The utility and limitations of an ultrasonic miniprobe in the staging of gastric cancer

The utility and limitations of an ultrasonic miniprobe in the staging of gastric cancer

Seisuke Okamura^a, Akemi Tsutsui^a, Naoki Muguruma^a, Soichi Ichikawa^a, Masahiro Sogabe^a, Yoshio Okita^a,
Tamotsu Fukuda^a, Shigehito Hayashi^a, Toshiya Okahisa^a, Hiroshi Shibata^a, Susumu Ito^a, and Toshiaki Sano^b

^aSecond Department of Internal Medicine, and ^bFirst Department of Pathology, The University of Tokushima School of Medicine, Tokushima, Japan

Abstract:To determine the utility and limitations of an ultrasonic miniprobe (UMP) in the staging of gastric cancer, we evaluated 46 patients who underwent endoscopic ultrasonography (EUS) using an UMP and who were histologically determined to have gastric cancers. In every case, UMP findings were compared with histopathological findings after treatment. The total accuracy of UMP relative to the depth of tumor invasion was71.7% (33/46 cases). Accuracy with respect to T1-m tumor diagnosis was 75.7% (22/29cases), and for T1-sm, 76.9% (10/13 cases), but accuracy for T2 tumor diagnosis was low, due to ultrasound attenuation. When the analysis was carried out based on the size of tumor, the accuracy for UMP was 50.0% (9/18cases) for all tumors over 20mm and 85.7%(24/28 cases) for all tumors smaller than 20mm. We conclude that UMP is suitable for investigation of tumor extension when the lesion is superficial and / or small gastric cancers which do not cause ultrasonic attenuation, but not when the tumor is large or located in certain sites, although conventional EUS is useful in some of these cases.
J. Med. Invest. 46:49-53, 1999

Keywords:endoscopic ultrasonography, ultrasonic miniprobe, gastric cancer

INTRODUCTION
Accurate staging is an important consideration in the management cases of gastric cancer, since this staging is the basis on which treatment decisions are made. Endoscopic ultrasonography (EUS) is generally considered to be the most accurate method for the locoregional staging of gastric cancer (1-3). This is due to its ability to visualize the different layers of the wall of the gastrointestinal tract (4), as well as its role in demonstrating lymph nodes (5).
Conventional EUS is performed with a side-viewing instrument containing a small ultrasonic transducer incorporated in the tip which generates high fre-quency ultrasonic images (7.5 to 20 MHz). Recently, a radial scanning ultrasonic miniprobe (UMP) has been developed (6-7). The UMP is inserted through the instrument channel of a standard endoscope with a diameter of 2.5mm and a rotating 12 or 20MHz transducer for radial imaging. The UMP permits the scanning of lesions under direct endoscopic visual-ization within the water-filled gastric lumen (8).
In this study, we assess the utility and limitations of a UMP in terms of the staging of gastric cancer.

MATERIALS AND METHODS
Subjects
Forty six patients who underwent EUS using a UMP in this department between April 1995 and July 1997 and who were histologically determined to have gastric cancers were included in this study. EUS using a UMP was performed prior to treatment after obtaining informed consent of the patient;25patients underwent gastrectomy and the remaining21 patients underwent endoscopic mucosal resec-tion. For all cases, the UMP findings were compared with the histopathological findings in endoscopically or surgically excised specimens after treatment (Table1).

High frequency UMP
The UMP used in this study was a model UM-3R (Olympus, Tokyo, Japan). The probe was inserted through the instrument channel of an endoscope of diameter 2.4mm with a 20MHz high frequency rotating transducer for radial imaging (8). The UMP permits the scanning of lesions under direct endo-scopic visualization within the water-filled gastric lumen. For the standard endoscopy, GIF-Q200 or GIF-Q230 (Olympus, Tokyo, Japan) endoscopes were used.

Diagnosis of the depth of tumor invasion
The depth of tumor invasion was determined according to the Union Intern Contra Cancrum TNM classification (9). Additionally, we classified T1 tumor as T1-m, where the tumor remained in the mucosa, or T1-sm, where the submucosa was invaded.
On EUS using a UMP, the diagnostic criteria for the depth of tumor invasion is the same as that for dedicated conventional EUS (1). Briefly, stage T1-m was diagnosed when the tumor invasion was limited to the mucosa, T1-sm was diagnosed when the tumor invasion was limited to the submucosa, stage T2when the wall layer structure was destroyed but the outer margin was smooth or only slightly irreg-ular, stage T3 when transmural tumor growth was detected, and stage T4 in cases of invasion into adjacent organs.

Diagnosis of regional lymph nodes metastasis
Lymph node metastasis was diagnosed based on previously established criteria (5), i.e. hypoechoic, roundish and well-demarcated nodes were assumed to be malignant, whereas hyperechoic and elliptoid nodes with indistinct margins were assumed to be benign.

RESULTS
Accuracy of UMP with reference to the depth of tumor invasion
Table2 shows the accuracy of UMP relative to the depth of tumor invasion in the 46 patients with gastric cancer. The total accuracy was 71.7% (33/46cases). The accuracy for T1-m tumor was 75.7%(22/29cases), and for T1-sm76.9% (10/13cases), but the accuracy relative to the T2 tumor was low due to ultrasound attenuation. When the analysis was carried out based on the size of tumor (Table3), the accuracy for UMP was 50.0% (8/19cases) for all tumors over 20mm and 85.7% (24/28cases) for all tumors smaller than20mm. When analyzed based on the location of the tumor (Table4), the accuracy for UMP was 86.7% (13/15 cases) for tumors in the antrum, 53.3% (8/15 cases) in the angles, 73.3% (11/15 cases) in the body and 100%(1/1case) in the fornix.

Accuracy of UMP with reference to the preoperative detection of regional lymph node metastasis
Of the 25 cases which underwent gastrectomy, 6 (24.0%) had lymph node metastasis.
Preoperative detection of regional lymph nodes with UMP was possible in only 2/6 cases, 33.3%(Table5).

Case reports
Case1:Figure1 shows the UMP image of a super-ficial depressed type gastric cancer in the anterior wall of the body (diameter 5mm). The focal area of tumorous thickening is observed to be limited to the mucosa, but the submucosa is intact. This case was thus diagnosed as having IIc type gastric cancer in stage T1-m, and as a result underwent endoscopic mucosal resection. The histological findings con-firmed the presence of a superficial depressed car-cinoma localizing in the mucosa (Figure2).
Case2:Figure 3 shows the UMP image of an ulcerative and infiltrated type gastric cancer in the posterior wall of the body (diameter 54mm). It was not possible to diagnose the depth of tumor invasion because of ultrasound attenuation. This case under-went gastrectomy. The histologic findings revealed an ulcerative and infiltrated carcinoma invading to the subserosa (Figure4).

DISCUSSION
Gastric cancer is a disease of the elderly in the Western world, and not all patients are sufficiently robust to undergo a gastrectomy. Furthermore, the huge difference in prognosis between T1 or T2 cancer and T3 or T4 cancer influences decisions regarding endoscopic or surgical treatment (10-13). As a result, the accurate staging of gastric cancer is of crucial importance in the management of this type of case. Various studies support the view that EUS is a very accurate technique in the staging of gastric cancer, and that it is superior to other imaging methods, such as extracorporeal ultrasonography and CT (1-3, 14-15).
The UMP is inserted through the instrument channel of a standard endoscope, and is then used to scan a lesion at anytime during an endoscopic examination. Moreover, the UMP permits the scan-ning of lesions under direct vision. In conventional EUS, diagnosis of small mucosal lesions is difficult, due to the fact that the lesion is either compressed by the balloon, or out of focus and, therefore not visualizable. Small mucosal lesions can be scanned with less difficulty than with a conventional EUS (6-8) and the UMP produces an image with finer resolution.
These advantages make this probe useful in assess-ing superficial and/or small gastric cancers. To our disappointment, studies using the conventional EUS were made on the basis of the TNM staging system, in which T1 includes both m and sm invasion depth, and does not distinguish between the two (1-3, 16). Some investigators have studied the differentiation of m from sm cancer, but its accuracy ranges from63.3% to 71.4% (17-19). In the present study, the accuracy of T1-m tumor diagnosis was75.7%, and T1-sm 76.9%. When the analysis was carried out based on the size of tumor, the accuracy for UMP was 85.7% for all tumors smaller than 20mm. In our opinion, the UMP will be useful for the determi-nation of a need for endoscopic mucosal resection in mucosal gastric cancer.
In this study, the accuracy of assessment was low for tumors over20mm in size, probably due to the ultrasound attenuation. Determination of the extent of tumor invasion is not always possible with UMP, due to the the limited depth of penetration (8). The accuracy of assessment was lower for tumors located in the vicinity of the gastric angle, probably due to the fact that it is difficult to place the probe parallel to the lesion in this region.
Detection of regional lymph node metastasis is possible, but is limited due to the small depth of visualization of the UMP (about 25 mm in diameter). In this study, the rate of preoperative detection of regional lymph node metastasis with UMP was 33.3%. The accuracy of conventional EUS in preoperative detection of regional lymph node metastasis ranges from 50% to 87% (1, 3, 16) due to the greater penetra-tion depth of ultrasound (about60mm diameter).
We concluded that the UMP is suitable for the investigation of tumor extension when the lesion is superficial or for small gastric cancers which do not cause ultrasonic attenuation, but not when the tumor is large or located in certain sites, although conventional EUS is useful in some of these cases. It is recommended that a UMP and an optimal frequency of sound be selected, taking into consid-eration the features of a given case, when EUS is performed for gastric cancer.

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Received for publication November 11, 1998 ; accepted December 16, 1998.

Address correspondence and reprint requests to Seisuke Okamura, M.D., Ph.D., Second Department of Internal Medicine, The University of Tokushima School of Medicine, Kuramoto-cho, Tokushima 770-8503, Japan and Fax:+81-88-633-9235.