AB017. SOH22ABS100. Post-treatment surveillance for low-grade appendiceal mucinous neoplasms (LAMN): results from a national surveillance programme

Background: Low-grade appendiceal mucinous neoplasm (LAMN) is an unusual tumour that can progress to pseudomyxoma peritonei (PMP). There is a lack of standardisation of surveillance for LAMN as the progression rate is unclear. The purpose of this study was to evaluate the rate of progression of LAMN to PMP in a national surveillance programme.

Methods: Data for all patients referred for LAMN surveillance from 2014 to 2020 was retrospectively collected. The surveillance regime consisted of annual computed tomography and tumour markers for a five-year period. Patients that progressed to PMP were identified.

Results: Of 65 patients with LAMN enrolled in the programme, 54 had completed at least one year of surveillance (66.7% female, median age 49 years). Of these, 11.1% (n=6) progressed to PMP. The median time to progression was 24 months (range, 13–24 months). In those that progressed, resection margins were positive in 33.3% (n=2), T staging was T4a in 83.3% (n=5), tumour markers were elevated in 50% (n=3). All remaining patients were disease free at a median follow-up of 36 months (range, 12–84 months). In those that did not progress, 1 patient (2.1%) had positive margins (P<0.001), 2 (4.2%) had elevated tumour markers (P<0.001) and 50% (n=24) had T4a disease (P=0.06).

Conclusions: Our findings suggest that, while LAMN has a relatively low risk of progression to PMP, the current surveillance regime is justified, with progression being highest within the first two years of diagnosis. Positive resection margins and elevated tumour markers appear to confer a higher progression risk.

Keywords: Appendiceal tumour; low-grade appendiceal mucinous neoplasm (LAMN); peritoneal malignancy; pseudomyxoma peritonei (PMP)