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November 2001. Review Article. Endoscopic Ultrasonography and Hydrosonography Conventional Radiology Computerized Tomography Ultrasonography Magnetic Resonance Imaging Positron Emission Tomography Angiographic Techniques
At the beginning of any cancer therapy, the tumor stage must be evaluated. The choice of therapy depends on the patient's prognosis, which generally is determined by the specific tumor stage. Therefore the preoperative imaging diagnosis is the basis for a tumor-stage-adapted therapy of each patient.
EUS ( Endoluminal Ultrasound ) is performed with 7 to 12 MHz scanners. Five layers of the gastric wall can be visualized endosonographically. The inner layer is hyperechoic. Thin hyperechoic and hypoecoic layers are alternating. The second hypoecoic layer represents the lamina muscolaris propria, which exists throughout the whole gastrointestinal tract. If the carcinoma does not penetrate this second hypoecoic layer, it has to be classified as T1. If the third hyperechoic layer is infiltrated, the carcinoma is classified as uT3. The serosa, which is the critical pathohistological layer, cannot accurately be visualized by ultrasound because of its thinness.
Lymph nodes are detectable when their diameter exceeds 3 mm. According to Kuntz's study tumor-infiltrated lymph nodes appear inhomogenueus and hypoechoic,similar to the primary gastric carcinoma, whereas inflammatory enlarged lymph nodes mostly appear homogeneous and hyperechoic ( 1 ). Recently miniprobe ultrasonography has shown an overall accuracy of 80% for T staging, but with more dependance on T stage. Therefore miniprobe US is a necessary device for those patients with early gastric cancer to whom mucosectomy should be offered, since differentiation of T1m and T1sm is possible (and necessary), and lymph node metastases occur in 3.3% (T1m) and 19.6% (T1sm) respectively. EUS and miniprobe ultrasonography correctly classifies lesions that are limited to the mucosa in 92% of cases. The combination of both techniques is therefore the method of choise for identifying mucosal carcinoma ( 3 ). EUS is useful in detecting destruction of the gastric wall due to lymphoma, as well as linitis plastica and other disorders. EUS is the method of choice for staging infiltrative gastric wall disorders. Differential diagnosis of gastric fold thickening (Menetrier's disease, linitis plastica and lymphoma) is sometimes difficult, or even impossible, if no histologic abnormalities are found. In those cases, large biopsy forceps may increase diagnostic yeld, or EUS-guided FNA may be considered ( EUS is helpful in guiding needless precisely through the gut wall into surrounding structures to obtain bioptic samples ). MALT lymphoma can be assessed by EUS and EUS can be useful in assessing the response to Helicobacter eradication. Also the ultrasonic miniprobe can be recommended as part of routine care in patients with gastric MALT lymphoma, both initially and during the follow up period ( 3, 4 ). Submucosal lesions of the gastrointestinal tract are best diagnosed by EUS. EUS can reliably distinguish between solid intramural lesions and extramural compressions. Furthermore EUS can suggest the nature of the tumor by determining the origin of the tumor and the corresponding layer ( e.g. a hypoecoic lesion in the fourth layer is pathognomonic for a stromal cell tumor ).Nevertheless different experts had low accuracy ( ranging from 56% to 77% ) in predicting malignancy ( 3 ). With EUS ascites in the bursa omentalis can be detected as an indirect sign of peritoneal metastases. In contrast to hydrosonography, liver metastases or peritoneal metastases distant from the stomach cannot adequately be detected by EUS.
Further repeated tumor evaluations during preoperative chemotherapy are easily performed with hydrosonography when the tumor has been correctly visualized once before.
On the other hand small gastric tumors (pT1, pT2 ) can be better differentiated with the high resolution capacity of EUS ( 12 MHz ) than with hydrosonography
( 1 ). Conventional radiology is particularly useful in scirrhous carcinoma characterized by diffuse, predominantly submucosal infiltration. It is manifested by irregular narrowing of the lumen and rigidity due to marked desmoplastic reaction, the so called linitis plastica. In advanced neplasms of this type, extension of the tumor to the transverse colon through the gastrocolic ligament may be manifested on barium enema studies by irregular tethering of the mucosal surface contour or luminal narrowing. When peritoneal dissemination occurs, an abdominal plain film can disclose the presence of associated ascites or separation, or narrowing and angulation of the mesenteric gas-filled bowel loops. Distant skeletal and pulmonary metastases from gastric cancer can be seen on skeletal survey and chest radiographs, respectively ( 6 ).
In the preoperative staging of gastric carcinoma the criteria to be assessed with CT include extension of the tumor along the wall and adjacent areas, lymph node metastases, and distant metastases
( 7 ). The sensitivity of US in identifying metastatic liver nodules is inversely proportional to the size of the tumor and can be optimized, as already discussed, with the introduction of contrast media in conjunction with the utilization of harmonic tissue imaging. Moreover under US guidance biopsies can be performed. Finally, intraoperative US may be considered the gold standard in liver nodule detection and may alter the preoperative plan in one third of the patients. ( 5 ).
In some cases MRI has demonstrated the ability to depict a primary tumor and define the degree of its invasion depth; it is a valid means of evaluating intraperitoneal extension. The administration of intravenous contrast medium increases, in most cases, the conspicuousness of gastric lesions, and with T1/ fat suppressions sequences facilitates the identification of an eventual intraperitoneal diffusion of the lesion. The regional lymph nodes and the peritoneal seedings enhance intensely if the fat signals is suppressed. The potential distribution of the anatomical sites of metastases, via the large venous and lymphatic drainage, can make FDG-PET very useful in the evaluation of nodal involvement in the upper to middle body. No large clinical studies have applied PET in the staging of gastric cancer. The diagnostic performance is encouraging in detecting lymph node invasion and is superior to the conventional imaging techniques. Changing in the 18 F-FDG uptake have been observed in tumors after chemotherapeutic treatments, and the rate of reduction of FDG uptake is correlated with prolonged survival. Further clinical studies are needed to investigate both the usefulness of the FDG-PET in gastric cancer staging and in predicting a favourable response to chemotherapy in advanced disease ( 12, 13, 15 ). Selective celiac and mesenteric angiography is not routinely performed before gastroduodenal resections. However, preoperative knowledge of the detailed vascular anatomy may be useful in patients who have undergone previous surgical gastrointestinal resections to identify major feeding vessels to the organs. Moreover gastric bleeding that is refractory to systemic and endoscopic control may be the first sign of a gastric cancer, or may occur during the follow up of an advanced or recurrent tumor. In these cases, angiographic superselective embolization may successfully stop the bleeding ( 14 ). References
1) C Kuntz, C Herfarth. Imaging diagnosis for staging of gastric cancer. Seminars in Surgical Oncology 1999; 17: 96-102.
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