Objective:

To report a rare case of mucinous adenocarcinoma with neuroendocrine differentiation and a HER2 R678Q mutation in the context of cancer of unknown primary.

Approach:

Key Findings:

• The patient exhibited early resistance to FOLFOX chemotherapy and trastuzumab, leading to disease progression.
• This case represents the first documented instance of HER2 R678Q mutation in mucinous adenocarcinoma of unknown primary origin.

Interpretation:

The findings highlight the need for comprehensive molecular testing in cancer of unknown primary to identify rare mutations that may influence treatment strategies.

Limitations:

• Limited understanding of the biological behavior and therapeutic responsiveness of HER2-mutant mucinous adenocarcinomas.
• The rarity of the case limits generalizability of findings.

Conclusion:

The case underscores the importance of integrating broad molecular testing into CUP diagnosis to uncover rare mutations with potential therapeutic implications.

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