Topic 12: Radionuclide therapy and dosimetry Flashcards
Ideal imaging radionuclide characteristics
1) x or gamma ray so it came out of the body without exposing the body to too much radiation 2)we want a particular energy range 3) Half life to get a good image but not be around for the rest of their life.
Ideal therapy radionuclide characteristics.
1) Radiation type has to be alpha, beta. we want to deposit that radiation in that tumour. 2) Energy of radiation : alpha has a short range in tissue, and beta has to be chosen to match size of tumour 3) Half-life: depends on uptake rate, typically days/weeks.
what is Iodine-131 used for?
Hyperthyroidism, and thyroid cancer
Why do we need imaging pre-treatment?
To track tumour staging and spread
Why do we need imaging during treatment?
Estimate of dosimetry - how much dose has been delivered to various organs etc.
Why do we need imaging post-treatment?
To assess treatment effectiveness
What hazards arise of people who have had radionuclide therapy
Radiation exposure of friends/family/children. Contamination Child-bearing Post mortem.
How do we compare dosimetry in x rays and nuclear medicine?
How do you calculate the dose in nuclear medicine?
MIRD method - Medical Internal radiation dose
Cumulated activity
Is defined as : The physiological data obtained by imaging the volume of interest over time.
Activity decays exponentially
- Effective half-life depends on
- Physical half-life of* radionuclide
- -* Biological half-life of radiopharmaceutical
Specific absorbed fraction (phi)
Need to divide into
- Non-penetrating radiation
- betas, electrons, low energy photons
- Almost all energy deposited in the volume containing radionuclide
- Penetrating radiation
- Gammas > 15 keV
- Only a proportion of energy deposited in volume containing radionuclide
What is the S value?
S is the absorbed dose per unit accumulated activity for a particular isotope and organ (units: Grays /MBq.s)
What is the simplified calculation of absorbed dose?
The simplified dose equation becomes:
(see image)
What are the limitations of the MIRD?
- Based on standard models of human anatomy
- Shape, size and location of organs
- Assumption that activity is distributed uniformly in an organ.
- Assumption that energy is deposited uniformly in an organ.
What is the problem with absorbed dose and what is the effective dose?
Absorbed dose calculations provide radiation dose for all organs in the body, with no perception of radiation risk for a study.
Effective dose combines organ dose with the biological effect of the radioactive emissions and the radiobiological sensitivity of the organ.
