# Patient Release Criteria After Radionuclide Therapy

## Introduction

Following radionuclide therapy, patients become temporary sources of radiation exposure to others. The NRC established patient release criteria in 10 CFR 35.75 to balance patient autonomy and comfort with public radiation safety. The 1997 revision allowed release based on either administered activity or measured dose rate, provided the projected dose to any individual is unlikely to exceed 5 mSv (500 mrem). Understanding these criteria is essential for safe therapy planning and patient counseling.

## Regulatory Framework

### 10 CFR 35.75 Release Criteria

Patients may be released when the total effective dose equivalent (TEDE) to any individual from exposure to the patient is unlikely to exceed 5 mSv (500 mrem). For a child or pregnant individual, the licensee should ensure the dose is unlikely to exceed 1 mSv (100 mrem). Release may be based on administered activity, measured dose rate at 1 meter, or patient-specific dose calculation.

### NUREG-1556, Vol. 9 and Regulatory Guide 8.39

Regulatory Guide 8.39 provides default activity and dose-rate values for release. The calculations assume an occupancy factor of 0.25 for the maximally exposed individual. Default release activities are specified for common therapeutic radionuclides: the I-131 default release activity is 1.22 GBq (33 mCi). Lu-177 is not specifically listed in the default tables, so the dose-rate method is used. The default dose rate at 1 meter for I-131 release is 0.07 mSv per hour (7 mrem per hour).

## Practical Application by Radionuclide

### I-131 Sodium Iodide (Thyroid Cancer and Hyperthyroidism)

I-131 is the most common therapeutic radionuclide requiring release assessment. Hyperthyroidism doses of 370 to 1,110 MBq (10 to 30 mCi) usually allow immediate release. Thyroid cancer ablation doses of 1.1 to 7.4 GBq (30 to 200 mCi) may require hospitalization until exposure rates fall below release criteria. Factors affecting clearance include renal function, hydration status, and thyroid remnant uptake. Patient-specific calculations using measured dose rates may allow earlier release than the default activity-based criteria.

### Lu-177 DOTATATE and Lu-177 PSMA

Administered activities are typically 7.4 GBq (200 mCi). The lower gamma energy of Lu-177 at 208 keV and its predominant beta emission result in lower external dose rates compared to I-131. Most patients can be released the same day after post-therapy imaging. Urine precautions are observed for 48 to 72 hours post-therapy.

### Ra-223 Dichloride

Ra-223 is an alpha emitter with minimal external radiation hazard. Administered activities are approximately 55 kBq/kg. Patients are releasable immediately after administration. Fecal contamination precautions are advised for 1 week because Ra-223 is excreted through the gastrointestinal tract.

### Y-90 Radioembolization

Y-90 is a pure beta emitter with negligible external radiation exposure. Patients are released immediately with no radiation isolation required. Minimal radiation safety instructions are needed for household contacts.

| Radionuclide | Typical Activity | Release Criteria | Isolation Needed | Key Precautions |
|---|---|---|---|---|
| I-131 (hyperthyroidism) | 10–30 mCi | Usually immediate (≤33 mCi) | Minimal (1–3 days distance) | Separate bathroom, avoid close contact |
| I-131 (thyroid cancer) | 30–200 mCi | Rate-based or activity-based | Yes (1–7 days depending on dose) | Separate bed/bath, hydration, avoid children/pregnant |
| Lu-177 DOTATATE/PSMA | 200 mCi (7.4 GBq) | Same-day (low external rate) | No formal isolation | Urine precautions 48–72 h |
| Ra-223 | 55 kBq/kg | Immediate (alpha emitter) | No | Fecal precautions 1 week |
| Y-90 microspheres | Variable (GBq) | Immediate (pure beta) | No | None significant |

## Patient-Specific Dose Calculations

### Measurement-Based Approach

The dose rate is measured at 1 meter from the patient with a calibrated survey meter. An occupancy factor is applied (0.25 default, or patient-specific if justified). The projected dose is calculated using the formula: D = D-rate x Tp x OF / ln(2), where Tp is the physical half-life. If the projected dose is less than 5 mSv, the patient may be released. The measurement, calculation, and release decision are documented.

### Factors Allowing Earlier Release

The actual clearance rate is often faster than the physical half-life alone because of biological clearance. Lower occupancy factors apply when the patient lives alone or can maintain distance from others. Patient-specific instructions that the patient will follow can be factored in. Aggressive hydration accelerating renal clearance further reduces the time to release.

## Radiation Safety Instructions for Released Patients

### I-131 Therapy Patients

Patients should maintain a distance of more than 1 meter from others, especially children and pregnant individuals. They should sleep in a separate bed for a specified duration, typically 3 to 7 days depending on the dose. A separate bathroom should be used, flushing twice, with good hygiene practiced. Prolonged close contact with children and pregnant individuals should be avoided. Disposable utensils should be used or dishes washed separately. Public transportation should be avoided for extended trips during the first 24 to 48 hours. The duration of precautions depends on the administered activity and individual clearance rate.

### Documentation Requirements

Written radiation safety instructions must be provided to the patient before release. Instructions must be specific to the radionuclide and administered activity. The record must include the measurement or calculation basis for the release decision. Patient or responsible party acknowledgment is documented. Instructions are retained for 3 years per NRC requirements.

## Special Situations

### Hospitalized Patients

Hospitalized therapy patients require a private room with a private bathroom. Room surveys are performed daily. Nursing time at bedside is minimized, with remote monitoring used when possible. Contamination control measures apply to linens, waste, and room surfaces. Visitor restrictions are enforced, especially for children and pregnant individuals.

### Breastfeeding Patients

Cessation of breastfeeding is required after I-131 therapy and is permanent for that child. For Lu-177 therapy, cessation is required for a period determined by dosimetric calculations. For Ra-223, there is minimal concern, but conservative recommendations apply. Patients must be counseled about breastfeeding implications before therapy administration.

### Patient Death After Therapy

The RSO and coroner or medical examiner are notified. The body may contain significant radioactivity, which affects autopsy considerations. Cremation restrictions are based on residual activity. NRC guidance in NUREG-1556 addresses handling of remains.

### Incontinence and Cognitive Impairment

Patients unable to follow safety instructions may require extended hospitalization. Catheterization reduces contamination risk. Additional caregiver instructions and more conservative release criteria are applied.

## Clinical Pearls

The NRC permits patient release when the projected dose to any individual is unlikely to exceed 5 mSv. For children and pregnant contacts, the target is 1 mSv.

Patient-specific dose calculations using measured dose rates and individualized occupancy factors often allow earlier release than default activity-based criteria.

Written radiation safety instructions must be provided and documented before release. Instructions should be tailored to the specific radionuclide and the patient's living situation.

Lu-177 therapies generally allow same-day release due to predominant beta emission and lower external dose rates compared to I-131.

## References

1. U.S. Nuclear Regulatory Commission. "10 CFR 35.75 -- Release of Individuals Containing Unsealed Byproduct Material or Implants Containing Byproduct Material." NRC Regulations.
2. U.S. Nuclear Regulatory Commission. "Regulatory Guide 8.39: Release of Patients Administered Radioactive Materials." Rev. 1, 1997.
3. Siegel JA, et al. "MIRD Pamphlet No. 20: The Effect of Patient Release Criteria on Radiation Doses to Family Members." *J Nucl Med*. 2002;43(3):354-361.
4. Sisson JC, et al. "Radiation Safety in the Treatment of Patients with Thyroid Diseases by Radioiodine." *Thyroid*. 2011;21(4):335-346.