Elsevier

Hepatology Research

Volume 36, Issue 4, December 2006, Pages 308-314
Hepatology Research

Microbubble-induced increase in ablation of liver tumors by high-intensity focused ultrasound

https://doi.org/10.1016/j.hepres.2006.08.013Get rights and content

Abstract

We studied the possibility of using high-intensity focused ultrasound (HIFU) together with a microbubble agent to treat hepatocellular carcinoma.

Development of liver tumors in rats was induced by administration of Dimethylnitrosamin (100 ppm). Rats with liver tumors were anesthetized, underwent laparotomy, and were given the microbubble agent Levovist or saline intravenously. After the injection, the liver was exposed to HIFU for 30 s (2.18 MHz, 600 W/cm2, 40 mm in diameter). Immediately after HIFU exposure, ultrasound images of the HIFU area were evaluated. Then the liver was excised and the volume of coagulated tissue was measured.

The mean volumes of hyperechoic areas after HIFU were as follows (mm3, Levovist versus saline: 355.3 ± 180.7 versus 47.4 ± 35.6, P < 0.001, n = 13). The volumes of liver tissue coagulated by HIFU were as follows (mm3, Levovist versus saline: 275.3 ± 120.0 versus 60.1 ± 23.6, P < 0.001, n = 13). On microscopic examination of areas exposed to HIFU, implosion cysts were seen, and many cancer cells were found to have been destroyed completely (loss of cell membranes or nuclei).

In conclusion, the microbubble agent Levovist can increase the volume of tissue coagulated by HIFU.

Introduction

High-intensity focused ultrasound (HIFU) is used to cause therapeutic coagulation and necrosis [1], [2]. In preliminary clinical studies, HIFU has been shown to be safe and effective against solid tumors, including those of the prostate, liver, breast, kidney, bladder, pancreas, and bone [3], [4], [5], [6], [7]. Especially in the treatment of localized prostate cancer, HIFU is a potential alternative to conventional therapies [8], [9], [10]. HIFU's role in oncology could increase as it becomes more widely available [11], as occurred with the development of noninvasive thermal ablation [12].

Hepatocellular carcinoma (HCC) is one of the most common malignancies worldwide, resulting in more than 500,000 deaths per year [13]. It is usually treated by surgical resection, liver transplantation, percutaneous ethanol injection (PEI), radio-frequency ablation (RFA), or transarterial chemoembolization (TACE) [14], [15], [16]. HIFU may also have a role in treating HCC, because it is only minimally invasive. However, HIFU as it is commonly used has an important disadvantage: adequate treatment of a liver tumor requires necrosis of a 0.5–1.0 cm margin of apparently healthy tissue adjacent to the tumor, but the extent of necrosis caused by a single HIFU exposure is so small that ablating a clinically adequate volume of liver-tumor tissue would require hundreds of exposures [17]. To overcome this disadvantage, we developed a more efficient method of HIFU exposure by using microbubbles.

In ultrasound imaging of liver tumors, microbubbles are used as contrast agents. They are used, for example, to characterize and evaluate responses to treatment after TACE, RFA and HIFU [18], [19], [20], [21]. Attention has recently been drawn to the interaction between ultrasound and the dynamic behavior of microbubbles [22], [23], [24]. Theoretically, microbubbles can change the acoustic properties of tissues and can cause ultrasound energy to be deposited more efficiently. Intravenously infused microbubbles can increase the efficiency of HIFU in the kidneys or livers of healthy rabbits [25], [26] but we know of no previous study of the effects of microbubbles on HIFU-induced coagulation of liver tumors.

We induced liver tumors in rats by injecting diethylnitrosamine, and exposed the tumors (in vivo) to HIFU. Then we studied the echogenicity of the affected areas and the amount of tissue that had been coagulated.

Section snippets

Animal model of liver cancer

Male Wister rats weighing 80–100 g were purchased from Nihon-Seibutu-Zairyo (Tokyo, Japan). To induce liver carcinogenesis, dimethylnitrosamin (DEN, Nacalai Tesque Co., Kyoto, Japan) was administrated at 100 ppm through the drinking water ad libitum. Every 2 weeks, the rats were anesthetized with 40 mg/kg sodium pentobarbital (Nembutal, North Chicago, IL, USA) and their livers were examined by transcutaneous gray-scale ultrasonography (12-MHz probe, M2540A EnVisor, Philips Medical Systems,

Tumors induced by DEN

Tumors developed in all rats after 8–12 weeks of DEN administration. Macroscopically, the tumors were irregularly shaped, white nodules (Fig. 2A).

Microscopically, multifocal nodules of moderately differentiated hepatocellular carcinoma (HCC) were seen. The nuclei were abnormally large, with coarse chromatin and conspicuous nucleoli. The cells had high nuclear/cytoplasmic ratios. The cells were arranged chiefly in a trabecular manner, and a pseudo-glandular structure was occasionally observed.

Discussion

The volume of liver-tumor tissue ablated by HIFU was about 4.5 times greater in the rats that had received Levovist™ than in those that had received saline. This finding is consistent with previous reports of the effects of microbubbles on tissue coagulation by HIFU [25], [26]. HIFU has been used in combination with low-dose radiotherapy to treat rectal carcinoma, with some success [30], and injecting microbubbles before HIFU might be useful in treating HCC.

Microbubble agents may be delivered

Acknowledgement

This study was supported by a Grant-in-Aid from the Ministry of Education, Science, Sports, Culture, and Technology of Japan.

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