The Clinical Significance of Occult Gastrointestinal Primary Tumours in Metastatic Cancer: A Population Retrospective Cohort Study

Malek B. Hannouf, Eric Winquist, Salaheddin M. Mahmud, Muriel Brackstone, Sisira Sarma, George Rodrigues, Peter K. Rogan, Jeffrey S. Hoch, Gregory S. Zaric

To estimate the incidence of occult gastrointestinal (GI) primary tumours in patients with metastatic cancer of uncertain primary origin and evaluate their influence on treatments and overall survival (OS). We used population heath data from Manitoba, Canada to identify all patients initially diagnosed with metastatic cancer between 2002 and 2011. We defined patients to have "occult" primary tumour if the primary was found at least 6 months after initial diagnosis. Otherwise, we considered primary tumours as "obvious". We used propensity-score methods to match each patient with occult GI tumour to 4 patients with obvious GI tumour on all known clinicopathologic features. We compared treatments and two-year survival data between the two patient groups and assessed treatment effect on OS using Cox regression adjustment. 83 patients had occult GI primary tumours, accounting for 17.6% of men and 14% of women with metastatic cancer of uncertain primary. A 1:4 matching created a matched group of 332 patients with obvious GI primary tumour. Occult cases compared to the matched group were less likely to receive surgical interventions and targeted biological therapy, and more likely to receive cytotoxic empiric chemotherapeutic agents. Having an occult GI tumour was associated with reduced OS and appeared to be a non-significant independent predictor of OS when adjusting for treatment differences. GI tumours are the most common occult primary tumours in men and the second most common in women. Patients with occult GI primary tumours are potentially being undertreated with available GI site-specific and targeted therapies.