In patients with follicular lymphoma (FL) circulating lymphoma cells (CLC) can be detected by quantitative real-time PCR with a high sensitivity and reproducibility. Sustained molecular remission is associated with a significantly longer relapse-free survival whereas evidence of PCR detectable MRD was associated with recurrent disease in several studies. Furthermore, we and other reported long-term remission in some FL patients being persistently PCR positive. However, molecular monitoring to guide therapy and to predict clinical relapses is not routinely performed or integrated in clinical studies. Between 1996 and 2006 a long-term molecular monitoring of CLC was done in all FL patients with a PCR-detectable t(14;18) translocation. CLC numbers were determined by a standardized quantitative real-time PCR for the detection of lymphoma cell specific t(14;18) rearrangement [
Dölken,L. et al.; Biotechniques 1998;25:1058–1064
]. The K-ras wild-type gene served as reference gene to determine the number of cells in a given sample. We identified 9 patients who developed a relapse after chemotherapy alone (n=4), or in combination with rituximab (n=2), radiotherapy (n=1) and autologous stem cell transplantation (n=2) and for whom at least 4 PBMNC samples (median 5 samples; range 4–8) were collected at different time points before and after the clinical relapse. All patients had either decreasing or stable numbers of t(14;18) positive cells within a period of at least 6 months before clinical relapse. Only one patient had an early relapse 4 months after start of initial chemotherapy, all other patients had a relapse 1 – 6 years later. Being in clinical remission 8/9 patients had CLC numbers < 100/105 PBMNC. 4/9 patients had a molecular remission before they relapsed. A median CLC increase of 2log (range 1–4log) in association with relapse could be observed in all patients after a molecular remission or a decreasing or a stable amount of circulating t(14;18) positive lymphoma cells was achieved. The corresponding clinical relapse was diagnosed at a median of 1.5 months later after the first PBMNC sample with a ≥1log CLC increase. We defined a molecular relapse by a 2log CLC increase in 2 consecutive blood samples within 6 months. 6/9 patients fulfilled these criteria when applying this definition. In 3/9 patients the kinetics of increase of CLC was below this arbitrary chosen threshold. In conclusion, increasing numbers of CLC precede clinical relapse in all patients. Only two thirds of relapses would have been classified by our definition of molecular relapse if this study had been performed prospectively. However, sensitive, reproducible and quantitative techniques are now available to detect very low levels of circulating lymphoma cells. A lasting molecular remission is of predictive value for an improved failure-free survival. Hence, the optimal timing and frequency of molecular monitoring remains further unclear especially when aimed at the prediction of clinical relapses and has to be confirmed in large prospective studies.
