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While planar imaging may be diagnostic (Fig. 10), in other cases SPECT is a necessary tool in determining the presence and extent of disease (Fig. 11).

If the radiopharmaceutical is to be used to monitor treatment response or detect relapse, it is also important to have a pre-treatment gallium scan to confirm that the tumor is gallium-avid. About 95% of patients with HD and between 70-80% of patients with NHL have positive uptake. Gallium imaging can be used to exclude distant metastases (Fig. 12). Recent chemotherapy and in particular G-CSF can cause significant alterations in gallium biodistribution (Fig. 13).

18F-FDG has higher sensitivity and specificity than gallium imaging and if PET is available should be the investigation of choice (Fig. 14). PET is more sensitive

Figure 11B. Coronal SPECT slices. Note how the disease extent can be readily seen (arrow), in contrast to the planar images. Extension into the mediastinum is now obvious (arrowhead).

than CT scanning in evaluating tumor burden at presentation, although anatomical localization remains important for treatment planning. The data supporting the routine use of FDG at presentation are more persuasive than they are for gallium and most centres which have PET available as a routine tool will use FDG imaging as part of their staging protocol.

Monitoring Response to Therapy

Anatomic imaging will often demonstrate residual tumor masses after therapy— often for years after completion of therapy. The exclusion of viable disease at these

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