F]fluorothymidine(FLT) another radiofluorinated thymidine analog. FLT is a marker taken up bythe cells and phosphorylated by thymidine kinase 1 which is an enzyme closelytied to cellular proliferation. FLT PET imaging provides a profound map of thelocal growth fraction of tumour cells and its SUV appears to be a promisingbiomarker for proliferation and cell kinetics in general (missing citation).Some studies found that both pre-treatment cell kinetics and pre-treatment cellgrowth are not predictive of fractionated radiotherapy outcome puttingtreatment planning based on proliferation biomarkers acquired at baseline intoquestion (missing citation); (missing citation). However the change in FLTstatus between measurements at baseline and after the start of radiotherapydoes indeed seem to be a biological predictor of therapy outcome making theproliferation rate during therapy a more important criterion than theproliferation at baseline (missing citation). Therefore additional imageacquisition during treatment is crucial for an adaptive radiotherapy based ontumour proliferation. Studies also showed a statistically significant relationbetween the epidermal growth factor receptor (EGFR) and a favorable therapyoutcome for patients with head and neck squamous-cell carcinoma (HNSCC) . Theexpression of the EGFR can be PET imaged using radiolabeled tyrosine kinaseinhibitors or antibodies against the extra-cellular domain of the receptor(missing citation).