Nuclear Medicine · Radiopharmaceuticals

Radiopharmaceuticals and Theranostics One-Liners

High-yield one-liners on diagnostic radiopharmaceuticals, therapeutic radionuclides and the theranostic paradigm linking paired imaging and therapy agents.

Rapid-revision one-liners covering key diagnostic and therapeutic radiopharmaceuticals, the theranostic concept, radionuclide selection principles and safety considerations.

Tc-99m generator
Tc-99m is eluted from a Mo-99/Tc-99m generator; Mo-99 has a 66-hour half-life and Tc-99m a 6-hour half-life, making daily elution practical for clinical use.
Tc-99m photon energy
Tc-99m emits a 140 keV gamma photon ideal for gamma camera detection, with no significant beta emission, minimising patient radiation dose for diagnostic imaging.
Tc-99m MDP bone imaging
Tc-99m methylene diphosphonate (MDP) localises by chemisorption to hydroxyapatite crystals; uptake reflects osteoblastic activity and regional blood flow.
Tc-99m sestamibi mechanism
Tc-99m sestamibi is a lipophilic cation taken up by mitochondria-negative transmembrane potential; used in myocardial perfusion, parathyroid and breast imaging.
Tc-99m macroaggregated albumin
Tc-99m MAA particles (10-90 micrometres) lodge in pulmonary arterioles for perfusion imaging; contraindicated in severe pulmonary hypertension and right-to-left shunts.
I-131 properties
I-131 emits both beta particles (therapy, mean range 0.4 mm) and 364 keV gamma photons (imaging); half-life 8.02 days; used for thyroid ablation and differentiated thyroid cancer therapy.
I-131 versus I-123 for imaging
I-123 (159 keV, 13-hour half-life, no beta emission) provides superior image quality with lower radiation dose for thyroid imaging; I-131 is preferred when concurrent therapy is intended.
Ga-68 properties
Ga-68 is a positron emitter (half-life 68 minutes) produced from a Ge-68/Ga-68 generator; ideal for PET imaging of neuroendocrine tumours (DOTATATE) and prostate cancer (PSMA).
Lu-177 properties
Lu-177 emits medium-energy beta particles (max range 2 mm) for therapy and 113/208 keV gamma photons for post-therapy imaging; half-life 6.7 days.
Lu-177 PSMA therapy
Lu-177 PSMA-617 (Pluvicto) is indicated for PSMA-positive metastatic castration-resistant prostate cancer after ARPI and taxane chemotherapy, based on the VISION trial showing improved overall survival.
Lu-177 DOTATATE therapy
Lu-177 DOTATATE (Lutathera) is peptide receptor radionuclide therapy (PRRT) for somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumours, showing improved progression-free survival in the NETTER-1 trial.
Theranostic pair principle
A theranostic pair uses the same molecular target with a diagnostic radionuclide for selection and dosimetry (e.g., Ga-68) and a therapeutic radionuclide for treatment (e.g., Lu-177), enabling personalised medicine.
Ga-68/Lu-177 PSMA pair
Ga-68 PSMA-11 PET/CT selects patients with sufficient PSMA expression; Lu-177 PSMA-617 delivers targeted beta therapy to the same PSMA-expressing lesions.
Ga-68/Lu-177 DOTATATE pair
Ga-68 DOTATATE PET/CT confirms somatostatin receptor expression and excludes non-avid disease before Lu-177 DOTATATE PRRT, ensuring therapeutic efficacy.
Y-90 properties
Y-90 is a pure high-energy beta emitter (max range 11 mm, half-life 64 hours) used in radioembolisation for hepatic malignancies; bremsstrahlung imaging or PET (minor positron branch) for post-treatment verification.
Ra-223 dichloride
Ra-223 is an alpha emitter mimicking calcium, targeting bone metastases in castration-resistant prostate cancer; alpha particles cause dense ionisation over short range (50-100 micrometres), sparing marrow.
Alpha versus beta therapy
Alpha emitters (Ra-223, Ac-225) deliver high linear energy transfer over 2-10 cell diameters with less collateral damage; beta emitters (Lu-177, I-131, Y-90) have longer range and cross-fire effect.
Renal protection in PRRT
Amino acid infusion (lysine and arginine) is administered during Lu-177 DOTATATE therapy to reduce renal tubular reabsorption and radiation dose to kidneys.
Radiation safety post-therapy
Patients receiving I-131 or Lu-177 therapy require radiation safety instructions; release criteria depend on residual dose rate and local regulatory limits, typically below 7 microSv/hour at 1 metre.
Ac-225 PSMA
Ac-225 PSMA is an emerging alpha-emitting theranostic agent showing efficacy in Lu-177 PSMA-refractory patients, though xerostomia is a significant dose-limiting toxicity.

Caution

Radiopharmaceutical indications, dosing and release criteria vary by jurisdiction, institutional protocol and current regulatory approval status. Always verify against the latest SNMMI, EANM or national regulatory guidance.

Exam pearl

In viva, always name the theranostic pair explicitly (e.g., Ga-68 PSMA-11 for selection, Lu-177 PSMA-617 for therapy) and state the trial evidence supporting therapeutic indications.

Viva questions

What is the theranostic principle?
The theranostic principle uses the same molecular target with a diagnostic radionuclide for patient selection and dosimetry, and a matched therapeutic radionuclide for targeted treatment, enabling personalised radionuclide therapy.
Compare alpha and beta emitters used in radionuclide therapy.
Alpha emitters such as Ra-223 and Ac-225 deliver high linear energy transfer over 50-100 micrometres, causing dense DNA damage with minimal collateral injury. Beta emitters such as Lu-177 and I-131 have longer ranges of 1-11 millimetres, providing a cross-fire effect useful for heterogeneous tumour uptake.
Why is amino acid infusion given during Lu-177 DOTATATE therapy?
Positively charged amino acids (lysine and arginine) competitively inhibit renal tubular reabsorption of the radiolabelled peptide, reducing radiation dose to the renal cortex and protecting against nephrotoxicity.
What trial established Lu-177 PSMA-617 for metastatic castration-resistant prostate cancer?
The VISION trial demonstrated improved overall survival and radiographic progression-free survival with Lu-177 PSMA-617 plus best standard of care versus best standard of care alone in PSMA-positive mCRPC after prior ARPI and taxane therapy.
Why is I-123 preferred over I-131 for diagnostic thyroid imaging?
I-123 emits a 159 keV gamma photon ideal for imaging with no significant beta emission, providing superior image quality and substantially lower radiation dose. I-131 has higher energy gamma emission and beta particles, making it less suitable for pure diagnostic imaging.

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