We found that the peripheral RE on CT of metastatic hepatic tumors is associated with the angiogenesis in tumor micro-environment and may predict a good chemotherapy response. The combination of liver resection with chemotherapy improved the survival of patients who had multiple hepatic metastases. There are several reports on the concept of peripheral RE on CT of CRC metastasis 20, 21). CT-based morphological criteria, including peripheral rim of enhancement of the hepatic metastatic tumor, was reported to have a strong association with the pathological response and survival. These investigations support our study.
Angiogenesis is associated with tumor aggressiveness and poorer prognosis in patients with hepatic tumors22,23). Tumor angiogenesis facilitates metastatic formation by providing mechanisms to increase the likelihood of tumor cells invading the blood circulation, and provides nutrients for tumor growth and survival at the metastatic site. The interaction of tumor cells with endothelial cells in tumor micro-environment has an essential role in tumor angiogenesis. Blood nutrient supply and tumor-related endothelial cells promote tumor cell proliferation and tumor growth24). The tumor micro-environment is essential for the formation of a newly metastatic lesion. Tumor cells and host-cells, such as endothelial cells or fibroblast, participate in tumor metastasis. Tumors that are not vascularized at the metastatic site are typically maintained as small dormant nodules, and the tumor volume remains constant because of a balance between cell proliferation and cell death. Thus, tumor growth is dependent on angiogenesis.
We assessed clinical angiogenesis of metastatic hepatic tumors using indirect imaging by enhanced CT. The clinical manifestation of peripheral RE on CT of the metastatic hepatic tumor was confirmed to correspond with tumor angiogenesis on abdominal angiography (Fig. 1). This was supported by the investigation of the correlation between angiographically assessed vascularity and blood flow in hepatic metastases from colorectal carcinoma25). Of note, the hemodynamics of contrast medium between abdominal angiography and enhanced CT images were different. Angiography images were taken in the direct celiac arterial phase, whereas enhanced CT images were taken in the indirect portal phase through the intravenous injection of contrast medium. However, there was a possibility that tumor staining on angiography and peripheral RE on CT was the same because both images may reflect newly formed angiogenic vessels.
There are several methods to monitor angiogenesis using conventional imaging such as contrast-enhanced US (ultrasound), CT, and MRI (magnetic resonance imaging). Enhanced CT is frequently used in the clinical setting, and can readily access metastatic hepatic tumors and surrounding tissues. Contrast-enhanced CT is useful to evaluate tumor angiogenesis by immunohistochemical quantification of the MVD in colorectal adenocarcinoma patients26). Evaluation of angiogenic vessels by the MVD is associated with microscopic tumor angiogenesis18). We found a strong relationship between microscopic tumor angiogenesis and peripheral metastatic tumor RE on CT.
Hepatic tissue including hepatocytes was fed by blood from the portal vein or hepatic artery, and the blood supply drained via the hepatic vein. Metastatic hepatic tumors may be supplied through angiogenesis via the portal vein or arterial blood flow. Metastatic hepatic tumors were reported to have a dual blood supply from both the portal vein and hepatic artery27,28). In our study, clinical angiogenesis was able to be assessed by not only peripheral RE on portal-phase CT, but also by arterial flow on celiac angiography. This suggested that metastatic hepatic carcinoma was fed from dual blood flow from the portal vein and hepatic artery. Therefore, the clinical manifestation of RE on CT of the metastatic hepatic tumor may be closely associated with tumor angiogenesis.
Angiogenic hepatic metastatic tumors responded well to systemic chemotherapy despite their aggressiveness. Angiogenic tumors may readily uptake anti-cancer drugs to the tumor through newly formed angiogenic vessels. Furthermore, immature angiogenic vessels are fenestrated29). Angiogenic factors, such as VEGF, which was first identified as vascular permeability factor30,31), not only stimulate endothelial cells lining nearby microvessels to proliferate and migrate, but also render these vascular endothelial cells hyperpermeable. Hyperpermeable vessels may readily leak plasma proteins and deliver anti-cancer drugs into the extravascular space.
Recent clinical anti-angiogenic therapies, such as angiogenic antibody, have been used in patients with unresectable metastatic hepatic disease32,33). Anti-angiogenic antibody therapy itself is insufficient for anti-cancer effects. However, a single infusion of anti-VEGF antibody reduced tumor perfusion, vascular volume, microvascular density, interstitial fluid pressure, and circulating endothelial cells in patients with rectal cancer34). This suggests that anti-angiogenic therapy has direct anti-vascular effects on human tumors. A combination of these drugs with anti-cancer drugs produces anti-cancer effects. Anti-angiogenic molecules, such as anti-VEGF antibody, remodel tumor-related endothelial cells into normal endothelial cells with a normal structure35,36). Anti-angiogenic molecules reshape pathologic vasculature into normal vasculature, which results in delivery of the anti-cancer drug to the tumor. Although we analyzed only a few patients using anti-angiogenic agents in this study, there were anti-tumor effects without anti-angiogenic agents. Anti-angiogenic therapy may not be associated with a direct tumor response, but rather maintenance of anti-tumor effects. Normalization of tumor-related vasculature may enable the sustained delivery of anti-cancer drugs.
Hepatic resection remains the only potential curative treatment for metastatic tumors and improves survival 37,38). Similarly, we found that resection of metastatic hepatic tumors improved OS (Fig. 4A, B).
When the groups were divided by hepatic resection, there were no significant differences between patients with RE-positive and -negative tumors with hepatic resection (Fig. 4A). In addition, there were no significant differences between patients with RE-positive and -negative tumors without hepatic resection (Fig. 4B). A higher response rate to systemic chemotherapy was observed in patients with RE-positive tumors, but the OS rate of patients with RE-positive tumors was not significantly different from that of patients with RE-negative tumors (Fig. 4C). This suggests that a higher response to systemic chemotherapy does not always lead to longer survival. After the initial higher response in our patients, additional surgical therapy prolonged survival. To improve survival, additional therapeutic strategies, such as maintenance chemotherapy, use of molecular targeted drugs, or immuno-check point inhibitors, are needed. Clinically, hepatic metastatic tumors can recur or develop other metastatic lesions, such as a lung metastases or peritoneal dissemination, during the follow-up period for RE-positive and -negative tumors, which may affect patient survival.
There were a few limitations in the present study. First, there were patients without clinical manifestations, such as lung metastasis or peritoneal dissemination, during the initial treatment period. Second, the statistical power was weak because the sample size was small. The observational period of 10 years was relatively long, but new molecular drugs, such as anti-VEGF antibody and anti-EGFR antibody, were not frequently used for the initial treatment. Further studies with a larger number of patients and a shorter time period are needed to confirm our results.