Nowadays PSMA PET-CT scanning is showing promising results for detection and staging of Prostate cancer.68Ga- PSMA and 18F-PSMA are commonly used for scanning.
Both have their pros and cons.
The recent inclusion of 99mTc-PSMA has created the opportunity of doing a scan with the help of SPECT-CT.
Currently,
to label PSMA for diagnostic purpose,
three are three radionuclides available,
68Ga,
18F and 99mTc with their pros and cons.
Radioisotope Gallium-68 is a choice of isotope for labelling with PSMA due to few specific properties; which includes availability in generator form,
long half-life of parent radionuclide enables year-long availability of generator and the possibility of 2-3 elutions in a single day with 8 hrs intervals.
The labelling procedure with 68Ga is quite simple which takes 10 to 15 minutes,
and also the Radiochemical purity of the product is in a range of 95 to 99.9% which is still dependent on the type and make of generator.
Parent radionuclide 68Ge breakthrough in final elution is very low,
which results in low radiation exposure to patients of a long life radionuclide and help to achieve good image quality.
Emission of higher energy photon by 68Ga results in higher radiation exposure to the patient as well as for the operator.
Correspondingly,
it also affects the image quality by dropping the spatial resolution.The short half-life of 67.5 minutes limits delayed imaging as well as possibilities of centralised radiopharmacy production and supply.
Radioisotope 18F is another choice for labelling PSMA for prostate cancer scanning.
It is cyclotron produced radioisotope which enables to get a sufficient amount of activity and makes possible to produce multiple doses in a single batch; which allows commercial supply to nearby centres.
The half-life of 18F is 109.8 minutes,
and it is quite better for diagnosis purpose,
and it also allows delay imaging.
Positron energy of 0.65 MeV allows a reasonable spatial resolution hence image quality is excellent as compare to 68Ga -PSMA.
Radiolabeling of 18F with PSMA is either direct substitution reaction (one step reaction) or a two-step procedure; it is quite complicated as compared to 68Ga-PSMA.
Time of synthesis depends upon the procedure,
for direct substitution reaction it is 35 to 40 minutes,
and for two-step synthesis procedure it is about 80 minutes.
Various synthesis modules are available in the market for the purpose(Cardinale et al.,
2017).
As per Cardinale (2017),
there are a lot of good things associated with 18F-PSMA,
and we can come out from limitations of 68Ga- PSMA,
but the cost of the cyclotron is very high,
so It is not possible for everyone to have a cyclotron on site.
Radioisotope 99mTc labelled with PSMA has been a new inclusion recently.
Its looking quite exciting as 99mTc is a workhorse of the Nuclear Medicine and most of the time available onsite.
The half-life of 99mTc is 6 hrs,
and photon energy is 140 keV.
Both these factors are quite favourable, as long half-life is suitable for the imaging purposes while photon energy is quite favourable with less chances of radiation burden for patients as well as for the opertor.
It can be labelled at the hospital radiopharmacy,
but it takes quite a long time in comparison of the other labelling method of PSMA with 68Ga and 18F.
Also,
there are multiple factors which can affect the radiochemical purity of 99mTc with PSMA like high 99Mo Breakthrough and possibility of Aluminium impurity in the eluted 99mTc.
A study was done by Lawal,
Ankrah,
Mokgoro,
Vorster,
Maes & Sathekge,2017 of comparing the 99mTc and 68Ga labelled PSMA,
and it was found that 99mTc has less sensitivity of picking the lesions in comparison of the 68Ga labelled PSMA.
Radioisotope-labelled 99mTc-PSMA can be an option for prostate imaging if 68Ga is not available (Lawal et al.,
2017).