12.3 Therapeutic Radiopharmaceuticals & Radiation Safety Precautions

12.3 Therapeutic Radiopharmaceuticals & Radiation Safety Precautions

Quick Answer: Therapeutic radiopharmaceuticals utilize targeted particulate radiation (beta or alpha particles) to destroy diseased tissue while sparing surrounding healthy structures. Key agents include I-131 for thyroid disease, Radium-223 for prostate bone metastases, and Lu-177 Dotatate/PSMA-617 for targeted molecular therapy. Administering these high-dose therapies requires stringent adherence to NRC regulations, strict inpatient radiation safety protocols, and proper calculation of patient release criteria to protect the public.

1. Principles of Targeted Radionuclide Therapy

Unlike diagnostic imaging, which relies on penetrating gamma or positron emissions to escape the body and reach a detector, therapeutic nuclear medicine aims to deposit the maximum radiation dose directly into the target tissue.

Emission Types

• Beta Particles (β⁻): Fast-moving electrons emitted from the nucleus (e.g., I-131, Lu-177, Y-90). They have a moderate linear energy transfer (LET) and a tissue penetration range of a few millimeters. This allows for the destruction of the targeted cell as well as adjacent tumor cells (the "crossfire" effect).
• Alpha Particles (α): Helium nuclei consisting of two protons and two neutrons (e.g., Ra-223). Alpha particles have an extremely high LET, depositing a massive amount of destructive energy over a very short range (less than 0.1 mm, or just a few cell diameters). They cause complex, double-strand DNA breaks that are nearly impossible for the cell to repair.

2. Key Therapeutic Radiopharmaceuticals

I-131 Sodium Iodide

Iodine-131 is the oldest and most widely used therapeutic radiopharmaceutical. It has a physical half-life of 8.02 days and emits both a high-energy beta particle (for therapy) and a 364 keV gamma ray (which allows for post-therapy imaging).

• Indications: It is used for the treatment of hyperthyroidism (Graves' disease, toxic multinodular goiter) at lower doses (typically 10-30 mCi) and for the ablation of thyroid remnants/cancer at much higher doses (30 to over 200 mCi).
• Mechanism: As an iodide ion, it is actively transported into thyroid follicular cells by the sodium-iodide symporter (NIS), where it is organified and trapped.
• Preparation: Patients must undergo strict dietary iodine restriction (low iodine diet) for 1-2 weeks prior to therapy and must withdraw from thyroid hormone replacement or receive recombinant human TSH (Thyrogen) to elevate their TSH levels, driving maximum uptake of the I-131.

Radium-223 Dichloride (Xofigo)

Radium-223 is an alpha-emitting radiopharmaceutical with a half-life of 11.4 days.

• Indications: It is specifically approved for the treatment of patients with castration-resistant prostate cancer (CRPC) that has metastasized to the bone, who have symptomatic bone disease but no known visceral metastatic disease.
• Mechanism: Radium is an alkaline earth metal in the same chemical group as calcium. As a calcium analog, it naturally targets areas of high bone turnover, such as osteoblastic bone metastases. Because alpha particles have such a short range, Ra-223 delivers heavy radiation directly to the bone tumor while causing minimal damage to the adjacent bone marrow, significantly preserving hematologic function.

Lutetium-177 Therapies

Lutetium-177 is a beta-emitter with a half-life of 6.7 days. It also emits low-energy gamma rays (113 and 208 keV) that allow for post-therapy imaging and dosimetry.

• Lu-177 Dotatate (Lutathera): A targeted therapy for somatostatin receptor-positive gastroenteropancreatic neuroendocrine tumors (GEP-NETs). It binds to the somatostatin receptors on the tumor cells and is internalized, delivering a lethal beta dose. To protect the kidneys from radiation damage during excretion, an intravenous amino acid solution (lysine and arginine) must be co-administered to competitively inhibit renal reabsorption of the radiotracer.
• Lu-177 PSMA-617 (Pluvicto): Used for the treatment of adult patients with PSMA-positive metastatic castration-resistant prostate cancer (mCRPC) who have already been treated with androgen receptor pathway inhibition and taxane-based chemotherapy.

3. Radiation Safety Precautions and Release Criteria

The administration of therapeutic doses involves significant radiation hazards. Strict protocols governed by the Nuclear Regulatory Commission (NRC) or Agreement States must be followed.

Written Directives

A Written Directive is a mandatory, legally binding medical prescription that must be completed and signed by an Authorized User (AU) before the administration of any therapeutic dose, or any I-131 dose greater than 30 μCi. It must detail the patient's name, the radiopharmaceutical, the route of administration, and the prescribed dose. Before administration, the patient's identity must be verified using two independent methods (e.g., name and date of birth).

Inpatient Therapy Precautions

When a patient receives a high dose of radiation (such as a >100 mCi I-131 ablation) and cannot be released immediately, strict inpatient isolation protocols are implemented:

• Private Room: The patient must be placed in a private room with a private bathroom. The room should ideally be at the end of a hallway to minimize exposure to staff.
• Room Preparation: The floors, countertops, and bathroom fixtures (toilet, sink, phone) must be completely covered with plastic wrap or absorbent, plastic-backed paper to prevent radioactive contamination from sweat, saliva, and urine.
• Nursing Care: Nursing staff should spend the absolute minimum time required in the room, practicing the principles of Time, Distance, and Shielding. All trash, linens, and food trays must remain in the room in designated radioactive waste bins until surveyed and cleared by the Radiation Safety Officer (RSO).
• Visitors: Visitors must be strictly limited (often prohibited for the first 24 hours). Pregnant women and children are absolutely barred from visiting.

10 CFR 35.75: Patient Release Criteria

A patient who has received a therapeutic radiopharmaceutical may only be released from the facility if the calculated Total Effective Dose Equivalent (TEDE) to any other maximally exposed individual (such as a family member) from exposure to the released patient is not likely to exceed 5 millisieverts (mSv) or 0.5 rem.

If the calculated dose is likely to exceed 1 mSv (0.1 rem), the facility must provide the patient with detailed, written instructions on how to maintain radiation doses to others As Low As Reasonably Achievable (ALARA). These instructions include:

• Sleeping alone in a separate bed.
• Using a separate bathroom, sitting down to urinate, and flushing twice.
• Maintaining distance from others (at least 6 feet).
• Washing clothes separately and washing hands frequently.
• Avoiding close contact with pregnant women and children.

The release determination is typically based on measuring the patient's dose rate at 1 meter with a survey meter immediately after administration and applying occupancy factors, or by calculating the expected clearance of the radioisotope based on its physical and biological half-life.