AB146. SOH24AB_181. Mucinous adenocarcinoma in the axilla of undetermined origin

Background: Cancer of unknown primary (CUP) accounts for 2–5% of all cancer diagnoses. Rarer still is mucinous adenocarcinoma (MAC), accounting for approximately 1% of all cancers. Outside of common sites such as colorectal and breast cancers, our knowledge of MAC depends largely on case reports and cases series. Currently, there is no standard of care for the surgical treatment of MAC of unknown origin.

Methods: Herein, we present a case of a 75-year-old male who, after presenting for an elective inguinal hernia repair, disclosed concerns for a new lesion in his right axilla which was removed at the same time. Investigations revealed a MAC of unclear origin, with immunohistochemistry suggesting either a breast or primary cutaneous mucinous adenocarcinoma (PCMC). The patient was asymptomatic with an otherwise normal examination. Extensive investigations failed to identify any primary source. Axillary node clearance was performed, resulting in 0/9 positive nodes. After multi-disciplinary team (MDT) discussion, this patient (who remains asymptomatic) will be kept under close clinical surveillance, with yearly computerized tomography scans.

Results: This case highlights two possible differential diagnoses that of breast or PCMC. The latter seems most likely, as no other source has been found. This case emphasises the importance of MDT discussion, the standardised approach to CUP, essential use of immunohistochemistry to narrow the search, and the lack of literature in this subject matter.

Conclusions: To our knowledge, no such presentations have been reported in the literature. Thus, our case report is a unique addition of an atypical presentation of MAC with axillary metastases with an unknown primary site.

Keywords: Primary of unknown origin; mucinous adenocarcinoma (MAC); primary cutaneous mucinous adenocarcinoma (PCMC); cancer of unknown primary (CUP); unclear origin