153P Mutational Landscape of Gastric Cancer (GC) in Adolescents and Young Adults (AYA) in Asia from 2015-2019

Gastric cancer (GC) is an aggressive cancer and it is thought that they have a worse prognosis in AYA due to delayed diagnosis and a more aggressive tumour biology. There is an increasing trend to adopt next-generation sequence (NGS) to identify tumour genomic alterations for precision medicine in AYA. We aim to study the differences in the mutational landscape of GC in AYA and how it correlates to outcomes in a single-centre high-volume Asian institution. Patients diagnosed between 16-39 years old with GC between 2015 to 2019 in our centre, National Cancer Centre Singapore, were included in this study. A total of 50 AYA GC patients were referred to NCCS between 1st January 2015 and 31st December 2019. 7 patients were excluded from this analysis (3 due to incomplete data and 4 were neuroendocrine tumours). The median age (n=43) was 35(22-39) with 22 males (51.2%) and majority (53.5%, n=23) having signet ring cell carcinoma. 28 patients (65.1%) had metastatic disease at diagnosis. Out of the 43 patients, only 60.5%(n=26) and 58.1%(n=25) were tested for programmed death-ligand 1(PD-L1) and DNA mismatch repair (MMR) respectively. 1 patient (3.8%) had MLH 1 loss and 1 patient (4.5%) had PD-L1 percentage of more than 50% by using combined positive score (CPS). 32 patients (74.4%) had the human epidermal growth factor receptor 2(HER2) tested, of which 5 were HER2 positive (15.6%). With a median follow-up of 11 months, median OS was 16.0 months (range 0.5- 111.3 months). There was no significant differences between subgroups with regards to age(p=0.931), presence of signet-ring histology (p=0.135), histology grade (p=0.384) and HER2 positivity (p=0.66). Gastric cancer in the young behaves differently and generally seem more aggressive in the young regardless of gender. Further studies to evaluate how the differences in the mutational landscape of gastric cancer in the adolescents and young adults impact prognosis and treatment paradigm.