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Brachytherapy HDR Iridium-192 Source

The high-dose-rate (HDR) iridium-192 source at the working end of a brachytherapy afterloader — a small radioactive source (typically a few millimeters long) sealed inside a stainless-steel capsule and welded to the end of a flexible drive cable. The afterloader drives the source from a shielded safe inside the unit, through a transfer tube, and into the applicator placed inside the patient for the treatment dose. After the treatment fraction, the source is retracted back into the safe.

Ir-192 is the dominant HDR brachytherapy isotope globally because of its convenient half-life (74 days), high specific activity (small-source-volume, high-dose-rate clinically deliverable), and well-characterized clinical dosimetry. The source is a planned consumable — it decays continuously and must be replaced approximately every 3 months as activity falls below clinically usable levels. Source replacement is a routine scheduled service event with substantial regulatory + operational ceremony around it (NRC source-receipt protocols, source surveys, dosimetric calibration of the new source against a known reference).

The brachytherapy installed base is dominated by three afterloader OEMs: Elekta (Flexitron), Varian (GammaMedplus iX, BRAVOS), and a smaller share for Eckert & Ziegler (BEBIG MultiSource). Source itself is supplied by a small set of specialist nuclide vendors (BEBIG, Mallinckrodt / Curium, Eckert & Ziegler) shipping into all afterloader brands.

Fits

Source models and physical specifications are platform-specific (the source must mate to the afterloader's drive cable interface), but the underlying Ir-192 isotope is generic. Representative platforms:

  • Elekta Flexitron — Ir-192 source for Flexitron-class afterloaders.
  • Varian GammaMedplus iX / BRAVOS — Ir-192 source for Varian afterloader platforms.
  • Eckert & Ziegler BEBIG MultiSource — see Eckert & Ziegler.

Distinctive technology

  • Ir-192 isotope — 74-day half-life, ~380 keV mean photon energy, gamma + beta emission (clinically gamma-relevant; beta absorbed in the source capsule).
  • Sealed source — Ir-192 metallic core inside a welded stainless-steel capsule. Capsule integrity is the primary radiation-safety control.
  • Drive-cable mounting — source welded or crimped to the end of a flexible drive cable that the afterloader pushes / pulls through the transfer tube.
  • Activity at delivery — initial activity typically ~10–12 Ci (370–440 GBq); decays to clinically usable lower bound over ~3 months.
  • Source dosimetry — consensus-based AAPM TG-43 dosimetric formalism applies to all clinical Ir-192 sources.

Failure modes

  • Decay — the planned, scheduled "failure" mode. Activity falls below clinically usable threshold; source-replacement event.
  • Drive-cable wear — flexible cable that pushes the source through the transfer tube wears with cumulative-treatment cycles. Cable-replacement events are part of routine afterloader PM.
  • Source-positioning drift — the source must be positioned at programmed dwell-positions inside the applicator with mm-precision; positioning-system wear can drift this.
  • Source-capsule integrity events — extremely rare but the most safety-critical failure mode. Capsule rupture would release Ir-192 contamination; routine wipe-tests at source-receipt and at periodic intervals verify capsule integrity.
  • Transfer-tube kinking / blockage — operational failure mode that can trap the source mid-treatment. Afterloaders include manual-retraction emergency procedures for this scenario.

Diagnosis

  • Daily afterloader QA — source-positioning verification, dwell-time accuracy, transit-time measurement.
  • Source-strength verification at receipt and at routine intervals — well-chamber measurement.
  • Wipe-test surveys — periodic verification that no Ir-192 contamination is present on the source surface (capsule integrity).
  • Drive-cable visual inspection at PM intervals.

Replacement path (source change)

A source-change event is a multi-stakeholder coordinated service event:

  • NRC / Agreement-State licensing — receipt of the new source documented; old source source-of-receipt traceability maintained throughout disposal.
  • DOT shipping — both incoming and outgoing source shipments are subject to DOT hazardous-materials regulations.
  • Vendor delivery + source-load procedure — typically a vendor technician + site physicist + RSO coordinated procedure.
  • Source-strength verification — well-chamber measurement of new source before clinical use.
  • TPS database update — treatment-planning system updated with new source-strength reference and dwell-time scaling factors.
  • Old source disposal — vendor-managed return shipping to the source manufacturer for decay-storage / recycling.

Field notes

  • Source-change cost is a predictable line item in any HDR brachytherapy program operating budget — quarterly source replacements with associated calibration and physics work.
  • Refurb afterloader due-diligence — afterloader chassis condition + drive-mechanism cycle history + most-recent source-positioning QA. Source itself is not transferred at refurb; new source for the new owner.
  • Decommissioning — afterloader retirement requires NRC license amendment and source-removal verification before facility release.

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