Assessment Of Residual Disease Lymphoma

A commonly asked question at the end of therapy is the extent of active disease in a residual tumor mass. For example, in patients with lymphoma, a radiologically detectable residual abnormality following therapy occurs in up to 88% of patients, more likely when the initial disease is bulky, the precise incidence depending on presenting histology (Fig. 6). With CT, which is conventionally used to assess response to treatment in lymphoma, it can be impossible to distinguish between active disease and fibrosis and because of this some patients unnecessarily receive radiotherapy. The proportion of residual masses that represent active lymphoma varies between series with relapse rates of ~20% in HL and 10% in NHL (69,70). Radiotherapy has been shown to have significant short- and more worrying long-term toxicity including ischemic heart disease, fibrosis, and radiation induced carcinomas/sarcomas. Refining the criteria for the use of radiotherapy would therefore be a major advance.

A number of studies have confirmed the superiority of 18FDG PET in determining the activity of residual masses in HL and NHL compared to both CT and 67gallium scanning. Of the 32 patients studied by de Wit et al. (71), 17 had a negative 18FDG PET. None of these patients relapsed, although the follow-up period was relatively short, with a median of 62.6 weeks. Bangerter et al. (72) performed 18FDG PET in 36 patients with residual masses. Twenty five of 27 patients with negative scans remained in clinical remission at 25 months. In those with a positive scan, four of nine patients remained in complete remission. Similar high positive and negative predictive values in the order of 90% have been reported, with a significantly higher specificity than CT, in more recent published series using state of the art dedicated PET (73).

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