Radionuclide Therapy

Radionuclides - Atomic Healers

• Deliver targeted cytotoxic radiation to diseased cells.
• Key Emission Types for Therapy:
  • $\beta^-$ particles: Moderate LET, range mm-cm (e.g., $^{131}$I, $^{90}$Y, $^{177}$Lu).
  • $\alpha$ particles: High LET, short range µm (e.g., $^{223}$Ra, $^{211}$At, $^{225}$Ac). Dense ionization.
  • Auger electrons: Very short range, high LET locally (e.g., $^{125}$I).
• Ideal Properties:
  • Physical: Optimal half-life, particulate emission, high LET.
  • Chemical: Stable chelation/conjugation.
  • Biological: High target uptake, rapid non-target clearance.

⭐ Therapeutic radionuclides primarily utilize beta-minus particles, alpha particles, or Auger electrons for cytotoxic effects due to their high Linear Energy Transfer (LET) and short path length.

Thyroid Therapies - Iodine's Impact

• Primary Radionuclide: $^{131}I$ (Iodine-131) - $\beta^-$ (therapeutic) & $\gamma$ (imaging) emitter.
• Mechanism: Selective uptake by thyroid cells; $\beta^-$ particles induce cytotoxicity.
• Key Indications & Doses:
  • Hyperthyroidism (Graves', Toxic Multinodular Goiter, Toxic Adenoma): 5-29 mCi.
  • Differentiated Thyroid Cancer (DTC - Papillary, Follicular):
    • Remnant Ablation: 30-100 mCi.
    • Metastatic Disease: 100-200 mCi.
• DTC Therapy Essentials:
  • Low Iodine Diet (LID): 1-2 weeks prior.
  • TSH Stimulation: Target TSH >30 $\mu$IU/mL (rhTSH or Thyroid Hormone Withdrawal).

⭐ For $^{131}I$ therapy in differentiated thyroid cancer, TSH stimulation (either rhTSH or thyroid hormone withdrawal) to levels typically >30 $\mu$IU/mL is crucial for enhancing uptake in thyroid remnants or metastatic tissue.

• Contraindications: Absolute: Pregnancy, breastfeeding.
• Common Side Effects: Radiation thyroiditis, sialadenitis, xerostomia.

NETs & Adrenals - Receptor Raiders

• PRRT (Peptide Receptor Radionuclide Therapy)
  • Targets: Somatostatin Receptors (SSTRs, esp. SSTR2)
  • Agents:
    • $^{177}$Lu-DOTATATE
    • $^{90}$Y-DOTATOC
  • Indications: SSTR+ tumors: GEP-NETs, pheochromocytoma, paraganglioma.
  • Mechanism: Receptor binding → internalization → β-particle emission → cell death.
  • 📌 $^{177}$Lu: Longer half-life (vs $^{90}$Y), preferred for larger tumors, less myelosuppression.

  ⭐ Peptide Receptor Radionuclide Therapy (PRRT) with $^{177}$Lu-DOTATATE is indicated for unresectable or metastatic, progressive, well-differentiated, somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs).

• $^{131}$I-MIBG Therapy
  • Targets: Norepinephrine Transporter (NET) system.
  • Indications: Malignant/metastatic pheochromocytoma, paraganglioma, neuroblastoma.
  • Mechanism: Uptake by NET → β & γ emission → cell death.
  • ⚠️ Thyroid blockade (e.g., Lugol's iodine) essential.

Bone Metastases - Skeletal Soothers


• Goal: Pain palliation in osteoblastic bone metastases.
• Mechanism: Radiopharmaceuticals target areas of high bone turnover (hydroxyapatite binding).
• Beta-Emitters ($\beta^-$):
  • $^{89}$Sr-chloride (Strontium-89): Pure $\beta^-$, T½ 50.5 days.
  • $^{153}$Sm-EDTMP (Samarium-153): $\beta^-$ & $\gamma$ (imaging), T½ 1.9 days. Dose: 1.0 mCi/kg (37 MBq/kg).
  • $^{186}$Re-HEDP (Rhenium-186): $\beta^-$ & $\gamma$.
• Alpha-Emitter ($\alpha$):
  • $^{223}$Ra-dichloride (Radium-223, Xofigo®): For CRPC with bone mets.

⭐ $^{223}$Ra-dichloride, an alpha-emitter, is unique among bone-seeking radiopharmaceuticals as it shows improved overall survival in patients with castration-resistant prostate cancer (CRPC) and symptomatic bone metastases, with no visceral metastases.

High‑Yield Points - ⚡ Biggest Takeaways

• Iodine-131 (I-131): Key for thyroid remnant ablation in differentiated thyroid cancer and treating hyperthyroidism.
• Lutetium-177 (Lu-177) DOTATATE: Targets somatostatin receptor-positive neuroendocrine tumors (NETs).
• Radium-223 (Ra-223): Alpha-emitter for bone metastases in castration-resistant prostate cancer (CRPC).
• Yttrium-90 (Y-90) microspheres: Used in radioembolization (TARE) for hepatocellular carcinoma (HCC).
• Samarium-153 / Strontium-89: Provide palliative pain relief from widespread bone metastases.
• Theranostics: Pairs diagnostic imaging (e.g., Ga-68) with targeted therapy (e.g., Lu-177).