title: Minor changes in effective half-life during fractionated 177Lu-octreotate therapy.
abstract: OBJECTIVE
  Fractionated (177)Lu-DOTA-octreotate therapy has been reported to be an effective treatment option for patients with generalized neuroendocrine tumors. In our clinic, full individual dosimetry is performed during the first therapy cycle, while dosimetry at later cycles is based on the 24 h uptake measurement assuming an unchanged effective half-life. Our aim was to evaluate this assumption and the variation in the 24 h uptake during therapy.
  
  
  METHODS
  Thirty patients, 13 women and 17 men, were included in the study.
  
  
  METHODS
  During the first therapy cycle the (177)Lu-concentration was measured with SPECT/CT over the abdomen at 24 h, 96 h and 168 h after infusion. The effective half-life was determined for the kidneys, liver and spleen. The procedure was repeated at cycle 4 or 5.
  
  
  RESULTS
  The median ratio between the effective half-lives of the latter and the first cycle was 0.97 and 1.01 for the right and left kidney, with a range of 0.89-1.01 (1st-3rd quartile) and 0.93-1.05, respectively.
  
  
  CONCLUSIONS
  The mean value of the ratios was slightly lower than one, indicating a tendency towards increased activity elimination during therapy. In individual patients, significant changes were found for all organs, often when a large tumor burden reduction occurred during treatment. Possible contributing factors appeared to be larger amounts of non-tumor bound tracer, improved organ function (kidneys), decrease of vessel obstruction (spleen), less scatter from large tumors and reduction of small metastases (liver and spleen).
  
  
  CONCLUSIONS
  With most patients it is safe to estimate absorbed doses to kidneys, liver and spleen from 24 h activity concentration assuming an unchanged effective half-life during therapy. Patients with risk factors for kidney dysfunction need to be monitored in more detail. Simplified dosimetry based on the assumption of unchanged effective half-life can function as guidance to the number of therapy cycles an individual patient can tolerate.
authors.affiliation: Department of Radiology, Oncology and Radiation Science, Section of Nuclear Medicine, Uppsala University Hospital, Sweden.
authors.forename: Ulrike
authors.initials: U
authors.lastname: Garske
authors.forename: Mattias
authors.initials: M
authors.lastname: Sandström
authors.forename: Silvia
authors.initials: S
authors.lastname: Johansson
authors.forename: Anders
authors.initials: A
authors.lastname: Sundin
authors.forename: Dan
authors.initials: D
authors.lastname: Granberg
authors.forename: Barbro
authors.initials: B
authors.lastname: Eriksson
authors.forename: Hans
authors.initials: H
authors.lastname: Lundqvist
mesh_terms: D019583:Dose Fractionation, Radiation; D005260:Female; D006207:Half-Life; D006801:Humans; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008297:Male; D064847:Multimodal Imaging; D018358:Neuroendocrine Tumors / Q000378:metabolism / Q000532:radiotherapy*; D015282:Octreotide / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000627:therapeutic use; D058958:Organs at Risk / Q000000981:diagnostic imaging; D049268:Positron-Emission Tomography; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics / Q000627:therapeutic use*; D017481:Receptors, Somatostatin / Q000378:metabolism; D013154:Spleen / Q000378:metabolism; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome
chemical_list: C554548:177Lu-octreotate; D019275:Radiopharmaceuticals; D017481:Receptors, Somatostatin; D015282:Octreotide