Clinical and biological heterogeneity of Grade 2 digestive neuroendocrine neoplasms: prognostic significance of the 10% Ki-67 index cutoff and implications for treatment strategies. A longitudinal study.

Background: Digestive neuroendocrine neoplasms (NENs) encompass a heterogeneous group of tumors with varying prognoses and clinical behaviors. Grade 2 (G2) tumors, defined by a Ki-67 index between 3% and 20%, are particularly challenging to manage due to their intermediate and variable biological behavior. Evidence suggests a distinct prognosis between G2 digestive NENs with a Ki-67 index < 10% and those with a Ki-67 index ≥ 10%.

Aim: To investigate the clinical and biological heterogeneity between Grade 1 (G1) and G2 digestive NENs, and within G2 tumors, with a focus on the prognostic significance of a 10% Ki-67 index cutoff.

Methods: This study involved a combined retrospective and prospective analysis of patients with low-grade G1 and G2 digestive NENs managed at IRCCS San Gerardo Hospital in Monza, Italy, between January 2000 and May 2024. Data on patient demographics, tumor characteristics, treatment modalities, and survival outcomes were collected and potential differences were analyzed between G1, G2 with Ki-67 index < 10% and G2 with Ki-67 index ≥ 10%.

Results: Out of a total of 113 enrolled patients, 69 (61%) had G1 tumors, and 44 (39%) had G2 tumors. Median tumor size at diagnosis was 19 mm (IQR: 12-25 mm), with primary lesions mainly localized in the pancreas (57% among G1 and 45% among G2). Most G1 tumors were diagnosed at stage I (29 patients, 42%), while the majority of G2 tumors were metastatic at diagnosis (24 patients, 54.5%). Patients with G1 tumors exhibited a slightly higher 5-year OS rate compared to G2 tumors (98.1% vs. 92.8% respectively, though not statistically significant), and a significantly longer median PFS (141 vs. 22 months, p = 0.0003). Within the G2 group, 31 patients (70%) had a Ki-67 index < 10%, while 13 (30%) had a Ki-67 index ≥ 10%, with comparable baseline characteristics. A Ki-67 index < 10% was associated with a significantly better median PFS (38 vs. 8 months for tumors with Ki-67 index ≥ 10% G2 tumors, p = 0.002). PFS after first-line medical therapy was significantly longer in patients with a Ki-67 index < 10%, compared to those with ≥ 10% (undefined vs. 16 months, p = 0.0085), as well as median post-surgical PFS (84 vs. 10.5 months, p < 0.0001). Multivariate analysis identified higher tumor grade, advanced stage at diagnosis, and absence of PRRT as independent predictors of worse outcomes.

Conclusions: The findings highlight the significant clinical heterogeneity within G2 digestive NENs. A Ki-67 index cutoff of 10% within G2 tumors may serve as a critical prognostic marker, with patients with a Ki-67 index < 10% exhibiting significantly better outcomes in terms of PFS. These results suggest that the Ki-67 index could play an essential role in guiding treatment strategies, emphasizing the need for personalized approaches in managing G2 digestive NENs.