Imaging Assessment of Pancreatic Cancer Resectability After Neoadjuvant Therapy: AJR Expert Panel Narrative Review

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Executive Summary

Pancreatic ductal adenocarcinoma (PDAC) remains a highly lethal malignancy, projected to become the second leading cause of cancer-related death by 2030. While upfront surgery followed by adjuvant chemotherapy was long the standard for resectable cases, there is a paradigm shift toward neoadjuvant therapy (NAT)—preoperative chemotherapy with or without radiation. This shift aims to improve the selection of surgical candidates and increase the rate of margin-negative (R0) resections.

The critical challenge identified in this review is the current inability of conventional imaging to reliably distinguish between viable residual tumor and post-therapeutic fibrosis or inflammation. Computed Tomography (CT), the current gold standard, frequently overestimates vascular involvement after NAT, leading to the potential underestimation of resectability. Evidence suggests that even when imaging shows "stable disease" without significant tumor shrinkage, patients may still achieve successful R0 resections. Consequently, surgical decisions should not rely solely on traditional downstaging criteria. Improved accuracy can be achieved through optimized CT protocols, the strategic use of MRI for liver metastasis detection, and the identification of specific imaging markers like the "perivascular halo."

1. Defining Borderline Resectable (BR) PDAC

The classification of PDAC has evolved from a binary (resectable vs. unresectable) system to include a "borderline resectable" (BR) category. This group consists of patients whose tumors involve adjacent vasculature but may become resectable following NAT.

Clinical Definitions and Guidelines

While multiple organizations provide criteria for BR disease, there are minor but significant differences in their restrictive nature:

The expert panel recommends the NCCN definition because it allows for a more comprehensive cohort of patients to potentially undergo surgical resection with vascular reconstruction.

2. The Role and Goals of Neoadjuvant Therapy (NAT)

NAT is increasingly utilized for patients at high risk for positive resection margins (R1). Its primary objectives include:

  • Candidate Selection: Identifying patients with aggressive or metastatic disease that emerges during initial chemotherapy, thereby avoiding futile surgery.

  • Tumor Volume Reduction: Downsizing tumors to increase the probability of R0 (microscopically margin-negative) resection.

  • Nodal and Micrometastatic Treatment: Reducing the rate of positive lymph nodes and treating occult micrometastases early.

  • Chemosensitivity Assessment: Using the tumor's response to NAT as a biological indicator of surgical benefit.

3. Limitations of Conventional Imaging

Current imaging techniques, particularly standard MDCT, face significant hurdles in assessing treatment response.

  • Fibrosis vs. Viable Tumor: PDAC is characterized by a dense desmoplastic reaction. After NAT, tumor cells are often replaced by fibrotic tissue, which mimics the appearance of a viable tumor on CT scans.

  • Lack of Downstaging: Radiographic downstaging is rare. In one landmark study, only 0.8% of patients had their disease downstaged to "resectable" on CT, yet 95% of those who underwent surgery achieved R0 resection.

  • Underestimation of Resectability: CT criteria used for initial staging are not valid after NAT, as they often overestimate vascular invasion due to treatment-related tissue changes.

4. Optimization Strategies for Imaging

To overcome the limitations of standard CT, the panel suggests several technical and interpretive improvements.

CT Protocol Optimization

A dedicated pancreatic MDCT protocol is mandatory. Key requirements include:

  • Phase Timing: Pancreatic parenchymal phase (40–50s) and portal venous phase (65–70s).

  • Dual-Energy CT (DECT): DECT improves tumor conspicuity and the assessment of the tumor-vessel interface through virtual monoenergetic imaging and iodine reconstruction.

  • Baseline Timing: Baseline CT should ideally be performed before biliary stent placement to avoid artifacts and inflammatory changes from post-ERCP pancreatitis.

MRI and Hybrid Imaging

  • MRI for Liver Metastasis: MRI is significantly more sensitive than CT for detecting small (< 1 cm) liver metastases (83% vs. 45%). The panel recommends systematic liver MRI for patients with nonmetastatic disease on CT to prevent unnecessary laparotomies.

  • PET/CT: Useful for assessing the biological activity of the treatment response, differentiating PDAC from benign pancreatitis, and identifying extrahepatic metastases. It should be used for "problem-solving" rather than routine staging.

5. Key Imaging Markers of Response

Specific findings on post-NAT imaging can indicate a favorable treatment response even in the absence of significant tumor shrinkage:

  • The "Halo Sign": A thin, low-attenuation rim (≤ 46.4 HU) surrounding a vessel where solid tumor once contacted it. This sign is independently associated with R0 resection.

  • Decrease in Tumor-Vessel Contiguity: Any partial regression of tumor contact with peripancreatic vessels, even if the tumor remains in contact.

  • Venous Burden Index: Evaluation of the circumferential extent and length of tumor-vascular contact, along with the degree of contour deformity. Regression in these parameters is a reliable indicator of response.

  • RECIST 1.1: While not always accurate for PDAC, any reduction in tumor size is generally associated with better surgical outcomes.

6. Surgical and Pathological Challenges

The infiltrative growth pattern of PDAC, characterized by discontinuous tumor cells, persists after NAT. This complicates the pathological assessment of "completeness" of resection.

  • R1 Resection Discrepancies: While R1 rates are reported low (4–30%) after NAT, local recurrence remains high (25–70%), suggesting that completeness of resection cannot be reliably diagnosed through current standard pathology.

  • Vascular Resection: While controversial, venous and arterial resections (including the modified Appleby procedure) are increasingly performed in tertiary centers. The mortality rate for pancreatic resection has decreased to < 4%, though morbidity remains high (> 60%).

7. Recommendations for Radiology Reporting

The panel discourages the reuse of baseline staging (Resectable/BR/LA) in post-NAT reports. Instead, reporting should focus on the change from baseline.

A Suggested Post-NAT Reporting Lexicon:

  • Tumor Size: Reported as same, increased, decreased, or not visible.

  • Arterial/Venous Interface: Focus on whether contact has increased or decreased; specifically note the presence of the halo sign.

  • Impression: Categorize as worsening, stable disease, or improving.

Conclusion

Radiologists and surgeons must recognize that stable imaging findings after NAT often mask a significant pathological response. Surgical exploration should be routinely advised for patients with stable disease or minor radiographic improvement, provided there are no other contraindications.