Limitations Of Fdgpet

FDG-PET can add valuable information at several stages of care of lymphoma patients. However, this modality also has limitations which must be recognized in order to benefit optimally from its use. First, while the majority of lymphomas are detectable by FDG-PET scanning, some tumors fail to accumulate FDG, rendering them silent by this modality. While the likelihood of FDG avidity can be predicted by histology, exceptions do exist. For example, although the vast majority of FLs show FDG avidity, intestinal FLs are much less likely to take up the tracer.18 Because of these exceptions, at this point, a baseline scan should

Figure 79.1 Relapsed large B-cell lymphoma obscured by ureteral obstruction. A patient with a history of large B-cell lymphoma underwent FDG-PET scanning in follow-up. The intense uptake demonstrated in the right abdomen was interpreted as ureteral obstruction with no evidence of tumor. A CT scan demonstrated a mass compressing the ureter, and biopsy confirmed relapse of lymphoma

Figure 79.1 Relapsed large B-cell lymphoma obscured by ureteral obstruction. A patient with a history of large B-cell lymphoma underwent FDG-PET scanning in follow-up. The intense uptake demonstrated in the right abdomen was interpreted as ureteral obstruction with no evidence of tumor. A CT scan demonstrated a mass compressing the ureter, and biopsy confirmed relapse of lymphoma be done in patients for whom FDG-PET will be used in follow-up.

A second limitation of FDG-PET involves difficulties of interpretation in areas of high physiologic tracer accumulation. One major example of this issue is seen in the urinary tract, as FDG is excreted via the kidneys. Tumor localized near the kidney, ureter, or bladder may not be visualized due to masking of FDG accumulation by urinary collecting structures (Figure 79.1). Alternatively, abnormal accumulations of urine may be interpreted as tumor mass, for example in the case of ureter obstruction from a cause unrelated to tumor. Another anatomic area that may present difficulty in imaging and interpretation is the cardiac region. Patients fast for several hours prior to FDG administration, therefore suppressing insulin levels. This leads to a conversion from glucose metabolism to fatty acid metabolism in adaptable tissues such as cardiac muscle, reducing FDG uptake in these tissues. In up to a third of cases, however, the heart may still take up significant amounts of FDG. While the radiologist can usually identify this uptake as cardiac, it may mask areas of malignancy in the paracardiac mediastinum. These issues emphasize the importance of concurrent anatomic imaging for optimal interpretation of FDG-PET.

Inflammatory lesions can on occasion mimic tumor on FDG-PET. Under most circumstances, inflammation can be differentiated from malignancy by intensity of uptake and spatial characteristics. In some cases, however, inflammatory lesions may masquerade as malignancy. Cases of suspected residual or recurrent tumor visualized by FDG-PET have been documented to be infection in multiple instances.4546 Furthermore, postradiation inflammation may cause significant FDG accumulation. While the characteristics of uptake usually identify the process, the localization to a former site of tumor involvement can be misleading. Zhuang and colleagues have suggested that changes in FDG intensity over time may accurately differentiate tumor from inflammation, with tumor tissue continuing to increase in intensity and inflammatory lesions decreasing in intensity over the course of several hours.47 These findings have yet to be confirmed in clinical practice.

Thymic uptake following chemotherapy, particularly in young patients with HL, is increasingly recognized as a possible cause of false-positive FDG-PET scans after therapy (Figure 79.2).48 In an appropriate clinical setting, the clinician can recognize this phenomenon of thymic rebound as a benign event unrelated to residual tumor. However, the location of the thymus in the mediastinum, an area of frequent lymphomatous involvement, may at times make this interpretation problematic. Similarly, bone marrow may show increased uptake during or following chemotherapy due to recovery of normal hematopoiesis (Figure 79.3). Again, the clinical scenario should in most cases make interpretation of this phenomenon clear-cut.

Figure 79.2 Thymic rebound. An FDG-PET scan was performed 9 weeks following completion of therapy in a patient with Hodgkin's lymphoma. The mediastinal uptake is typical of benign thymic hyperplasia, and the patient remains in a complete remission

Figure 79.3 Physiologic uptake of FDG in a patient with large B-cell lymphoma. A patient with large B-cell lymphoma underwent baseline FDG-PET demonstrating uptake in mediastinal tumor (a). A scan performed during treatment to evaluate disease response showed resolution of FDG uptake in malignant tissues but an increase in uptake of the tracer in bone marrow, reflecting hematopoietic recovery (b). Following completion of therapy, no tracer uptake is detected in tumor or bone marrow, but physiologic cardiac uptake is demonstrated (c)

Figure 79.3 Physiologic uptake of FDG in a patient with large B-cell lymphoma. A patient with large B-cell lymphoma underwent baseline FDG-PET demonstrating uptake in mediastinal tumor (a). A scan performed during treatment to evaluate disease response showed resolution of FDG uptake in malignant tissues but an increase in uptake of the tracer in bone marrow, reflecting hematopoietic recovery (b). Following completion of therapy, no tracer uptake is detected in tumor or bone marrow, but physiologic cardiac uptake is demonstrated (c)

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