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Radiopharmacy · Generator

⁹⁹Mo/⁹⁹ᵐTc Generator

Snapshot

The ⁹⁹Mo/⁹⁹ᵐTc generator brings ⁹⁹ᵐTc to every department. ⁹⁹Mo (half-life ~66 h) is adsorbed on an alumina column and decays to ⁹⁹ᵐTc (6 h), which is eluted with sterile saline as sodium pertechnetate. Because it is a transient equilibrium, ⁹⁹ᵐTc regrows after each elution, allowing repeated daily elutions.

Pertechnetate (⁹⁹ᵐTcO₄⁻) is washed off the column while ⁹⁹Mo remains bound. Yield is highest ~24 h after the previous elution (equilibrium). Purity checks guard against parent and column breakthrough.

⁹⁹Mo 66 hParent
⁹⁹ᵐTc 6 hDaughter
Transient eq.Elute ~daily
Reference values
  • ⁹⁹Mo: T½ 66 h, β⁻; about 87% of decays feed ⁹⁹ᵐTc and 13% go directly to ground-state ⁹⁹Tc; γ-rays at 181, 740 and 778 keV.
  • ⁹⁹ᵐTc: T½ 6.01 h, isomeric transition, 140-keV γ in about 89% of decays (the rest mainly internal conversion).
  • The column holds 5–10 g of alumina; ⁹⁹ᵐTc is eluted as Na⁹⁹ᵐTcO₄ in 0.9% saline.
  • ⁹⁹ᵐTc peaks about 23 h after an elution (t_max = ln(λ_Tc/λ_Mo)/(λ_Tc − λ_Mo)); thereafter A_Tc ≈ 0.96 × A_Mo.
  • ⁹⁹Mo breakthrough limit (USP/NRC): 0.15 µCi ⁹⁹Mo per mCi ⁹⁹ᵐTc at administration; Ph. Eur. 0.1% of total activity.
  • Aluminium: ≤10 µg/mL (USP) or ≤5 µg/mL (Ph. Eur.); eluate pH 4.5–7.5 (USP).
  • Practical elution yield is 80–90%; a typical label sets eluate expiry at 12 h.
  • About 24 h after an elution only about 28% of the technetium atoms in the eluate are ⁹⁹ᵐTc; the rest are ⁹⁹Tc.
Worked example

Ingrowth and elution yield after 24 h

Given. A generator holds 20 GBq ⁹⁹Mo at a complete elution at 08:00 (T½ ⁹⁹Mo 65.9 h, ⁹⁹ᵐTc 6.01 h, 87% branching, elution efficiency 85%). What can be eluted at 08:00 next day, and when is ⁹⁹ᵐTc maximal?

  1. λ_Mo = 0.693/65.9 = 0.01052 h⁻¹; λ_Tc = 0.693/6.01 = 0.1153 h⁻¹.
  2. Equilibrium factor K = 0.87 × λ_Tc/(λ_Tc − λ_Mo) = 0.87 × 0.1153/0.1048 = 0.957.
  3. A_Tc(24 h) = K × 20 × (e^(−0.01052×24) − e^(−0.1153×24)) = 0.957 × 20 × (0.777 − 0.063) = 13.7 GBq on the column.
  4. Eluted = 13.7 × 0.85 = 11.6 GBq.
  5. t_max = ln(0.1153/0.01052)/(0.1153 − 0.01052) = 2.395/0.1048 = 22.9 h.

Answer. About 11.6 GBq can be eluted at 24 h; the column content peaks at about 23 h, so daily elution gives near-maximal yield.

Worked example

⁹⁹Mo breakthrough at the time of administration

Given. At 08:00 an eluate contains 30 GBq ⁹⁹ᵐTc and 1.5 MBq ⁹⁹Mo. The USP/NRC limit is 0.15 kBq ⁹⁹Mo per MBq ⁹⁹ᵐTc at administration. Until when can doses from it be given?

  1. Ratio at elution = 1.5 MBq / 30,000 MBq = 5×10⁻⁵ = 0.05 kBq/MBq — passes.
  2. ⁹⁹ᵐTc decays faster than ⁹⁹Mo, so the ratio grows as e^((λ_Tc − λ_Mo)t) = e^(0.1048t).
  3. Limit reached when e^(0.1048t) = 0.15/0.05 = 3 → t = ln3/0.1048 = 10.5 h.
  4. By comparison the Ph. Eur. limit (0.1% = 1 kBq/MBq) would need a factor of 20: t = ln20/0.1048 = 28.6 h.

Answer. Doses from this eluate meet the USP/NRC limit only until about 18:30 (10.5 h), earlier than the 12-h label expiry.

Two graphs: after an elution technetium-99m grows in to a maximum at about 23 hours and then decays in step with molybdenum-99; with elution every 24 hours the technetium activity forms a sawtooth whose peaks fall as molybdenum decays.
Figure. ⁹⁹Mo/⁹⁹ᵐTc transient equilibrium calculated from the half-lives (⁹⁹Mo 65.9 h, ⁹⁹ᵐTc 6.01 h; NNDC): ⁹⁹ᵐTc peaks about 23 h after elution and then decays with the parent at 0.957 × the ⁹⁹Mo activity, so eluting once a day gives near-maximal yields that fall each day with ⁹⁹Mo decay. Panel B assumes complete elution.
Line graph of the molybdenum-99 to technetium-99m activity ratio rising exponentially with hours after elution from 0.05 kBq per MBq, crossing the USP and NRC limit of 0.15 at about 10.5 hours, with the Ph. Eur. limit of 1 kBq per MBq shown far higher.
Figure. Worked example from this page: an eluate at 0.05 kBq ⁹⁹Mo per MBq ⁹⁹ᵐTc passes at elution, but the ratio triples in about 10.5 h because ⁹⁹ᵐTc decays faster than ⁹⁹Mo, so it crosses the USP/NRC limit (0.15 kBq/MBq at administration) before the 12-h label expiry. The Ph. Eur. limit (0.1%, 1 kBq/MBq) is much less strict. Half-lives from NNDC.

How it works

  • ⁹⁹Mo adsorbed as molybdate on an alumina (Al₂O₃) column decays to ⁹⁹ᵐTc; saline elutes sodium pertechnetate (⁹⁹ᵐTcO₄⁻) while molybdate stays bound.
  • Transient equilibrium: after an elution the daughter activity grows, peaks about 23 h later and then decays with the 66-h parent half-life.
  • The eluate is sterile, pyrogen-free sodium pertechnetate with technetium in the +7 oxidation state; generator elution is itself an aseptic manipulation.

Quality of the eluate

  • ⁹⁹Mo breakthrough — USP and US NRC: ≤0.15 kBq ⁹⁹Mo per MBq ⁹⁹ᵐTc (0.15 µCi/mCi) at the time of administration, measured on every eluate. Ph. Eur. (EU/UK): ≤0.1% of total radioactivity (1 kBq/MBq).
  • Alumina breakthrough is checked colorimetrically: Al³⁺ ≤10 µg/mL (USP) or ≤5 µg/mL (Ph. Eur.).
  • The radiochemical form must be pertechnetate (Ph. Eur. ≥95% of radioactivity); pH 4.5–7.5 (USP) or 4.0–8.0 (Ph. Eur.).

Pitfalls

  • Excess ⁹⁹Mo breakthrough gives unnecessary patient dose from the 740/778-keV photons and β⁻ particles, and degrades images.
  • Long intervals between elutions raise ⁹⁹Tc carrier in the eluate; for this reason exametazime (HMPAO) needs eluate no more than 2 h old from a generator eluted within the previous 24 h.
  • Alumina in the eluate impairs labelling (for example flocculation of sulfur colloid).
In the clinic — why the physics matters
  • Elute regularly (daily) to maximise yield and minimise ⁹⁹Tc carrier — ⁹⁹Tc competes with ⁹⁹ᵐTc for stannous reduction and can lower labelling efficiency.
  • ⁹⁹Mo breakthrough is checked on every eluate: the eluate is assayed in a lead pot that stops the 140-keV photons but passes the 740/778-keV ⁹⁹Mo photons.
  • Excess aluminium flocculates ⁹⁹ᵐTc-sulfur colloid (lung uptake) and agglutinates labelled red cells — hence the aluminium limit.
  • ⁹⁹ᵐTc-exametazime (HMPAO) needs eluate no more than 2 h old from a generator eluted within the previous 24 h.
Pertechnetate eluate specifications: USP (US) and Ph. Eur. (EU/UK)
TestUSP / US NRCPh. Eur. (fission ⁹⁹Mo)
⁹⁹Mo≤0.15 kBq/MBq (0.015%) at administration; every eluate≤0.1% of total radioactivity
¹³¹I≤0.05 µCi/mCi≤5×10⁻³ % (numerically the same)
¹⁰³Ru, ⁸⁹Sr, ⁹⁰Srsee monograph5×10⁻³ %, 6×10⁻⁵ %, 6×10⁻⁶ %
α emitters≤0.001 nCi/mCi≤1×10⁻⁷ % (numerically the same)
Aluminium≤10 µg/mL≤5 µg/mL
pH4.5–7.54.0–8.0
Radiochemical puritypertechnetate≥95% as pertechnetate
Eluate expiryper label, typically 12 hper SmPC
Common pitfalls & misconceptions
  • This is transient (not secular) equilibrium — half-lives differ ~11-fold, daughter tracks parent after ~24 h.
  • The daughter never exceeds the parent here — only 87% of ⁹⁹Mo → ⁹⁹ᵐTc, so A_Tc = 0.957·A_Mo (slightly less).
  • Only ~90% of ⁹⁹ᵐTc transitions give the 140-keV imaging photon.
  • Eluate specific activity (fraction that is ⁹⁹ᵐTc) falls the longer since last elution.
In depth
  • The time of peak daughter activity is t_max = ln(λ_d/λ_p)/(λ_d − λ_p): 22.9 h for ⁹⁹Mo/⁹⁹ᵐTc. The column already holds about 60% of that peak 6 h after an elution, which is why a second elution in the afternoon is worthwhile when demand is high.
  • In transient equilibrium A_d/A_p = BR × T_p/(T_p − T_d); with an 87% branching ratio this gives 0.957, so the ⁹⁹ᵐTc activity stays slightly below the ⁹⁹Mo activity — the daughter never overtakes the parent in this generator.
  • ⁹⁹Tc (T½ 2.1×10⁵ y) accumulates on the column from the 13% direct branch and from decay of ⁹⁹ᵐTc, so the ⁹⁹ᵐTc fraction of technetium atoms falls the longer the column stands (about 28% at 24 h). A first elution after a weekend therefore carries more ⁹⁹Tc carrier.
  • Because ⁹⁹ᵐTc decays faster than ⁹⁹Mo, the ⁹⁹Mo/⁹⁹ᵐTc ratio grows by e^((λ_Tc − λ_Mo)t) — about 2.85-fold in 10 h — so the USP/NRC limit must be checked for the time of administration, not only at elution.
  • The USP/NRC ⁹⁹Mo limit (0.015%) is about seven times stricter than Ph. Eur. (0.1%). The limits for ¹³¹I and α emitters are numerically identical in the two pharmacopoeias; aluminium (10 vs 5 µg/mL) and pH (4.5–7.5 vs 4.0–8.0) differ.
  • US NRC 10 CFR 35.204 requires the licensee to measure the ⁹⁹Mo concentration in each eluate and to report any eluate that exceeds the limit; for ⁸²Sr/⁸²Rb generators ⁸²Sr (≤0.02 kBq/MBq) and ⁸⁵Sr (≤0.2 kBq/MBq) are measured before the first patient each day.
  • The EANM cGRPP guideline classes generator elution, kit reconstitution and QC sampling as aseptic manipulations to be done in a grade A environment (grade C background for a laminar-flow cabinet, grade D for an isolator), with vial stoppers wiped with a sterile disinfectant that is allowed to evaporate before puncture.

Sources: Saha, Fundamentals of Nuclear Pharmacy, 7th ed. (2018), Ch. 5 · Cherry, Sorenson & Phelps, Physics in Nuclear Medicine, 4th ed. (2012) · TechneLite US prescribing information (2019) · Ph. Eur. monograph 0124 · 10 CFR 35.204 · Gillings et al. 2021 (PMID 33580358)

Sources

  1. Saha GB. Fundamentals of Nuclear Pharmacy. 7th ed. Cham: Springer; 2018.
  2. Cherry SR, Sorenson JA, Phelps ME. Physics in Nuclear Medicine. 4th ed. Philadelphia: Elsevier Saunders; 2012.
  3. Lantheus Medical Imaging. TechneLite (technetium Tc 99m generator) US prescribing information. 2019.
  4. European Pharmacopoeia. Sodium pertechnetate (⁹⁹ᵐTc) injection (fission), monograph 0124. Strasbourg: EDQM (current edition).
  5. US Nuclear Regulatory Commission. 10 CFR 35.204: Permissible molybdenum-99, strontium-82 and strontium-85 concentrations.
  6. Gillings N, Hjelstuen O, Ballinger J, et al. Guideline on current good radiopharmacy practice (cGRPP) for the small-scale preparation of radiopharmaceuticals. EJNMMI Radiopharm Chem. 2021;6:8.
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