Percutaneous microwave ablation applications for liver tumors: recommendations for COVID-19 patients

Abstract

Microwave ablation (MWA) is an alternative locoregional therapy to surgical resection of solid tumors in the treatment of malignancies, and is widely used for hepatic tumors. It has a slightly higher overall survival (OS) rate compared to external beam radiation therapy (EBRT), and proton beam therapy (PBT), and better long-term recurrence-free OS rate compared to radiofrequency ablation (RFA). In this paper, current commercial devices, most recent noncommercial designs, and the principles behind them alongside the recently reported developments and issues of MWA are reviewed. The paper also provides microscopic insights on effects of microwave irradiation in the body. Our review shows that MWA is a safe and effective, minimally invasive method with high ablation completion rates. However, for large tumors, the completion rates slightly decrease, and recurrences increase. Thus, for large tumors we suggest using a cooled shaft antenna or multiple antenna placements. Comparisons of the two common ablation frequencies 915 MHz and 2.45 GHz have shown inconsistent results due to non-identical conditions. This review suggests that 915 MHz devices are more effective for ablating large tumors and the theory behind MWA effects corroborates this proposition. However, for small tumors or tumors adjacent to vital organs, 2.45 GHz is suggested due to its more localized ablation zone. Among the antenna designs, the double-slot antenna with a metallic choke seems to be more effective by localizing the radiation around the tip of the antenna, while also preventing backward radiation towards the skin. The review also pertains to the use of MWA in COVID-19 patients and risk factors associated with the disease. MWA should be considered for COVID-19 patients with hepatic tumors as a fast treatment with a short recovery time. As liver injury is also a risk due to COVID-19, it is recommended to apply liver function tests to monitor abnormal levels in alanine aminotransferase (ALT), aspartate aminotransferase (AST), total bilirubin, and other liver function indicators.

Keywords: Ablation; COVID-19; Hepatocellular carcinoma; Liver; Microwave; Percutaneous; Tumor.

Figure 1

The comparison of peak temperature achieved for ex vivo MWA of porcine liver at the two common frequencies 915 MHz and 2.45 GHz, at two different powers 50 W and 80 W, and at different distances from a cooled shaft antenna [58].

Figure 2

The wave pattern for the electric component of an infinitesimal dipole antenna at 915 MHz and 2.45 GHz with an arbitrary constant current amplitude for both frequencies.

Figure 3

Schematic representation of the common antenna designs.

Figure 4

Axial and radial schematics of a coaxial-based double slot choked antenna designed for hepatic microwave ablation (MWA).

Figure 5

Schematic representation of a coaxial monopole antenna inserted through a biopsy needle. The active length of the antenna and the insertion depth through the needle can be adjusted according to the tissue properties.

Figure 6

Common commercial microwave ablation systems.