This is a multicenter retrospective cohort study designed to compare the efficacy differences between two treatment sequence-"first-line surufatinib and second-line somatostatin analogs (SSA)" versus "first-line SSA and second-line surufatinib"-in patients with advanced neuroendocrine tumors (NETs). The primary endpoint is progression-free survival (PFS) from the initiation of first-line therapy to progression on second-line treatment. Secondary endpoints include PFS for each individual line of therapy, safety profiles, and exploration of influencing factors. This study aims to identify the optimal treatment sequence and to provide real-world evidence for optimizing individualized treatment strategies for patients with advanced NETs, thereby informing clinical decision-making in routine practice.

Inclusion Criteria:

• Histopathologically confirmed locally advanced unresectable, metastatic, or postoperative recurrent neuroendocrine tumors (NETs), any primary site.
• Grade G1 or G2 based on Ki-67 index and/or mitotic count.
• Somatostatin receptor 2 (SSTR2) positive confirmed by immunohistochemistry (IHC) or somatostatin receptor imaging (e.g., 68Ga-PET/CT).
• Received one of the following two sequential treatment patterns:

Cohort A (Surufatinib → SSA): First-line surufatinib monotherapy followed by second-line long-acting somatostatin analog (SSA; lanreotide or octreotide long-acting release formulation) after disease progression.

Cohort B (SSA → Surufatinib): First-line long-acting SSA monotherapy followed by second-line surufatinib after disease progression.

• Each line of treatment duration at least 1 cycle (surufatinib ≥4 weeks; SSA ≥1 injection).
• Complete baseline clinical data, treatment start/end dates, and serial imaging evaluation reports available to determine progression-free survival for each line.
• Age ≥18 years at first-line treatment initiation.
• Permitted concomitant treatments during study drug administration (not considered as exclusion criteria):

Best supportive care (e.g., antidiarrheals, analgesics, hepatoprotective agents, symptomatic treatment for hormone secretion).

Local palliative interventions for focal lesions (e.g., transarterial embolization/chemoembolization, ablation for liver metastases) or cytoreductive surgery, provided they do not interrupt systemic study treatment or violate protocol.

For functional NETs, short-acting somatostatin analogs as rescue therapy for symptom control (frequency and dose to be recorded).

Exclusion Criteria:

• Received any combined systemic therapy (e.g., chemotherapy, other targeted therapy, immunotherapy) in addition to surufatinib or SSA during first-line or second-line treatment.
• Missing key baseline data (e.g., pathological grade, primary tumor site) or incomplete treatment/follow-up imaging data precluding accurate assessment of progression-free survival.
• Presence of active, uncontrolled serious infection or another active malignancy at the start of first-line treatment that may interfere with assessment of neuroendocrine tumor progression or patient survival prognosis (except cured skin basal cell carcinoma or carcinoma in situ).

Patients with histopathologically confirmed advanced G1/G2, SSTR2-positive NETs who received either first-line surufatinib followed by second-line SSA (Cohort A) or first-line SSA followed by second-line surufatinib (Cohort B). Patients with incomplete data for PFS assessment or who received combined systemic therapy during first-line or second-line treatment are excluded. Approximately 500 patients will be enrolled from six tertiary hospitals in China.