Estimating the amount of FDG uptake in physiological tissues

The Association Between Liver and Tumor [18F]FDG Uptake in Patients with Diffuse Large B Cell Lymphoma During Chemotherapy

PURPOSE

The aim of this study was to explore the association between liver, mediastinum and tumor 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) uptake during chemotherapy in diffuse large B cell lymphoma (DLBCL).

PROCEDURES

Nineteen patients with proven DLBCL underwent positron emission tomography (PET)/X-ray computed tomography scan at baseline, 1 week and 2 cycles after rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone (R-CHOP) therapy, and again after chemotherapy completion. The mean and maximal standardized uptake value (SUVmean and SUVmax) of the liver and mediastinum were measured and correlated with the tumor SUVmax, SUVsum, whole-body metabolic tumor volume (MTVwb), and total lesion glycolysis (TLG).

RESULTS

At baseline, both the liver and mediastinum SUVmean and SUVmax correlated inversely with the tumor MTVwb or TLG (p < 0.01 or 0.001). The liver SUVmean and SUVmax increased significantly after 1 week of R-CHOP therapy and remained at the high level until chemotherapy completion. The mediastinum SUVmean and SUVmax remained stable during chemotherapy. The tumor SUVmax, SUVsum, MTVwb, and TLG decreased significantly after 1 week of R-CHOP therapy. The change of the liver SUVmean correlated inversely with the change of tumor MTVwb and TLG after 1 week of chemotherapy (p < 0.05, respectively). The intersubject variability of liver and mediastinum [¹⁸F]FDG uptake ranged from 11 to 26 %.

CONCLUSIONS

The liver [¹⁸F]FDG uptake increased significantly after R-CHOP therapy. One of the possible reasons is the distribution of a greater fraction of the tracer to healthy tissues rather than tumor after effective chemotherapy. The variability of the liver [¹⁸F]FDG uptake during chemotherapy might affect the visual analysis of the interim PET scan and this needs to be confirmed in future studies with a large patient cohort. In addition, the intersubject variability of the liver and mediastinum [¹⁸F]FDG uptake should be considered.

Determination of the Input Function at the Entry of the Tissue of Interest and Its Impact on PET Kinetic Modeling Parameters

Quantitative positron emission tomography (PET) imaging is employed with several measurement protocols all relying on the a priori determination of the input function (IF). The standard technique to determine IF is by blood sampling. However, a unique IF determined in a subject for a given PET study, either defined by sampling or in the images, and commonly utilized for all analyzed tissues in that study equally at rest and during interventions, is expected to provoke biases in the rate constants and in tissue blood volume. The determination of a specific IF at the site of the tissue to be analyzed enhances PET accuracy and renders PET imaging less invasive.

The total amount of uptake may affect the input function: a theoretic approach about 18F-FDG PET imaging

INTRODUCTION

The aim of this theoretic approach is to establish an analytical relationship between the total amount of uptake (TAU), either pathological or physiological, and the time constants "αi" that describe the time decay of the tracer input function (IF). The proposed approach uses parameters of a (population-based) arterial plasma IF for 18 F-FDG PET imaging.

METHODS

A previously published formula provides an estimate of the ratio "p" of 18 F-FDG molecules that are irreversibly trapped in an 18 F-FDG-positive tissue during a whole PET examination, to the number of injected molecules. Then, the change in the magnitude of the IF time constants is derived, involving a corrective procedure.

RESULTS

Trapping of injected tracer molecules affects each αi that is increased by a factor of 1/(1-p), with p ranging between zero and, theoretically, less than 1. This result is illustrated in a patient showing an intense uptake in the mediastinum at initial staging of a diffuse large B-cell lymphoma (DLBCL).

CONCLUSIONS

An analytical relationship between TAU and the IF time constants is available in 18 F-FDG PET imaging, showing that TAU may actually affect the IF.