The Mystery of FDG-PET Avidity In Follicular Lymphoma. Does Microenvironment Play the Key Role?

¹⁸F-FDG-positron emission tomography (PET) is a powerful tool for imaging of various lymphomas. Follicular lymphoma (FL) is the most common indolent lymphoma with usually slow progressive course, where growing is dependent on accumulation of cells with defect in apoptosis. In spite of its indolent nature, FL belongs to tumours with high ¹⁸F-FDG avidity. Semi-quantitative evaluation of PET activity can be defined as a standard uptake value (SUV max). Up to now, it is unclear, why FL is so highly ¹⁸F-FDG avid and whether SUV max can be of some prognostic value.

We tried to correlate PET activity defined as SUV max with grade, tumor growth activity (Ki67), and prognosis using progression free survival (PFS). FL has, however, generally low proliferative activity. Therefore, other cells are expected to be responsible for ¹⁸F-FDG avidity in this disease. We selected suitable cells of “microenvironment”, and tried to correlate their numbers wit SUV max.

Patients with newly diagnosed FL having PET with defined SUV max were included in this retrospective study. Diagnosis of FL including grading (grade 1–3) was confirmed by experienced hematopathologist on original lymph node samples and proliferation activity was evaluated by immunostaining with Ki67. In 25 cases, material was available and tissue microarrays (TMA) were done; populations of CD34 (endothelial marker), CD3 (global T-lymphocytes), CD8 (cytotoxic lymphocytes), FOXP3 (T-regulatory lymphocytes), CD23 (follicular dendritic cells) and CD68 (lymphoma associated macrophages) were evaluated. Ki67 as well as numbers of non-malignant elements were given in % of positive cells. Lymph node samples chosen for TMA were taken in our institution only, to avoid interlaboratory variability.

Data from 73 FL patients of stage II-IV were analyzed. Median age was 57 (31-76) years, and grading distribution was as follows: grade 1, 2 and 3 were observed in 40, 21 and 12 patients. Median follow up of the whole group was 45 months (1-73). PET activity defined as SUV max had median 7,8 (0-22,2), proliferation activity measured by Ki67 (n=53) ranged between 1,5-80% with median 25%. SUV max did not seem to correlate with grade or Ki67; moreover, SUV max did not predict course of follicular lymphoma in terms of PFS. No differences in gender, age or FLIPI were observed between subgroups with high and low SUV max. Additionally, 25 samples were available for TMA and subpopulations of microenvironment were evaluated. Although the number of samples was limited, we observed a tendency of positive correlation between amount of CD34+ cells (angiogenesis marker) and CD68+ (lymphoma associated macrophages) with SUV max (using cut off 8,0). Unfortunately, statistical significance could not be reached in these subpopulations (p 0.11 and 0.13). Surprisingly, we could identify strong negative correlation between number of interfolicullary localized CD8+ cells (cytotoxic lymphocytes) and SUV max using cut off 8 and 9 (p 0.02 and 0.003).

Our data support the assumption, that proliferation activity, although various in FL, seems have nothing to do with ¹⁸F-FDG avidity. On the contrary, we observed strong correlation of CD8+ cells and SUV max and certain tendency that cells involved in angiogenesis (CD34+) and inflammatory response (CD68+) may influence SUV max as well. Based on our pilot results, we suggest that the microenvironment gives global ¹⁸F-FDG activity in FL. The microenvironment, however, is a mixture of various cells, and that could be the reason why SUV max does not seem to be a prognostic tool in this disease. Our preliminary results require confirmation by further research.

No relevant conflicts of interest to declare.