Vascularization of small liver metastases.

The portal vein contributes little to the blood supply of established liver metastases in the presence of a patent hepatic artery. However, its role in the perfusion of early metastatic liver disease remains controversial. We have quantified the relative contribution of the portal vein and hepatic artery to the internal circulation of small liver tumours. These studies were conducted in a new rat model of liver metastases generated by the intraportal injection of cultured tumour spheroids. A total of 633 lesions were studied in 18 animals using radioisotope tracer microspheres. Nodules ranged from 0.5 to 6.0 mm in diameter, 92 per cent being 3.0 mm or less. The ratio of radioactivity in tumour compared with normal tissue (T:N ratio) was determined after simultaneously injecting microspheres into the portal and arterial circulation of each animal. The mean hepatic artery T:N ratio was significantly greater than that of the portal vein for all tumour sizes studied (P less than 0.01). Nodules 0.5 mm in diameter had a mean (s.e.m.) hepatic artery T:N ratio of 1.50 (0.12) in comparison with a mean portal vein ratio of 0.13 (0.04). Tumour nodules 2.0 mm in diameter had a mean (s.e.m.) hepatic artery T:N ratio of 1.26 (0.11) in comparison with a mean portal vein ratio of 0.06 (0.01). The overall T:N ratio for both routes decreased with increasing tumour size. These results indicate that hepatic tumours as small as 0.5 mm already have an established internal vasculature perfused predominantly by the hepatic artery. In contrast, the portal vein contributes insignificantly to the internal perfusion of these lesions. These results have significant implications for adjuvant locoregional therapy in gastrointestinal cancer.