Consequences of a Surveillance Strategy for Side-branch Intraductal Pancreatic Mucinous Neoplasms: Long-term Follow-up of One Thousand Cysts.

Objective: To quantify the rate of progression in surveilled cysts and assess what factors should indicate delayed resection.

Background: Side-branch intraductal papillary mucinous neoplasms (SB-IPMNs) are increasingly discovered, making it challenging to identify which patients require resection, thus avoiding inappropriate treatment. Most incidental lesions are surveyed, yet the consequences of that decision remain uncertain.

Methods: A prospectively maintained database of pancreatic cystic neoplasms was queried for patients with SB-IPMN. Patients with ≥2 imaging studies >6 months apart were included. Clinically relevant progression (CR-progression) was defined by symptoms, worrisome/high-risk stigmata, or invasive cancer (IC). Growth ≥5 mm in 2 years is considered CR-progression; size ≥3 cm alone is not.

Results: Between 1997 and 2023, 1337 patients were diagnosed with SB-IPMN. Thirty-seven (2.7%) underwent up-front surgery; 1000 (75.0%) had >6 months of surveillance.The rate of CR-progression was 15.3% (n = 153) based on size increase (n = 63, 6.3%), main-duct involvement (n = 48, 4.8%), symptoms (n = 8, 5.0%), or other criteria (n = 34, 3.4%). At a median follow-up of 6.6 years (interquartile range: 3.0-10.26), 17 patients (1.7%) developed IC. Those with CR-progression developed IC in 11.1% (n = 17) and high-grade dysplasia (HGD) in 6.5% (n = 10). Nearly half of the cancers were not contiguous with the surveyed SB-IPMN.Size ≥3 cm was not associated with HGD/IC ( P = 0.232). HGD/IC was least common in CR-progression determined by size growth (6.3%) versus main-duct involvement (24%) or other (43%, P < 0.001)Patients with CR-progression demonstrated improved survival (overall survival) with resection on time-to-event ( P < 0.001) and multivariate Cox regression (hazard ratio = 0.205, 0.096-0.439, P < 0.001) analyses. Overall survival was not improved with resection in all patients ( P = 0.244).

Conclusions: CR-progression for SB-IPMNs is uncommon, with the development of cancer anywhere in the pancreas being rare. Initial size should not drive resection. Long-term and consistent nonoperative surveillance is warranted, with surgery currently reserved for CR-progression, knowing that the majority of these still harbor low-grade pathology.