Our patient showed no abnormalities on the CT study. Nevertheless, significant nodal and extranodal (suprarenal gland) extension of his disease was discovered upon performing a PET study.
PET with 2- [fluorine 18] fluoro-2-deoxy-D-glucose (FDG) is increasingly being used in combination with CT to evaluate thoracic and abdominopelvic malignancies . A common systemic malignancy involving the pelvis is NHL. Whole-body PET is useful in the detection of a wide variety of both primary and metastatic malignancies because of the high glycolytic rate that the malignant tissue presents. The presence of FDG uptake in benign inflammatory conditions may limit the specificity of PET. Sensitivity for the detection of malignant lesions is about 97% and the positive predictive value is 94%. This technique is promising both in determining the nature of a localized lesion, as well as in defining the systemic extent of a malignant disease .
A number of studies have shown FDG-PET to be useful and in fact superior to CT for primary staging and assessment of disease extension in both Hodgkin's disease and NHL. The technique is reported to have sensitivities of 82% to 99% and specificities of 99% to 100% . Although data regarding the use of in-line FDG-PET-CT systems in evaluating lymphoma are inconclusive, preliminary results appear to indicate that this technique is useful when a guided biopsy procedure is needed . In one study that compared the diagnostic performance of PET alone, CT alone, and fused images for restaging or follow-up of patients with malignant lymphoma, 50 patients with NHL were included. In this study, the interpretation of PET alone (sensitivity = 86.1%, specificity = 99.4%, accuracy = 91.0%), and fused images (98.0%, 99.4%, and 99.2%, respectively) yielded significantly better diagnostic performance than CT alone (59.4%, 96.1%, 91.0%; P < 0.001). In particular, findings in cervical, supraclavicular and extranodal regions were more accurately identified using PET (P < 0.05) . With regard to staging, FDG-PET is more sensitive and specific than conventional staging methods in FDG avid lymphomas such as Hodgkin's lymphoma and most aggressive NHLs.
In assessing a patient's response to therapy, FDG-PET at the end of treatment seems to aid considerably in differentiating between residual masses with and without residual lymphoma. Concerning treatment planning, meanwhile, in the context of a combined-modality therapy, radiotherapy for lymphomas is moving towards more conformal techniques to reduce the irradiated volume and to include only the macroscopic lymphoma. In this context, accurate imaging is essential, and FDG-PET in combination with CT scan is increasingly being used. The availability of PET and CT scanners suited for virtual simulation has aided in this process .
The limitations of FDG-PET in detecting lymphomas have included variable FDG uptake in low-grade lymphomas; physiologic activity in muscles, bone marrow, bowels, and the urinary system; and FDG uptake in inflammatory or infectious processes, any of which may mask or mimic tumor signals . Another limitation in the analysis of the pelvis and the urinary tract is the physiological excretion of radiotracers .
This case suggests the need for extensive lymphoma staging, and especially the need for PET implementation, in order to make an accurate diagnosis of the extension of the disease and to properly design a course of treatment.