According to the 8th edition of the American Joint Committee on Cancer (AJCC) and Union for International Cancer Control (UICC) Cancer Staging, histopathological stage I pancreatic cancer (PC) is defined, based on a histopathological assessment of a resected specimen, as a tumor ≤40 mm in diameter without pathological lymph nodal (pN0)/major arterial involvement and the absence of clinically evident distant metastasis (M0) [1]. [2], Histopathological stage I PC is further subdivided into two categories: Stage IA (pT1: ≤20 mm maximum diameter, pN0) and Stage IB (pT2: 20 mm–40 mm maximum diameter, pN0) [1,2]. Generally, histopathological stage I PC is rare owing to difficulty in early detection (IA, 7.7 %; IB, 9.4 % among resected PCs) and is considered a relatively favorable status from a prognostic standpoint [3]. OS rates in relation to histopathological stage IA and IB have been reported to be more favorable than those with more highly advanced disease, with 5-year OS rates of 39.2 %, 33.9 %, 27.6 %, 21.0 %, and 10.8 % in patients with Stage IA, IB, IIA, IIB, and III, respectively [3].

The recent emergence of a neoadjuvant treatment (NAT) strategy for PC has increased the number of patients who undergo surgical resection after chemotherapy and/or chemoradiotherapy for whom histopathological assessment is conducted under posttreatment conditions [4,5]. Posttreatment histopathological stage I is separately categorized as ypStage I as a counterpart of histopathological stage I without NAT (pStage I) [6,7]. Histopathological stage I disease consists of four distinct subcategories, namely, pStage IA, pStage IB, ypStage IA, and ypStage IB, based on the presence or absence of NAT, and stratified according to tumor diameter, with a cut-off value of 20 mm. NAT has remarkably effect on the histopathological features of the resected specimen as well as on oncological outcomes [[8], [9], [10]] and a significant association has been reported between tumor size and oncological outcomes [11]. The four subcategories included in histopathological stage I disease have widely ranging oncological backgrounds; therefore, the prognostic significance of histopathological stage I may vary among the four subcategories. While limited information is available regarding this issue, clarifying the oncological inhomogeneity among histopathological stage I disease is important for a deeper understanding of the underlying pathophysiology to improve clinical care in the field of PC. Therefore, this study aimed to investigate the surgical outcomes of histopathological stage I PC, highlighting oncological differences between the statuses in pStage I and ypStage I.