Background: Adenoid cystic carcinoma (ACC) is a heterogenous disease with limited therapeutic drug options. Identifying effective therapies and defining aggressive subtypes that might benefit from earlier intervention over surveillance remains clinically pertinent. NOTCH pathway-activated ACC is associated with worse outcomes. Pembrolizumab is approved by the FDA for tumour mutational burden-high (TMB-H) [≥10 mutations/megabase (mut/Mb)] solid tumour using the FoundationOne CDx (F1CDx) assay. We describe the distribution of TMB and characterise its relationship with NOTCH gain-of-function (GoF) mutation and survival in 2 cohorts with locally advanced (LA) or recurrent/metastatic (R/M) ACC. Methods: 124 tumours from the UK NHS (NHS group) and 139 tumours from cBioPortal; MSK metTropism (MSK group) were evaluated as independent cohorts. TMB (mut/Mb) of FFPE tumour DNA was calculated either by F1CDx or its precursor FoundationOne NGS in the NHS group, or MSK-IMPACT NGS in the MSK group. NOTCH1/2 mutations were classed as GoF if predicted to disrupt the NRR/PEST domains. Overall survival (OS) was measured from diagnosis of unresectable LA-R/M ACC. P-values were computed using two-tailed Mann-Whitney U and log-rank tests. Results: Median age at diagnosis of LA-R/M ACC was 58.2 yrs in the NHS group and 53.7 yrs in the MSK group. 69/124 (56%) tumours in the NHS group and 69/139 (50%) in the MSK group were from female patients. In patients in whom the site of biopsy was known, 47/95 (50%) tumour samples in the NHS group and 100/139 (72%) in the MSK group were from metastases. Median TMB was lower in the NHS group than the MSK group (1.26 [IQR 0 – 2.52] vs 1.96 [IQR 0.87 – 3.46] mut/Mb, p = 0.007). TMB was ≥10 mut/Mb in 1/124 (0.8%) in the NHS group and 0/139 in the MSK group. NOTCH1/2 GoF mutation was seen in 16/124 (13%) in the NHS group and 22/139 (16%) in the MSK group. Median TMB for tumours with NOTCH1/2 GoF mutation was 2.52 mut/Mb (IQR 1.82 – 3.84) in the NHS group compared with 4.38 mut/Mb (IQR 3.33 – 4.89) in the MSK group (p = 0.03). For tumours with NOTCH1/2 GoF mutation in the NHS group, median OS reduced with TMB > median of 2.52 mut/Mb (0.7 v 2.6 yrs, p = 0.02). For the MSK subgroup with NOTCH1/2 GoF mutation, median OS was 1.6 yrs with TMB > median of 4.38 mut/Mb but 2.4 yrs with TMB less than this (p = 0.7). Conclusions: LA-R/M ACC has a low TMB profile overall; only 1 TMB-H tumour was detected using F1CDx or FoundationOne NGS, underlining the lack of efficacy of immunotherapy seen in ACC. Median TMB was higher in NOTCH-activated ACC in both the NHS and MSK groups. Differences in inter-group biology, site of disease biopsied and assay type may account for the variation in TMB between the 2 groups. Higher TMB significantly correlated with reduced survival in tumours with NOTCH1/2 GoF mutation in the NHS group. TMB may have value in further stratifying patients with LA-R/M ACC to guide initiation of drug therapies.