Cancer of unknown primary site (CUP) often presents with atypical clinical behaviours that make identifying the primary tumour challenging. In this paper, we discuss the case of a patient with brain metastases from a CUP, which was ultimately diagnosed as pleomorphic carcinoma of the lung following autopsy. Clinically, the development of peritoneal carcinomatosis had reduced suspicion of a primary lung neoplasm, despite imaging tests initially suggesting this. However, the patient had previously undergone lumboperitoneal (LP) shunt insertion. The sequence of clinical events and autopsy findings strongly indicated that the LP shunt facilitated the metastatic spread. In addition to this case report, we provide a systematic review of the literature on the role of ventriculoperitoneal (VP) and LP shunts in the metastatic dissemination of tumours. Although the spread of primary brain tumours via VP or LP shunts is rare, albeit well-documented, this phenomenon has been reported only twice for secondary tumours.