الفهرس 
Anatomy of the StomachOnly 14 pages are availabe for public view
Abstract
Gastric cancer is a leading cause of cancer death worldwide. Complete resection offers the only chance for permanent control, and accurate staging and evaluation of treatment response are crucial for appropriate management. Positron Emission Tomography (PET) is increasingly used to complement anatomic imaging in cancer management.
Staging of gastric cancer typically makes use of a variety of imaging modalities, such as computed tomography (CT), magnetic resonance imaging (MRI), endoscopic ultrasounds (EUS), and combined positron tomography (PET-CT), as well as laparoscopic staging and cytogenetic analysis of peritoneal fluid in appropriate patients.
Unlike conventional medical imaging, which is generally utilized for its depiction of anatomy and/or blood flow, nuclear medicine imaging has the unique ability to provide information regarding physiologic and metabolic activity. It is able to fulfill this task through the use of various radiotracers, which are radioactive compounds which are expected to localize to specific areas and become involved in different bodily processes. These radiotracers have very different and more complex functions than the contrast media used in CT or MRI for example, which merely travels through the vasculature and demonstrates areas of blood perfusion.
One of the more recent major advances in the field of nuclear medicine has been Positron Emission Tomography, known as PET. When combined with CT, PET is able to provide excellent physiologic information without sacrificing anatomic detail, which is presently the case for most other applications of nuclear medicine. This has enabled PET-CT to revolutionize oncologic imaging and thus the general management of many types of cancers. PET-CT also has other applications in the evaluation of certain cardiac and neurologic conditions as well as inflammatory processes.
The value of PET-CT has been of increasing interest among clinicians and data has supported its increased use in the detection, staging, and management of a variety of malignancies. During and after therapy, PET-CT may be useful in determining response to chemotherapy. It may be helpful for restaging and diagnosing recurrence at an earlier time or with greater certainty.
What all PET radiotracers have in common is their process of radioactive decay. The radiotracers, which are produced using cyclotrons, are all unstable in that they carry an excess of positive charge. In decaying to attain a stable state, the radiotracer molecules emit a positively charged electron, called a positron. After traveling a short distance from its origin (usually only a few mm) to expend excess energy, the positron comes into contact with its opposite particle, an electron, and the two cancel each other out in a reaction referred to as annihilation. The crucial point for PET imaging is that the annihilation reaction produces two 511 keV photons which are emitted at that spot in directions approximately 180 degrees from each other. These photons are detected by a special detector which surrounds the patient. While one photon alone could have derived from any location and would not provide any localizing information, the simultaneous (known as “coincidental”) detection of two photons localizes the source to a location somewhere along the line connecting the sites of impact of the two photons. With enough photon pairs producing enough intersecting lines, a computer is able to reconstruct the precise location from which the photons are being emitted.
PET is a promising modality with increasing use across a wide variety of malignancies. It is increasingly used in GI cancers. PET may be used as an option for greater specificity in characterizing suspected disease in gastric cancer; however, anatomic imaging remains the standard recommendation. Some data supports the use of PET in gastric cancer staging, particularly in characterizing distant metastases or lymphatic metastases beyond compartment I or II.
In several gastric cancer histologies, however, the metabolic differential between tumor and normal tissue is not as stark as with other malignancies, making the conceptual utility of PET less clear. Mucinous carcinoma, signet ring cell carcinoma, and poorly differentiated adenocarcinomas typically have less prominent FDG uptake.
Because surgical treatment is a major prognostic factor, effort to accurately determine the invasiveness of a gastric lesion is crucial. Regardless of the imaging modality used, loss of the fat plane between a gastric mass and adjacent organs is suggestive of invasion. For this reason, PET imaging is not particularly helpful in determining the T stage. The resolution of PET is limited by volume averaging of metabolic signal, with prominent uptake averaged across several millimeters.
PET seems an excellent adjuvant therapy to detect these anatomically small but potentially metabolically active focuses of metastatic disease. However, the relatively poor spatial resolution of PET makes it less effective because of the difficulty of distinguishing compartment I and II nodes from the primary tumor itself. The real value of PET may be in the detection of ”distant” metastatic disease in compartments III and IV and not amenable to surgical resection with a standard D2 lymphadenectomy. Identification of further spread with PET imaging may influence surgical planning for a more aggressive lymphadenectomy or the decision to avoid surgery altogether as fatal and unnecessarily morbid.