Role of Positron Emission Tomography In Mantle Cell Lymphoma

Fluorodeoxyglucose positron emission tomography (FDG-PET) is a functional imaging tool routinely used for staging and response assessment in aggressive lymphomas. Even if mantle cell lymphoma (MCL) is considered FDG avid, current data on the use of PET in this histotype are scant. We retrospectively analysed 43 PET scans in 22 MCL patients (pts) and compared the results to standard contrast-enhanced computerized tomography (CT) in order to define PET sensitivity and specificity in this rare lymphoma.

Twenty-two pts with biopsy proven diagnosis of MCL underwent PET in different phases of disease for a total of 43 scans. PET scans were co-registrated with a simultaneous CT and compared to standard contrast-enhanced CT performed within 4 weeks. Nineteen PET (44.2%) were performed for disease staging at diagnosis or relapse, while 24 (55.8%) for response assessment. Negative PET were included in the analysis for response assessment only if a previous positive scan was available.

PET and standard CT results were concordant in 16/43 cases (37.2%). Discrepancies were found in 27/43 cases (62.8%) and were grouped in four categories: false negative PET (5 cases), PET positive with a disease extension minor or major than CT (7 and 9 cases, respectively) and PET positive with negative CT (6 cases). At staging PET was concordant with CT in 4/19 (21%) cases. In 5/19 cases (26.3%) PET was negative in all disease sites documented by standard CT. In the remaining 10 cases (52.7%) PET was positive but discordant with disease extension evaluation by CT, determining upstaging and downstaging in 5 scans (26.3%) each. Involvement of gastro-intestinal tract was detected only in 4/19 cases (21%). PET performed for response assessment was concordant with CT in 12/24 cases (50%). In 6/12 concordant cases (25%) both CT and PET were negative; in the remaining 6/12 cases (25%) PET was positive and disease extension evaluation was concordant with standard CT. Six out of 12 discordant cases (25%) presented different disease extension evaluation; in the other six cases standard CT was negative while PET documented minimal residual disease (MRD). PET sensitivity and specificity in the entire cohort were 86.5% and 100%, respectively; positive predictive value was 100%, while negative predictive value was 54.5%. No difference in PET sensitivity was found in nodal and extra-nodal sites. No statistical significant correlation was found between PET sensitivity and proliferation index; nevertheless, no pts with Mib-1>50% presented either false negative PET scan or disease extension underestimated by functional imaging.

This analysis shows a PET sensitivity (86.5%) lower than expected if compared with other aggressive lymphomas. High specificity registered in this cohort is likely to be ascribed to simultaneous CT co-registration, that allows identification of false positive cases. A false negative PET scan at initial staging was registered in 21% of cases. A gastro-intestinal tract involvement, expected to be near universal in advanced stage MCL, was detected by functional imaging only in 21% of pts at disease onset or relapse. Either downstaging or upstaging of disease burden seldom result in different therapeutic approach since localized MCL is a rare entity. Minimal residual disease, detected by functional imaging in half of standard CT negative pts at response assessment, has uncertain role in MCL, since the disease remains incurable and there is no clear evidence that MRD positivity requires treatment. All these findings advise against utilization of functional imaging for routine management of MCL. Considering heterogeneity of FDG-uptake in MCL, PET is likely to be more reliable in pts presenting high proliferation index. Moreover, current data suggest a prognostic value of baseline FDG-avidity. Further investigations are needed to confirm these preliminary data in larger prospective trial, separating MCL from other aggressive histotypes.

No relevant conflicts of interest to declare.