Is Late FDG-PET/CT Imaging Better Than Early FDG-PET/CT Imaging to Distinguish Between Malign and Inflammatory Changes in Lymph Nodes in Patients With Muscle Invasive Bladder Cancer Scheduled for Radical Cystectomy?

Background: Approximately 900 patients are diagnosed with invasive bladder cancer (BC) in Denmark each year. Approximately 50% are diagnosed with non-muscle invasive BC (T1) and approximately 50% are diagnosed with a muscle invasive BC (T2-4). Male-female ratio is 75% - 25% and the median age among all diagnosed patients at time of diagnose is 73 years (range 32 - 98) (national data from September 1st 2014 to august 31st 2015).1 One third of all diagnosed patients will be treated with surgical removal of the urinary bladder, radical cystectomy (RC). Additional 14% will receive curative intended radiotherapy. At the Department of Urology, Aarhus University Hospital, approximately 120 patients will be treated with RC due to BC every year.

2-[18F]-Fluoro-2-deoxy-D-glucose (FDG) - positron emission tomography (PET) /computed tomography (CT) is standard for primary staging at Aarhus University Hospital, Denmark, when evaluating patients with muscle invasive BC before radical curative intended treatment.

Evaluation of disease stage is done by investigation for lymph node metastasis and non-local metastases separately.

FDG is a radioactively marked glucose analogue used as a tracer for cells with high glucose uptake. The uptake of glucose in most malignant cells are much greater than the glucose uptake in normal cells. Because of this, FDG can be used as a tracer in the diagnosis of cancers and hereby also BC. However, FDG is not a cancer specific tracer. Cells of the immune system like neutrophil leucocytes, macrophages and lymphocytes also have high glucose uptake and thus, high FDG uptake. Being a glucose analogue, the uptake of FDG in the cells is similar to the uptake of glucose by, the transport protein GLUT-1. Inside the cells FDG is phosphorylated into FDG-6-phosphate by the enzyme hexokinase. FDG-6-phosphate is not metabolized further and is trapped inside the cells. Malignant cells have a tendency to up-regulate both GLUT-1 and hexokinase which results in increase FDG uptake in cancer cells. Hereby optimizing imagine of primary tumour and possible metastasis but also possible inflammatory changes.

After intravenous injection of FDG, the distribution of FDG in the body can be visualized by performing a PET-scan. FDG-PET has a high sensitivity in detecting even small malignant changes but is not very specific to show the anatomical localization of these changes. Thus, the FDG-PET is combined with a CT-scan, which has a high sensitivity in anatomical localization. Hybrid PET/CT scanners are now widely used for imaging.