The Role of Positron Emission Tomography in Esophageal Cancer

摘要

The most common methods for staging esophageal cancer are endoscopic ultrasonography (EUS) and computed tomography (CT). EUS is well established in differentiating between early tumor stages and more advanced primary lesions. When combined with fine needle aspiration, EUS has become an important tool in assessing the regional lymph nodes, as well. EUS has its limitations, esophageal obstruction makes passage of the endoscope beyond the tumor nearly impossible and with a narrow field of evaluation, it is not useful for detecting metastatic disease. CT allows for assessment of local tumor invasion while simultaneously providing information regarding distant disease. Its usefulness locally, however, is limited. CT and EUS yield anatomic visualization. Fluorodeoxyglucose (FDG)-positron emission tomography (PET) can provide functional information and is an effective diagnostic modality in esophageal cancer. Its role in the management of esophageal cancer includes staging as well as potential utility in the evaluation of neoadjuvant therapy response and in follow-up after definitive therapy. FDG-PET will likely be more readily used in combination with anatomical imaging like CT to provide additional diagnostic information to aid radiation oncologists in target delineation and planning. In addition, FDG-PET has also been shown to have prognostic value that can be applied to patient management and aid in development of emerging therapies. Approximately 14,550 people will be diagnosed with esophageal cancer in the United States in 2006. Of these patients, about 13,770 will ultimately die of the disease. 1 While the national incidence has been slowly increasing over the past 25 years at roughly 4.5 cases per 100,000 people per year, the death-to-incidence ratio continues to be relatively high, at over 0.90. 2 The main reason for the poor prognosis is that esophageal cancer is largely asymptomatic in its early stages. Thus, most cases are diagnosed when the disease has become either locally advanced, with disease spreading to local draining lymph nodes, or distantly metastatic. 3 Historically, it has been difficult to distinguish potentially curable from likely incurable disease. This inaccuracy in staging makes both local and distant recurrences relatively common, even after aggressive multimodality treatment using combinations of surgery, chemotherapy, and radiation therapy. While improving curative therapy continues to be an objective, accurate staging is crucial, both to guide therapy and to predict prognosis. Proper staging allows selection of patients who can potentially benefit from local treatment, while sparing those patients with metastatic disease from undergoing intensive multimodality local therapy. 3 In addition, the prognostic information obtained through accurate staging aids both the physician and the patient regarding treatment decisions. Finally, the results of clinical trials of therapy for esophageal cancer have been severely confounded by stage migration as a direct result of inaccurate techniques in staging. In esophageal cancer, the most common methods for staging are endoscopic ultrasonography (EUS) and computed tomography (CT) of the chest and abdomen. EUS is well established in differentiating between early tumor stages and more advanced primary lesions. 4 When combined with fine needle aspiration, EUS has become an important tool in assessing the regional lymph nodes, as well. 5 The accuracy of EUS for evaluating primary tumor and nodal status has been reported to be 85% and 75%, respectively while the sensitivity has been reported to be in the range of 85% to 95% for primary tumor evaluation and 70% to 80% for nodal evaluation. 6 , 7 EUS has its limitations, esophageal obstruction makes passage of the endoscope beyond the tumor nearly impossible and with a narrow field of evaluation, it is not useful for detecting metastatic disease. 3 CT allows for assessment of local tumor invasion while simultaneously providing information regarding distant disease. Its usefulness locally, however, is limited, with sensitivity for staging primary tumor and nodal disease reported to be about 50% and 60% to 87%, respectively. 7 While CT and EUS yield anatomic visualization, positron emission tomography (PET) is a nuclear medicine imaging modality that allows for measurement of physiologic and biochemical processes. 8 PET can potentially determine quantitative information regarding blood flow, receptor status, and metabolic processes, depending on the radiopharmaceutical selected. 8 Regarding the role of PET in esophageal cancer imaging, many esophageal tumors exhibit increased cellular metabolism and are associated with increased glycolysis, as well as increased glucose transporter proteins. 9 A positron emitting radiotracer, 2-[18