The impact of intraoperative ultrasonography on the management of disappearing colorectal liver metastases

Hepatic resection for colorectal liver metastases (CRLM) is currently the only treatment option that can offer a chance of long-term survival, with 5-year survival rates of 40% [1–3], and exceeding 50% in selected patients [4–6]. However, resectability is the limiting factor; indeed, only 10–25% of patients with CRLM are candidates for surgical resection at the time of presentation [7]. More recently, the introduction of new and more effective chemotherapy regimens combined with targeted agents have improved the response rate over that of standard chemotherapy alone, from 30 to 60% [8–10]. For this reason preoperative chemotherapy for CRLM has been widely used and an increasing number of patients receive chemotherapy prior to liver resection, either as neoadjuvant strategy for initially resectable CRLM [11], or as conversion chemotherapy in patients with initially unresectable CRLM in attempt to convert them into surgical candidates [12,13]. The extensive use of chemotherapy may cause the shrinkage of CRLM and sometimes makes such lesions impossible to identify in radiological imaging studies. These lesions are called ‘missing’ or ‘disappearing’ CRLM [14]. Disappearing liver metastases (DLM) are defined as a disappearance of liver metastases on cross-sectional imaging after administration of preoperative chemotherapy, which means a complete radiological response or complete clinical response (CCR). This phenomenon has been reported by several centers and can occur in 5–25% of patients who undergo preoperative systemic chemotherapy [15–21]. Patients with multiple CRLM, with size <1 cm and those undergoing prolonged preoperative chemotherapy, presented significantly higher risk of developing DLM [18]. Different reported rates of DLM may depend on the quality and type of cross-sectional imaging [22]. Indeed preoperative chemotherapy can induce parenchymal changes to the liver by increasing fatty content, defined as steatosis or steatohepatitis. In that setting the background liver appears less dense, with lower contrast between the parenchyma and the hypovascular metastases, hindering their detection [14,22]. Compared with computed tomography (CT), magnetic resonance imaging (MRI) with liver-specific contrast agents, presents higher sensitivity and better specificity to detect and differentiate CRLM, and can be considered as the best modality to image CRLM missing on CT scan, especially in case of chemotherapy-induced steatosis or steatohepatitis [14,22,23].