TACE
TRANSARTERIAL CHEMOEMBOLIZATION

The therapeutic rationale for TACE rests on a fundamental biological difference between normal liver tissue and HCC — a difference in blood supply that creates the opportunity to selectively starve tumour tissue while largely preserving the surrounding hepatocytes. Understanding this vascular biology explains why TACE works, why it works specifically for HCC (and not for most other liver tumours), and why the degree of superselective catheter placement determines treatment outcome.

The Dual Blood Supply of the Liver

The liver is unique among abdominal organs in receiving blood from two separate vascular systems — the hepatic artery (a branch of the coeliac axis, delivering oxygenated blood from the aorta) and the portal vein (carrying nutrient-rich blood from the intestines, spleen, and pancreas). In a healthy liver, approximately 70–75 percent of hepatic blood flow arrives via the portal vein, and only 25–30 percent through the hepatic artery. This portal-dominant supply means that normal hepatocytes derive most of their oxygen and nutrients from portal venous blood — with the hepatic artery playing a supplementary role. This dual supply is the liver's protection against hepatic arterial occlusion: when the hepatic artery is embolised, normal hepatocytes can continue to survive on portal venous flow, preventing wholesale liver necrosis from arterial embolisation.HCC's Arterial Dependence: The Therapeutic Window

As HCC develops and grows beyond 1–2cm, it undergoes a characteristic angiogenic transformation — sprouting new blood vessels from the hepatic arterial system through a process driven by vascular endothelial growth factor (VEGF) and other pro-angiogenic signals. Tumours above 1cm progressively acquire an exclusively or predominantly arterial blood supply — their rapidly proliferating cells require the high-pressure, oxygen-rich blood of the arterial system, and the sluggish, oxygen-depleted portal venous blood that serves normal hepatocytes cannot sustain tumour growth. By the time an HCC is 2–3cm or larger, it is receiving virtually 100 percent of its blood supply from hepatic arterial branches — new, fragile, often chaotic tumour vasculature visible as arterial enhancement on contrast CT and MRI. This arterial exclusivity is the biological foundation of TACE: by injecting chemotherapy and embolic agents selectively into the arterial branches feeding the tumour, the interventional radiologist can deliver concentrated cytotoxic drug directly to the tumour while simultaneously depriving it of its blood supply, without significantly compromising the portal venous supply that sustains normal hepatocytes. The degree of tumour devascularisation achieved depends critically on how superselectively the catheter can be positioned — a catheter placed in the proper hepatic artery distributes embolic agent throughout the entire right and left liver; a catheter placed at the segmental feeding artery confines embolisation to the affected segment; a catheter placed in the sub-segmental tumour-feeding artery maximises tumour necrosis while minimising normal hepatocyte damage.

Lipiodol: The Brilliant Drug Carrier

One of the most clinically important elements of conventional TACE is Lipiodol (ethiodised oil, Guerbet) — an iodised poppy seed oil with a 40-year history of hepatic interventional use. When Lipiodol is injected into the hepatic artery as part of the TACE mixture, it exhibits a selective retention phenomenon: it is rapidly cleared from normal hepatic parenchyma (washed out by portal blood flow within days) but selectively retained within HCC tumour tissue for weeks to months. The mechanism of this selective retention involves the Kupffer cells of the tumour-associated sinusoids phagocytosing Lipiodol droplets, the lack of functional bile canaliculi in HCC preventing Lipiodol clearance, and the leaky, hyperpermeable tumour vasculature allowing Lipiodol penetration into the tumour interstitium. This selective retention means that chemotherapy mixed with Lipiodol (doxorubicin-Lipiodol emulsion or cisplatin-Lipiodol suspension) is carried into the tumour and retained there for extended periods — the Lipiodol acts as a chemotherapy depot, releasing drug slowly within the tumour over days to weeks at concentrations dramatically higher than achievable through systemic intravenous dosing. Lipiodol-retained within the tumour is also radiopaque on plain X-ray and CT — making it a visible marker of drug delivery that allows post-TACE CT to quantify how completely the tumour has been permeated by the Lipiodol-chemotherapy mixture and to identify viable tumour zones that have not taken up the Lipiodol.

The selectivity principle: TACE works because HCC is fed exclusively by hepatic arterial branches, while normal hepatocytes are sustained by portal venous flow. The interventional radiologist's technical skill in achieving superselective catheter placement — positioning the catheter tip in the tumour-feeding sub-segmental artery rather than a lobar or segmental vessel — maximises tumour drug delivery and embolisation while minimising normal hepatocyte damage and post-TACE liver function deterioration. The quality of superselection is one of the most important determinants of both efficacy and safety, which is why TACE volume and operator experience matter so profoundly.

The combination of TACE followed by ablation (RFA or MWA) is the dominant treatment strategy for HCC in the 3–5cm range at Chinese liver cancer centres — and is supported by the highest quality randomised controlled trial evidence of any combination strategy in interventional HCC oncology. Understanding why this combination works and what the evidence shows is essential for patients with HCC in this size range.

Why TACE Before Ablation Improves Outcomes

The synergy between TACE and subsequent ablation (RFA or MWA) arises from several complementary mechanisms. TACE eliminates the heat-sink effect: by embolising the tumour-feeding arterial branches, TACE reduces tumour vascularity — decreasing the blood flow within and around the tumour that would otherwise dissipate thermal energy during ablation (the heat-sink effect). For perivascular HCC tumours, this TACE-mediated devascularisation makes thermal ablation effective in locations where it would otherwise fail due to heat dissipation by adjacent blood vessels. Lipiodol delineates the tumour: the Lipiodol deposited by TACE marks the tumour volume precisely on post-TACE CT, providing an accurate three-dimensional map that guides electrode placement for ablation — ensuring the ablation zone covers the Lipiodol-delineated tumour with adequate margins. TACE sensitises tumour cells to thermal injury: the ischaemic stress imposed on tumour cells by TACE-induced arterial embolisation reduces their resistance to thermal injury, so that ablation temperatures that would not be lethal to well-perfused tumour cells become lethal to ischaemically stressed cells — a synergistic thermal sensitisation effect. These three mechanisms combine to make TACE + ablation substantially more effective than either modality alone for HCC in the 3–5cm range.