Histological findings of the bare area of the liver
In case 1 patient, the histological specimens were obtained from the bare area. H&E staining revealed no direct tumor invasion into the diaphragm. The serial sections were also stained by the Masson trichrome and Sirius red stains to clarify the structure of the connective tissues between the liver and the diaphragm. Sparse connective tissues were lying in a width of 100–200 m between the liver and the diaphragm in low-magnification micrographs (Fig. 1a, 1c, and 1e). The high-magnification micrographs showed that the liver parenchyma was covered with liver serosa (visceral peritoneum) and approximately 100 mm-thick dense sub-serosal tissues. In addition, the diaphragm tendon and its epimysium were covered with parietal peritoneum (Fig. 1b, 1d, and 1f). In case 2 patient, a huge liver tumor was located underneath the right diaphragm (Fig. 2a). The right liver was resected with the partial right diaphragm, which had adhered to the liver tightly (Fig. 2b). H&E staining revealed no direct tumor invasion into the diaphragm. The necrotic tumor was located just below the dense sub-serosal tissues of the liver (Fig. 2c). The diaphragm and liver were covered with a monolayer of parietal peritoneum and visceral peritoneum, respectively (Fig. 2d). Unlike the case 1 patient, the sub-serosal tissues of the liver were hypertrophic. This was done partially because of the necrotic tumor after chemotherapy.
The multilayered structure of the bare area was composed of the liver parenchyma, sub-serosal connective tissue, liver serosa, parietal peritoneum, retroperitoneal connective tissue, epimysium of the diaphragm, and the diaphragm, in order from the liver to the diaphragm (Fig. 3). There were 200–300-µm-thick connective tissues between the liver parenchyma and epimysium of the diaphragm of the bare area, and the liver serosa was close to the parietal peritoneum that caused peritoneal fusion. There are two layers in the dissection of the bare area in surgical procedures, an outer layer of the fused peritoneum (near the diaphragm) and an inner layer of the fused peritoneum (near the liver).
Macroscopic findings of the bare area of the liver
In order to examine the possibility of recognizing the multilayered structure of the bare area intraoperatively, a cadaver and two cases of laparoscopic hepatectomy were analyzed. In the cadaver study, when dissecting the bare area near the diaphragm, the surface of the liver was seen to be covered with dense fibrous tissues (Fig. 4a). On the other hand, when dissecting near the liver, the raw surface of the liver was exposed (Fig. 4b). In the case of laparoscopic bare area dissection, surgeons could sometimes recognize the line between the spare connective tissue on the diaphragm side, and the dense fibrous tissue on the liver side when the liver exerted traction is in the counter direction of the diaphragm (Fig. 4c). When dissecting the bare area near the liver (the inner side of the fused peritoneum), the liver surface covered only by a thin connective tissue was exposed (Fig. 4d). Therefore, it was possible to recognize the multilayered structure of the bare area and select the dissection layer under the magnified view of laparoscopy.
Case presentation of a colorectal liver metastatic tumor located directly beneath the right diaphragm
The case 3 patient is presented to demonstrate the importance of selecting the appropriate dissection layer of the bare area for good surgical therapeutic effect. Preoperative CT revealed a tumor 10 mm in diameter, located directly underneath the right doom of the diaphragm (Fig. 5a and 5b). Because of a possible direct invasion of the tumor into the right diaphragm, the bare area was dissected carefully near the diaphragm (the outer layer of the fused peritoneum) (Fig. 5c). There was no gross invasion observed, and the right liver was dissected from the diaphragm at the outer layer of the fused peritoneum with a dense white connective tissue (Fig. 5d). The multilayered structure of the bare area was recognizable with the high definition images of laparoscopy.
The low-magnification micrographs revealed that the tumor was located quite close to the liver serosa, although there was no direct invasion into the diaphragm (Fig. 6a, 6c, and 6e). However, tumor cells expanded to the sub-serosal connective tissue in the high-magnification micrographs (Fig. 6b, 6d, and 6f). In this surgery, the bare area was dissected at the outer layer of the fused peritoneum, and the parietal peritoneum and the partial retroperitoneal connective tissue were removed with the liver. Therefore, surgical margins of approximately 500 m were reserved.