Objective <p>To identify the predicting factors that are associated with portal venous system invasion (PVSI) from pancreatic ductal adenocarcinoma (PDAC) after neoadjuvant therapy (NAT) by comparing the degree of tumor contact on preoperative CT images.</p> Methods <p>This retrospective study included patients with PDAC who underwent pancreatic CT before and after NAT, as well as subsequent pancreatectomy, between April 2011 and January 2023. Two radiologists reviewed axial and coronal CT images that were obtained before and after NAT to determine the degree of tumor contact with the portal venous system (portal, superior mesenteric, and splenic veins), whether no contact, ≤ 180°, or &gt; 180° (contact category), as well as the presence or absence of the contour deformity (deformity category) in consensus. We used the logistic regression analysis to identify the predicting factors for PVSI and receiver-operating-characteristic (ROC) analysis to assess the diagnostic performance for predicting PVSI.</p> Results <p>This study evaluated 44 patients (median age, 72 years; 27 men) and pathologically confirmed PVSI in 16 (36%) patients. The logistic regression analysis revealed that pathological tumor size, pathological N stage, AJCC staging, tumor size that was measured on CT image after NAT, and deformity category after NAT were associated with PVSI (for all categories, <i>P</i> &lt;.05). The area under the ROC curve was 0.88 (95% confidence interval: 0.75–0.96) in the deformity category after NAT, and this value was the highest among other factors (0.77–0.84).</p> Conclusions <p>The presence of contour deformity after NAT was strongly associated with PVSI and had the highest diagnostic performance for predicting PVSI.</p>