Nuclear Medicine and Internal Dosimetry

Nuclear medicine (NM) / molecular imaging (MI) is a medical speciality that uses radioactive tracers of nanogram levels, in order to diagnose, confirm or exclude various pathologies and diseases in the body or for staging of malignancies and follow-up after therapy. The key thing is that it is the physiological process that is visualized. The radioactive tracer i.e. the radiopharmaceutical, is administered mainly as an intravenous injection or orally or intradermally. Imaging of the radiation emitted from the radionuclide of the radiopharmaceutical is carried out using a gamma camera (planar or SPECT- Single Photon Emission Computed Tomography) or PET (Positron Emission Tomography). In combination with imaging of the morphology using a CT (Computed Tomography) fused images can be produced displaying the physiological process and the morphology in the same image simultaneously.

Even though the amount of the radioactive substance administered to the patient is of nanogram or even lower levels, the activity of the radionuclide and the radiation exposure of the patient and the staff may be of concern. It is very important that the clinical examination or the biomedical study are medically justified and the procedure optimised. In order to do this justification assessment, the radiation exposure and absorbed dose to various organs and tissues have to be thoroughly determined. The organ and tissue doses are calculated based on biokinetic models that are obtained by means of repeated gamma camera or PET-camera measurements of the distribution of the activity in the body and models of various physiological processes.

The Nuclear medicine unit of the Medical radiation physics research group, Malmö, Lund university has a long tradition of performing biokinetic studies and producing dosimetric models. Already in the early 1980ies the project “Radiation Dose to Patients from Radiopharmaceuticals” begun and has since then developed and expanded with the support from the Swedish Radiation Protection Authority (SSM) and the International Commission on Radiological Protection (ICRP).

We have in addition to performing biokinetic studies of different radiopharmaceuticals on patients and healthy volunteers, developed and optimised activity quantification methods as well as developed dosimetric models based on Monte Carlo simulations and compartmental modelling. 

So far, biokinetic studies have been performed on the following radiopharmaceuticals and dosimetric models been produced: Tc-99m MIBI, Tc-99m octreotide, C-14 urea, C-14 triolein, C-14 glucose, C-14 glycocholic acid, I-123 ioflupane and F-18 choline.

The following doctoral theses has been produced by our group:

• Martin Andersson, “Radiation dose to patients in diagnostic nuclear medicine. Implementation of   improved anatomical and biokinetic models for assessment of organ absorbed dose and effective dose.” 2017,  (PhD Thesis). Department of Medical Radiation physics, University of Lund, Malmö University Hospital, Malmö
• Marie Sydoff, “Quantification methods for clinical studies in nuclear medicine - Applications in AMS, PET/CT and SPECT/CT”, 2013, (PhD Thesis). Department of Medical Radiation physics, University of Lund, Malmö University Hospital, Malmö
• Marcus Söderberg, “Image quality optimisation and dose management in CT, SPECT/CT and PET/CT”, 2012,  (PhD Thesis). Department of Medical Radiation physics, University of Lund, Malmö University Hospital, Malmö
• Mikael Gunnarsson, “Biokinetics and radiation dosimetry of 14C-labelled triolein, urea, glycocholic acid and xylos in man. Studies related to nuclear medicine “breath tests” using accelerator mass spectrometry”, 2002, (PhD Thesis). Department of Medical Radiation physics, University of Lund, Malmö University Hospital, Malmö
• Sigrid Leide Svegborn, “Radiation exposure of the patient in diagnostic nuclear medicine. Experimental studies of the biokinetics of 111In-DTPA-D-Phe1-octreotide, 99mTc-MIBI , 14C-triolein, and 14C-urea, and development of dosimetric models.”, 1999 (PhD Thesis). Department of Medical Radiation physics, University of Lund, Malmö University Hospital, Malmö

Right now, the research unit is focusing on biokinetic studies and dosimetric models of new radiopharmaceuticals such as F-18 flutemetamol, clinically used for the visualisation of Alzheimer´s disease with PET/CT. 

Continuously, the optimisation of nuclear medicine procedures on breastfeeding patients and pregnant patients are carried out in order to protect the child and the fetus/embryo from unwanted exposure.  By collecting breast milk samples from breastfeeding patients and measuring the activity in it after an administration of a radioactive substance to the patient, the organ and tissue doses as well as the effective dose to an infant who ingests the breast milk is determined. Based on this, breastfeeding interruption intervals can be recommended.

In addition to this, several projects regarding the radiation exposure of nuclear medicine staff are on-going. Equivalent doses to fingers and eye lenses are determined by various methods and optimisation on the working techniques are studied.

The SSM Dose Catalogue and the ICRP Task Group 36

Our research group continually updates radiation dose estimates for patients undergoing nuclear medicine examinations. This work is supported by the Swedish Radiation Safety Authority (SSM) and is a prerequisite for our long-term commitment to the International Commission on Radiological Protection (ICRP). This work has resulted in a Swedish dose catalogue of about 210 different radio-pharmaceuticals containing biokinetic data and models as well as extensive tables of absorbed doses to various organs and tissues and effective doses. These data are available through the SSM web page and the ICRP Publication 53 (ICRP 1988), ICRP Publication 62 (ICRP 1993), ICRP Publication 80 (ICRP 1998) and ICRP Publication 106 (ICRP 2008). The year 2015 ICRP published “A Compendium of Current Information Related to Frequently Used Substances” in ICRP Publication 128 (ICRP 2015). Currently, the main work is related to the biokinetic and dosimetry for 11C-PIB, 11C-choline, 18F-FMISO, and 68Ga-DOTA-TOC, -NOC, -TATE as well as producing a revised version of the ICRP publication 128.

Included in this work is the making on recommendations for optimal collection of biokinetic data and investigations of parameters that affect the absorbed dose to the bladder from radioactive substances.

The recently established ICRP computational framework for internal dosimetry will be implemented into nuclear medicine and the so-called reference dosimetry, by our group. This work consists of improvement of the anatomical and biokinetic models for assessment of organ absorbed doses and effective dose. In the transition of the ICRP framework for nuclear medicine the biokinetic representation will shift from descriptive models to more physiological realistic compartmental models. The anatomical models are mathematical models and are used to estimate the energy absorbed in the body from each radioactive decay. The anatomical models have been updated from stylised mathematical models described by linear and quadratic equations to voxelised adult phantoms and non-uniform rational B- spline preadult phantoms. New sets of phantoms creating a more realistic representation of the ICRP anatomical and physiological data for use in radiological protection is produced. Currently, focus lies on the development and implementation dosimetry software IDAC-Dose for absorbed dose and effective dose calculations as well as on the revised version of ICRP publication 128. This work is performed in collaboration with researches from several different prestigious institutions from Umeå and Malmö/Lund in Sweden, Munich in Germany, Seoul in Republic of Korea, Osaka in Japan, Miami, Florida in USA and Bogotá in Colombia.