies of adult LDLT have failed to demonstrate a negative prognostic impact of small differences in the graft weight, although the concept of SFS graft or syndrome has survived. This is partly because most clinical studies are performed in an intent-to-treat fashion; that is, a clinical practice is coupled with appropriate safety measures for graft selection and technical modifications, including outflow maximization and inflow modification. Many confounding factors potentially interfere with the true negative effect of SFS grafts. In this context, Moon et al. should be congratulated for their article. They have shown that older donor age, presumably accompanied by low adaptability of the graft tissue, affects graft prognosis only when it is combined with an SFS graft. Unfortunately, there remains some room for discussion concerning their analytical method. They censored patient death due to pneumonia as unrelated to graft function and included graft failure due to surgical complications or disease recurrence as potential results of graft function. The definition of graft loss