The Use of FDG-PET in the Initial Staging of Patients with Follicular Lymphoma (FL). A Retrospective Study From the Fondazione Italiana Linfomi

[¹⁸F]fluorodeoxyglucose - Positron Emission Tomography (FDG-PET) is confirmed as a useful functional imaging tool for staging and response assessment in Hodgkin lymphoma and Diffuse Large B-Cell Lymphoma. Despite FL is accounted among FDG-avid lymphomas few studies have been performed to investigate how FDG-PET can be used in initial staging of FL patients. We conducted a retrospective analysis to investigate the role of FDG-PET in the initial staging of FL.

The study was designed as a retrospective unplanned analysis of patients with newly diagnosed FL enrolled in the FOLL05 phase III trial (NCT00774826) and randomized to one of the three study arms (R-CVP. R-CHOP, R-FM). To be included in the study patients should have confirmed eligibility for the FOLL05 trial, have available data on clinical presentation, treatment details and results, and on follow-up. Baseline staging had to be performed with contrast-enhanced Computed Tomography (CT), with CT-PET, and with Bone Marrow (BM) biopsy. For study purposes disease extension at baseline was defined independently for both CT and PET using on local report and interpretation; only for difficult cases images were centralized and reviewed. For each exam nodal sites (NS) were counted according to the FLIPI schema. NS were considered as positive if greater than 1.5 cm in their maximum transverse diameter at CT or, using PET, if FDG avid or if they had disappeared at the end of treatment. Extranodal sites (ENS) were counted on an organ basis and were considered positive at CT in case of nodular involvement or in case of organ enlargement not otherwise justified. Extranodal involvement at PET was considered for sites showing avidity for FDG not justified by conditions other than FL. Conventional Ann Arbor (AA) staging was based on CT scan assessment only. PET and CT scan results were compared using the kappa-statistic measure of interrater agreement (IR), and the level of agreement was defined by the Koch Landis scale.

Among 534 patients enrolled in the FOLL05 trial, 122 cases fulfilled eligibility criteria for this study. All but 2 cases were confirmed as FDG avid at PET scan; these two cases were not used for staging comparison. Median age of patients was 57 years (range 33–74), 33% were older than 60 years, 48% were males. Bone marrow biopsy was positive in 52%. Using CT, AA stage was III-IV in 77% of cases. Fifty-two percent, 36%, and 12% of cases had less 0–4, 5–8, and >8 NS, at CT, respectively. Overall CT scan allowed the identification of 48 ENS in 36 patients (30%); 2 or more ENS were described in 8 patients (6%); most frequent ENS were spleen (52%), liver (10%), skin/soft tissue (8%) and GI tract (6%). Using PET 38%, 37%, and 25% of cases had 0–4, 5–8, >8 NS, respectively. PET allowed the identification of 88 ENS in 55 patients (46%) and 2 or more ENS were found in 17 patients (14%). Most frequent ENS at PET were bone (35%), spleen (30%), GI tract (10%), and skin/soft tissue (7%). Classifying patients according to the number of NS (0–4, 5–8, >8) agreement between CT and PET was fair (IR= 61%, Kappa=0.39). Agreement between CT and PET for ENS (grouped as 0, 1, and >1) was also fair (IR= 63%, Kappa=0.31) and improved to moderate when also details on BM histology were considered (IR 82% K=0.4). When PET was used independently of CT to define AA stage a moderate agreement was achieved with CT (IR=76%, kappa= 0.58): in particular 22 cases (18%) were upstaged with PET while only 6 (5%) were downstaged; seventeen out of 25 (68%) patients were reclassified by PET as stage III-IV from a previous localized stage. Looking at FLIPI, 22%, 41%, and 37% were classified by CT as having score of 0–1, 2, and 3–5, respectively. The use of PET modified CT based FLIPI index in 26% of cases, with a substantial agreement between PET and CT (IR=74%, kappa=0.61). FLIPI2 index is not affected by the use of PET.

The results of this study confirm FL as a FDG-avid disease. The use of PET for the initial staging of patients with FL seems to provide a more accurate definition of disease extent compared with CT. The clinical usefulness of adding PET to the initial staging of FL needs to be further investigated.

Di Raimondo:Celgene: Honoraria, Speakers Bureau; Janssen: Honoraria, Speakers Bureau.