Unsealed Source Therapy: I-131 Protocols & Patient Release Criteria

Unsealed Source Therapy: I-131 Therapeutic Physics & Radiobiology

Sodium iodide ($^{131}\text{I}$) therapy is the cornerstone of unsealed source radionuclide therapy in nuclear medicine, exploiting the physiological iodine trapping and organification mechanisms of thyroid follicular cells mediated by the sodium-iodide symporter (NIS). Understanding the balance between its therapeutic beta emissions and its penetrating gamma emissions is essential for effective clinical treatment and rigorous radiation protection.

Physical Decay Kinetics & Particulate Emissions

$^{131}\text{I}$ decays with a physical half-life of $T_p = 8.02\text{ days}$ ($192.5\text{ hours}$) via beta-minus ($\beta^-$) emission ($100\%$) to stable $^{131}\text{Xe}$:

${}_{53}^{131}\text{I} \longrightarrow {}_{54}^{131}\text{Xe} + \beta^- + \bar{\nu}_e + \gamma$

The beta spectrum is polyenergetic, dominated by a transition with maximum energy $E_{\beta,\max} = 606\text{ keV}$ ($89\%$ branching ratio; minor transition at $E_{\beta,\max} = 334\text{ keV}$, $7\%$), yielding an average beta energy of $\bar{E}_\beta = 192\text{ keV}$. In soft tissue, these beta particles exhibit a mean range of $0.8\text{ mm}$ and a maximum range of $2.2\text{ mm}$.

Because the linear energy transfer (LET) of beta particles leads to dense localized ionization events along their sub-millimeter tracks, beta emissions deposit $>90\%$ of the total absorbed radiation dose delivered to thyroid tissue. This localized energy deposition induces clustered, double-strand DNA breaks in thyrocytes, driving apoptosis, cellular senescence, and follicular necrosis. The short tissue range provides steep dose gradients, largely sparing adjacent vital structures including the parathyroid glands and recurrent laryngeal nerves.

Penetrating Gamma Emissions & External Hazard

Decay to excited states of $^{131}\text{Xe}$ results in the emission of penetrating gamma photons. The principal gamma photon has an energy of $364\text{ keV}$ ($81\%$ abundance), with minor high-energy gamma emissions at $637\text{ keV}$ ($7.2\%$) and $284\text{ keV}$ ($6.1\%$). These photons have a half-value layer (HVL) of approximately $3.0\text{ mm}$ in lead and $6.3\text{ cm}$ in soft tissue.

While the $364\text{ keV}$ gamma emission enables diagnostic planar scintigraphy and SPECT/CT post-therapy imaging, it escapes the patient's body readily. This penetrating radiation constitutes a substantial external exposure hazard to family members, caregivers, nursing personnel, and members of the public. The specific gamma-ray constant for $^{131}\text{I}$ is:

$\Gamma = 2.2\ \frac{\text{R}\cdot\text{cm}^2}{\text{mCi}\cdot\text{hr}} = 0.0595\ \frac{\mu\text{Sv}\cdot\text{m}^2}{\text{MBq}\cdot\text{hr}} \approx 0.22\ \frac{\text{mR}}{\text{hr}} \text{ per mCi at } 1.0\text{ meter}$

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Clinical Indications, Administered Activities & Dosimetric Ceilings

Therapeutic dosages of $^{131}\text{I}$ are administered orally as sodium iodide capsules or liquid solution across distinct clinical regimes:

1. Benign Thyroid Disease (Hyperthyroidism)

• Graves' Disease & Toxic Multinodular Goiter: Administered activities typically range from $370\text{ to } 1{,}110\text{ MBq}$ ($10\text{ to } 30\text{ mCi}$).
• Dosimetric Objective: Delivers an absorbed radiation dose of $80\text{ to } 300\text{ Gy}$ ($8{,}000\text{ to } 30{,}000\text{ rad}$) to hyperfunctioning thyroid tissue. Dosages are calculated based on thyroid mass estimated by palpation or ultrasound ($m$), fractional 24-hour radioiodine uptake ($U_{24}$), and effective half-life ($T_{\text{eff}}$), utilizing Marinelli's equation:

$A\,(\mu\text{Ci}) = \frac{\text{Desired Dose (rad)} \times \text{Thyroid Mass (g)}}{73.8 \times \bar{E}_\beta\,(\text{MeV}) \times T_{\text{eff}}\,(\text{days}) \times U_{24}}$

2. Differentiated Thyroid Carcinoma (DTC)

• Post-Surgical Remnant Ablation: Administered following total thyroidectomy to destroy residual normal thyroid bed tissue, utilizing $1.11\text{ to } 3.7\text{ GBq}$ ($30\text{ to } 100\text{ mCi}$). This eliminates baseline thyroglobulin production to facilitate recurrence surveillance.
• Adjuvant Treatment of Suspected/Confirmed Nodal Metastases: Typically $3.7\text{ to } 5.55\text{ GBq}$ ($100\text{ to } 150\text{ mCi}$).
• Distant Metastatic Disease (Lungs, Bone): High-activity therapy utilizing $5.55\text{ to } 7.4\text{ GBq}$ ($150\text{ to } 200\text{ mCi}$), or personalized dosimetric activities determined through blood and whole-body clearance dosimetry.

Critical Organ Dosimetric Safety Limits (Benua-Leeper Criteria)

When administering high activities ($>5.55\text{ GBq}$ / $>150\text{ mCi}$), personalized dosimetry must respect two deterministic organ dose thresholds:

1. Red Marrow Absorbed Dose Ceiling: The absorbed dose to the blood (a validated surrogate for hematopoietic red bone marrow) must remain $< 2.0\text{ Gy}$ ($< 200\text{ rad}$) to avert fatal myelosuppression and aplastic anemia.
2. 48-Hour Whole-Body Retention Ceiling: Retained whole-body activity at $48\text{ hours}$ should not exceed $4.44\text{ GBq}$ ($120\text{ mCi}$), or $2.96\text{ GBq}$ ($80\text{ mCi}$) in patients with diffuse lung metastases, to limit the risk of radiation pneumonitis and pulmonary fibrosis.

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NRC Patient Release Regulations: 10 CFR 35.75 & Biokinetic Modeling

The Regulatory Standard (10 CFR 35.75(a))

Under Title 10 of the Code of Federal Regulations, Part 35.75(a) (10 CFR 35.75(a)):

A licensee may authorize the release from its control of any individual who has been administered unsealed byproduct material or implants containing byproduct material if the Total Effective Dose Equivalent (TEDE) to any other individual from exposure to the released individual is not likely to exceed $5.0\text{ mSv}$ ($0.5\text{ rem}$).

In 1997, the NRC amended 10 CFR 35.75, eliminating the historical, inflexible $1{,}110\text{ MBq}$ ($30\text{ mCi}$) confinement rule and replacing it with a dose-based performance standard. This allows outpatient release of patients receiving higher activities provided rigorous biokinetic or behavioral controls ensure household contacts do not exceed $5\text{ mSv}$.

Release Criteria Compliance Pathways (NRC Regulatory Guide 8.39, Revision 1)

The NRC's acceptable methods, formerly in NUREG-1556 Volume 9, now appear in Regulatory Guide 8.39, Revision 1 (2020). Licensees typically use one of three approaches:

1. Activity Threshold Table (Default Release)

Under RG 8.39 Table 1, a patient given $^{131}\text{I}$ may be released without further calculation if the administered or retained activity does not exceed: $\mathbf{A_{\text{retained}} \le 1{,}220\text{ MBq}} \quad (\mathbf{33\text{ mCi}})$

2. Dose Rate Threshold Table (Default Release)

A patient may be released immediately if the measured exposure rate at $1.0\text{ meter}$ from the patient does not exceed: $\mathbf{\dot{X}_{1\text{ m}} \le 0.07\text{ mSv/hr}} \quad (\mathbf{7.0\text{ mrem/hr}})$

3. Patient-Specific Dose Calculation Method

When administered activity exceeds $33\text{ mCi}$ (e.g., $100\text{ to } 200\text{ mCi}$ for thyroid carcinoma), release is authorized if a patient-specific calculation proves the projected dose to any individual will remain $\le 5.0\text{ mSv}$.

Patient-Specific Calculation (RG 8.39 Appendix)

For I-131 the guide models the first 8 hours (0.33 day), when the patient may travel home in close contact with others, with an occupancy factor $E_1 = 0.75$ and physical decay only, and the remaining time with two clearance compartments and $E_2 = 0.25$:

$D(\infty) = \frac{34.6\,\Gamma\,Q_0}{(100\text{ cm})^2}\left\{E_1 T_p (0.8)\left(1 - e^{-0.693(0.33)/T_p}\right) + e^{-0.693(0.33)/T_p}\,E_2\left[F_1 T_{1\text{eff}} + F_2 T_{2\text{eff}}\right]\right\}$

where $\Gamma = 2.2\ \text{R}\cdot\text{cm}^2/(\text{mCi}\cdot\text{h})$, $Q_0$ is the administered activity in mCi, $T_p = 8.04\text{ days}$, the factor 34.6 converts days to hours and integrates the exponential ($24 \times 1.443$), and the result is in rem when the tissue-dose approximation of 1 R ≈ 1 rem is used. RG 8.39's default compartments are:

Patient-specific uptake measurements can replace the defaults, and the licensee then keeps a record of the basis for release.

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Written Instructions & Mandatory Breast-Feeding Interruption (10 CFR 35.75(b))

The 1 mSv Instruction Threshold

Under 10 CFR 35.75(b), the licensee is legally required to provide the released patient (or parent/guardian) with written instructions on actions recommended to maintain doses to other individuals ALARA if:

The Total Effective Dose Equivalent (TEDE) to any other individual is likely to exceed $1.0\text{ mSv}$ ($0.1\text{ rem}$).

Typical Instructions

Instructions commonly cover the following, for a period set from the patient's dose estimate:

• Sleep in a separate bedroom in a bed isolated by at least $2\text{ meters}$ from shared walls for a specified duration ($3\text{ to } 7\text{ days}$).
• Maintain a minimum distance of $2\text{ meters}$ ($6\text{ feet}$) from family members and coworkers; avoid public transportation, workplaces, and public gatherings.
• Avoid all close contact with children and pregnant individuals.
• Bathroom hygiene: Double-flush the toilet after every use, sit while urinating to avoid splashing, wash hands with copious water, and rinse bathroom sinks.
• Contamination mitigation: Use dedicated dinnerware or disposable utensils; wash patient clothing and bed linens separately.

Breast-Feeding & Lactation Mandates

The lactating breast concentrates iodide through the sodium-iodide symporter (NIS), so a nursing infant could receive a thyroid dose large enough to damage or ablate the gland.

Under 10 CFR 35.75(b), if the dose to a nursing infant or child could exceed $1.0\text{ mSv}$ ($0.1\text{ rem}$) assuming no interruption of breast-feeding, the instructions must include guidance on interruption or discontinuation of breast-feeding and information on the consequences of not following it. For $^{131}\text{I}$ (sodium iodide), NRC guidance (RG 8.39, Table 3) recommends:

Complete cessation of breast-feeding for this child.

Patients are also usually advised to stop breast-feeding several weeks before therapy so that breast uptake declines; breast-feeding may resume with a future child.

Regulatory Record Retention (10 CFR 35.2075)

Under 10 CFR 35.2075, the licensee keeps a record of the basis for release when the dose was calculated using retained rather than administered activity, an occupancy factor below 0.25 at 1 m, biological or effective half-lives, or tissue shielding, and a record that breast-feeding instructions were given when the infant's dose from continued breast-feeding could exceed 5 mSv. Both records are kept for 3 years after release.

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Inpatient Room Preparation & Precautions for Non-Released Patients

When a patient cannot be released under 10 CFR 35.75—due to domestic circumstances (lack of separate bedroom/bath, living with infants/pregnant individuals), severe urinary incontinence, physical incapacitation, or calculated potential dose exceeding $5\text{ mSv}$—inpatient hospitalization is required.

Room Engineering & Surface Containment

• Isolation Room Assignment: A private room equipped with a dedicated private bathroom. Adjoining rooms must be evaluated for structural shielding and occupancy.
• Surface Protection: Because $^{131}\text{I}$ is excreted in high concentrations in sweat, saliva, and urine ($>80\%$ of unabsorbed activity excreted in urine within 48 hours), all contact surfaces must be covered with absorbent paper backed with heavy impermeable polyethylene plastic:
  • Floors from patient bed to bathroom
  • Telephone, nurse call pendant, television remote control, and door handles
  • Bed mattress and pillows encased in waterproof vinyl covers
  • Toilet bowl, flush handle, sink, and faucet handles wrapped in plastic

Radiation Postings & Boundary Demarcation

• Post "Caution - Radioactive Materials" signs on entrance doors per 10 CFR 20.1902.
• Survey adjacent hallways and adjoining patient rooms. Establish a demarcated radiation perimeter line where exposure rates drop below $0.02\text{ mSv/hr}$ ($2\text{ mrem/hr}$) to ensure compliance with the public dose limit in unrestricted areas under 10 CFR 20.1301.

Nursing Radiation Protection Protocols (ALARA)

• Personnel Dosimetry: Staff likely to exceed the 20.1502 monitoring thresholds wear whole-body dosimeters, and many programs badge all nurses caring for therapy patients.
• Shift Rotation & Maximum Stay Times: Nursing shifts are rotated to distribute dose. Maximum permissible nursing stay times are calculated based on post-administration survey meter readings:

$t_{\text{stay}} = \frac{D_{\text{administrative limit}}}{\dot{X}_{\text{bedside}}}$

• Mobile Bedside Shielding: Mobile lead barriers (commonly about $1/2$ to $1$ inch of lead) reduce the $364\text{ keV}$ dose rate behind them by roughly an order of magnitude or more; the actual transmission is checked with a survey meter because broad-beam buildup makes it higher than narrow-beam HVLs predict.
• Visitor Restrictions: Hospital policies typically exclude children and pregnant visitors and limit adult visits in time and distance, keeping each visitor's dose within the limits that apply (1 mSv per year for members of the public, or up to 5 mSv for a visitor when the authorized user has determined before the visit that this is appropriate, under 10 CFR 20.1301(c)).

Personnel Bioassay Monitoring (10 CFR 20.1502 & Regulatory Guide 8.20)

Under 10 CFR 20.1502(b), internal exposure must be monitored for adults likely to receive an intake exceeding $10\%$ of the applicable ALI. Regulatory Guide 8.20, Revision 2 (2014) gives the NRC's acceptable radioiodine bioassay program:

• When bioassay is necessary: when a worker handles, in any 3-month period, more than 1 mCi of volatile I-131 on an open bench, 10 mCi in a properly working fume hood, or 100 mCi in a glovebox (ten times these amounts if the iodine is bound to a nonvolatile agent). Sealed capsules are treated as sealed unless one is opened or crushed.
• Timing: for I-131 the maximum bioassay interval is 2 weeks; for infrequent work, a thyroid count is done within that interval but not sooner than about 6 hours after handling (unless early emergency assessment is needed).
• Predetermined action levels (thyroid content):
  • Above 1.0 µCi (37 kBq): investigate the operation, take corrective actions, and repeat a direct thyroid measurement within 24 hours.
  • Above 5.0 µCi (185 kBq): do all of the above, refer for medical consultation as soon as possible (within 2–3 hours of a known intake if thyroid blocking may help), repeat measurements about weekly until below 1 µCi, and restrict the worker if further work could exceed the limits.

Radioactive Waste Management (10 CFR 35.92)

All contaminated linens, paper gowns, disposable dining trays, and cleaning materials must be segregated in dedicated yellow radioactive waste containers. Under 10 CFR 35.92, byproduct material with a physical half-life of 120 days or less may be held for decay-in-storage and disposed of without regard to radioactivity once a survey at the container surface with an appropriate survey instrument, set on its most sensitive scale with no interposed shielding, shows that radiation levels cannot be distinguished from background. All radiation labels are removed or obliterated (except on materials inside containers that will be managed as biomedical waste after release), and disposal records are kept for 3 years (35.2092). There is no fixed holding time; for $^{131}\text{I}$, practice is often to hold waste for about 10 half-lives (about 80 days), but the background survey is what governs release.

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Worked Numerical Examples: Outpatient Release & Inpatient Stay-Time

Example 1: Patient-Specific Outpatient Release Evaluation

A patient with differentiated thyroid carcinoma receives $5.55\text{ GBq}$ ($150\text{ mCi}$) of $^{131}\text{I}$ after total thyroidectomy. Using RG 8.39's default thyroid cancer parameters, estimate the dose to a household member.

1. Inputs: $\Gamma = 2.2$, $Q_0 = 150\text{ mCi}$, $T_p = 8.04\text{ d}$, $E_1 = 0.75$, $E_2 = 0.25$, $F_1 = 0.95$, $T_{1\text{eff}} = 0.32\text{ d}$, $F_2 = 0.05$, $T_{2\text{eff}} = 7.3\text{ d}$.

2. Prefactor: $\frac{34.6 \times 2.2 \times 150}{10{,}000} = 1.1418$

3. Terms, with $e^{-0.693(0.33)/8.04} = 0.9720$:

• First 8 hours: $0.75 \times 8.04 \times 0.8 \times (1 - 0.9720) = 0.1353$
• Extrathyroidal: $0.9720 \times 0.25 \times 0.95 \times 0.32 = 0.0739$
• Thyroidal: $0.9720 \times 0.25 \times 0.05 \times 7.3 = 0.0887$
• Sum: $0.2979$

1. Dose: $D(\infty) = 1.1418 \times 0.2979 = 0.340\text{ rem} = 3.40\text{ mSv}$

2. Determination: $3.40\text{ mSv} \le 5\text{ mSv}$, so release is permitted under 35.75(a); $3.40\text{ mSv} > 1\text{ mSv}$, so written instructions are required under 35.75(b). The same model gives about $4.5\text{ mSv}$ for $200\text{ mCi}$. Because default parameters were used, no 35.2075(a) record is needed; if measured uptake, retained activity, or reduced occupancy had been used, the basis for release would be recorded.

Example 2: Inpatient Nursing Stay-Time Calculation

An unreleased inpatient receives $7.4\text{ GBq}$ ($200\text{ mCi}$) of $^{131}\text{I}$. Survey at bedside ($0.5\text{ m}$) indicates an exposure rate of $80.0\text{ mR/hr}$. The hospital RSO establishes an administrative nurse exposure limit of $0.10\text{ mSv}$ ($10\text{ mrem}$) per 12-hour shift.

1. Calculate maximum stay time without shielding:

$t_{\text{unshielded}} = \frac{10\text{ mrem}}{80.0\text{ mrem/hr}} = 0.125\text{ hr} = 7.5\text{ minutes}$

1. A mobile shield with about $12\text{ mm}$ of lead (roughly 4 narrow-beam HVLs of about $3\text{ mm}$ at $364\text{ keV}$) is placed between the bed and the nurse. Estimate the transmission as $(1/2)^4 = 1/16$:

$\dot{D}_{\text{shielded}} \approx 80.0\text{ mrem/hr} \times \frac{1}{16} = 5.0\text{ mrem/hr}$

$t_{\text{shielded}} = \frac{10\text{ mrem}}{5.0\text{ mrem/hr}} = 2.0\text{ hours}$

Because broad-beam buildup makes the real transmission somewhat higher, the physicist confirms the dose rate behind the shield with a survey meter before posting stay times. Even so, shielding extends the allowed bedside time from minutes to hours.

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Comparison Table & Clinical Pitfalls

Clinical Practice Pitfalls

• Permitting Breast-Feeding Interruption Instead of Cessation: Unlike diagnostic $^{99m}\text{Tc}$ where milk collection can resume after 24 hours, $^{131}\text{I}$ therapy requires permanent cessation for that child due to irreversible neonatal athyreosis risk.
• Neglecting Renal Function in Release Calculations: The two-compartment release model assumes normal renal clearance ($T_{b1} \approx 8\text{ hours}$). Patients with renal impairment (low GFR) retain extrathyroidal $^{131}\text{I}$ significantly longer, easily doubling whole-body dose and invalidating standard release models.
• Releasing Patients with High Uptake Without Calculation: Administering $1.11\text{ GBq}$ ($30\text{ mCi}$) to a patient with severe Graves' disease ($80\%$ uptake) retains far more activity at 48 hours than administering $3.7\text{ GBq}$ ($100\text{ mCi}$) to an athyreotic cancer patient ($2\%$ uptake).
• Counting Too Early or Too Late: A thyroid count within the first few hours misses iodine not yet taken up, while waiting beyond the 2-week interval lets retained I-131 clear and decay, underestimating the intake.