Does anyone know what kind of radiopharmaceuticals delivers more absorbed dose (mSv/MBq) to the patients? SEPT or PET drugs?
PET radionuclides deliver more dose than SPECT mainly due to higher gamma energies but also emission of the positrons (which actually deliver the biggest percentage in a PET scan compared to the photons). For example for FDG the dose per MBq is 0.019 mSv/MBq while for Tc-99-DTPA is 0.008 mSv/MBq. The dose equivalent is specific to the tracer used. So the dose for Tc99-DTPA is different than the dose for Tc-99-MAG3(0.01 mSv/MBq).
Dear Konstantinos G. Zeimpekis,
Thank you for your answer, in terms of higher energy you are right but as you know higher energies have lower interaction (absorbed or cross section) with the tissues therefore PET tracer should deliver lower dose to the total body + their half lives are lower than SPECT tracer
This is partially true. As i said the highest percentage of dose delivery in a PET scan is due to the positrons. The positrons are charged particles so their Linear Energy Transfer is higher, meaning higher dose in a very localised area.That being said, i am just giving the general idea of dose equivalent which takes all the parameters into account (from half-life to biodistribution and biokinetics) and PET radionuclides have higher dose equivalent than the SPECT tracers. 511 keV they penetrate of course easily the body tissues but they give rise to more scattering - secondary electrons cascade - effects before reaching the detector. An analogous is done in CT : if you use 120 kV instead of 100 kV the dose to the patient will be around 44% higher.
Dear Konstantino,
What an Interesting data!
Do you have any data about 67Ga vs 68Ga?
bests
67 Gallium Citrate dose equivalent is 0.12 mSv/MBq. 68-Ga-DOTATOC is 0.021 mSv/MBq. You can also check the ICRP publication 80 or even better articles on JNM that have to do with PET tracers dosimetry. Once again the dose equivalent isn't fixed for a radionuclide but depends on the ligament that you are engaging it to.
in this paper:
Exendin-4–BasedRadiopharmaceuticalsfor
GlucagonlikePeptide-1ReceptorPET/CTand
SPECT/CT
111Indium absorbed doses were 5 times higher than 68Ga
http://jnm.snmjournals.org/content/51/7/1059.full
Yes, Indium 111 is one (if not the only) of the exceptions (for example scans with In-111 like octreoscan for somatostatin receptor scintigraphy). This has to do with the longer half-life of Indium 111 (2.8 days) and lot of energies emitted during decay (171 keV and 245 keV). In addition it is not excreted that fast by the body compared to other radionuclides so retention time within the body is longer.
Some data is available in Table 5
Fred A. Mettler, Jr, etal. Effective Doses in Radiology and Diagnostic Nuclear Medicine:
Radiology 2008; 248:254 –263
DOI: http://dx.doi.org/10.1148/radiol.2481071451
As pointed out above: It is not only the radionuclide and its decay scheme/branch and half-life that matters it also depends on the biological half-life of the radiopharmaceutical and the biodistribution of it.
The generic answer to this very broad question is: "Yes"
That's an excellent remark. This is the reality in Nuclear Medicine where we use hybrid imaging. CT is always needed for attenuation correction. CT dose optimisation should be considered (even very low dose CT can extract the desired attenuation maps). CT Iterative Reconstruction can play a significant role in minimising the dose as well.
A very useful discussion. It is also worth noting that the dose will be dependent on the administered dose of the radiopharmaceutical that is required for effective imaging. This clearly depends on its PK/PD profile (how fast it's cleared, time to accumulate in ROI etc). With the push from Regulatory bodies to minimise dose, this is under more scrutiny and newer agents being developed, along with better imaging algorithms, should reduce dose, particularly in PET.
Probably the best resource is RADAR (http://www.doseinfo-radar.com/RADARLit.html). Like Ian has stated, the absorbed dose from a particular radiopharmaceutical depends on many factors. It is difficult to say whether SPECT tracers or PET tracers expose a patient to more or less absorbed dose. Some SPECT and PET tracers are 'dirtier' than others which mean that some of the emitted radiation is unused for imaging purposes and instead deposits absorbed dose. Lastly, before comparing SPECT and PET tracers, it is important to understand the 'critical organ' which limits the 'therapeutic ratio'. It is foreseeable that some tracers are limited in administered activity to avoid normal tissue toxicity or decrease the chance for a secondary cancer.
Informative discussion. Thank you all for contributing. In the context of an imaging application, the imaging equipment should also be factored in (as Ian and Joseph eluded to). Because of PET's superior counting efficiency (counts per sec per MBq), much less activity is required (on the order of 1/100 to 1/10th activity). Both in PET and SPECT, advances in image reconstruction (e.g. iterative versus filtered-back-projection) can be leveraged to decrease the injected activity without compromising image quality (1/4 to 1/2 activity). Cardiac dedicated SPECT cameras (for example) have higher counting efficiency than traditional planar cameras, and can also be leveraged to decrease activity (1/4-1/10 activity). In clinical practice however, these advances are commonly leveraged to improve image quality rather than dose reduction.
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