I 131 therapy of the thyroid is mostly followed by a planar or SPECT scan with a gamma camera and High Energy collimator.
The thought is that a better quality scan with I 123 would be feasible and could be used for Dosimetry thus omitting the need for a I 131 scan after therapy.
Can this be done and/or is there any experience with this already? Please your input
Dear Peter van Loon ,
when googling for that topic, you will see that I-123 is indeed used for imaging applications.
I-123 decay time is quite short (about half a day) and the gamma energies are about 160keV, 440keV and 530keV. So they can escape from the body tissues and one is able to use them for imaging.
Transition of the nucleus to a stable one is performed by electron capture. There are no alphas or betas involved.
https://www.gehealthcare.de/-/jssmedia/facaf0e84b9d4b9495bcf9b89f98ebe9.pdf?la=de-de
I-131 decay time is about 8 days. Gamma energies are about 365keV, 637keV and 284keV. So they also can escape from the body tissue and are used for imgaing, as you mentioned above.
The striking difference between I-123 and I-131 is that the I-131 nucleus transition is performed via ß-decay, i.e. an electron emission having energies of up to about 606 keV.
https://www.gehealthcare.de/-/jssmedia/ffac097e9feb4ac9baac4ffa3fd06639.pdf?la=de-de
This ß-emission is used for Tyroid therapy.
Due to the short range of the betas (sub to very low mm region) the deposition of the energy dose is restricted to the place of deposited/enriched I-131 in the tissue (e.g. the tyroid).
Most of the gammas are able to escape hat region; they do not significantly contribute to the therapy; but still affect the nearby tissue due to their dose deposition. They are used for imaging of the deposited Iodine.
One may use in addiiton I-123 for that imaging purpose, but that makes no sense, because the additional I-123 will not contribute to the therapy (absence of betas) and will only add unwanted dose to the healthy tissue.
Thus I-123 is no surrogate for I-131 in thyroid (ablation) therapy.
For diagnostic and 'dosing' application one may use I-123 instead of I-131; but one has to 'hurry up' because its short half life...
So far my comments;
best regards
G.M.
Dear Gerhard your contribution is very much appreciated (also showing GE Isotope Docs). it seems technically (the imaging) we can use both. Of course I 123 will not be useable for therapy since no Beta's.
A experienced Thyroid specialist (MD) said there is a real discrepancy in distribution of a diagnostic dose of I 123 versus a relatively very high dose of I 131, thus even in diagnostics there is a bias. For now the answer to my question is no way as i understand.
Also, if you're planning on administerng the 123I a few days before the actual therapy with 131I, it usually is the case that the uptake of iodine the first time, for dosimetry, might not be representative of the uptake a few days later, for treatment. Many factors, such as supression of medication to increase iodine uptake, diet changes and so forth contribute to that effect.
Dear Peter,
The usage of I123 mainly for the dosimetry planning for the high dose of I131. The image quality of low dose I123 is more superior compared to low dose I131. Better image quality related to the ability to define the shape of lesion on localized and metastati and with the combination of voxel based dosimetry could mitigate the problem with dosimetry planning using I131.
For post therapy imaging, I think it would be enough only with I131.
Hope this helps
Syahir
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