FDGPET for Predicting Treatment Response

Evaluation of PET for assessment of treatment response has been studied more extensively in HD and NHL than in any other tumor. Differentiation of viable tumor from fibro-sis in a residual posttreatment mass is a common problem in lymphoma that is seen in more than 85% of patients with HD and approximately 40% of the patients with NHL.

Initial studies have assessed response to treatment in heterogeneous populations including both HD and NHL patients. Cremerius et al.324 reported better specificity and positive predictive value (PPV) for FDG-PET (92% and 94%, respectively) as compared with that of CT (17% and 60%) in 27 patients.

Zinzani et al.325 studied 44 patients with HD or aggressive NHL who had residual abdominal disease and showed that the 2-year progression-free survival (PFS) rate was 95% for the PET-negative group and 0% for the PET-positive group.

A PPV of 100% for PET, as compared with 42% for CT, was found in 54 patients with HD and aggressive NHL assessed after therapy by Jerusalem et al.326 The negative predictive values (NPV) of PET (83%) and CT (87%) were not significantly different. Recurrence was noticed in the same study, with both positive PET and CT in 26% of patients and with negative PET and positive CT in only 10%. The PFS (2-year) rate with negative PET and negative CT was 87%, with positive CT and negative PET only 60%, and finally, with positive PET regardless of the CT, 0%.

Mikhaeel et al.327 also found in 45 patients with aggressive NHL that the relapse rate was 17% for PET-negative patients and 100% for PET-positive patients compared with 25% for CT-negative patients and 41% for CT-positive patients. The progression-free survival (PFS ) for 1 year was 83% for PET-positive and 0% for PET-negative patients.

Spaepen et al.328 evaluated 93 patients with NHL and reported that all patients who had persistent FDG uptake relapsed, with a 2-year PFS rate of 85% of patients with negative PET findings. The 2-year PFS rate was 4% in patients with positive PET findings. Weihrauch et al.329 studied the predictive value of PET in 28 patients with HD who had residual masses after treatment. The 1-year PFS was 95% for the PET-negative group as compared with 40% for the positive group. Spaepen et al.330 evaluated 60 patients with HD with or without masses at the end of first-line treatment; the 2-year disease free survival (DFS) rate was 4% for the PET-positive group and 85% for the PET-negative group.

Kostakoglu et al.331 compared FDG-PET after the first cycle of chemotherapy and after completion of chemotherapy. PET had greater sensitivity and PPV for predicting relapse after the first cycle. PET after the first cycle had a lower false-negative rate (13%) than the posttherapy PET (35%), possibly reflecting the presence of a small but still detectable tumor load of resistant cells early, but not late, in the therapy course (Table 33.14).

In general, in NHL and high-grade HD, a positive PET at the end of first-line therapy is highly suggestive of disease and requires intensive confirmatory investigation. A negative PET does not exclude the presence of minimal residual disease or future relapse and requires close follow-up. However, in early HD, a negative PET can be used to define complete response (CR) with favorable prognosis, even in the presence of residual masses on CT (see Figure 33.7A,B). A positive PET, especially if located in a site different from the residual mass, should be assessed with caution, and benign or inflammatory etiologies should also be considered in the differential diagnosis of persistent disease.330

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