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Linac EPID Portal-Imaging Panel

The electronic portal-imaging device mounted opposite the linac treatment head — a flat-panel detector that captures the megavoltage X-ray beam transmitted through the patient during treatment delivery. EPIDs serve dual purposes: treatment-verification imaging (confirming patient setup matches the treatment plan before delivery) and dose-distribution verification (in-vivo dosimetry for IMRT / VMAT QA programs).

EPID technology evolved from camera-based fluorescent screens in early designs to amorphous-silicon flat-panel detectors that became the dominant architecture from the mid-2000s onward. Modern EPIDs are essentially MV-spectrum-tuned versions of the same a-Si TFT flat-panel architecture used in diagnostic DR, with thicker copper plates and metal-plate-plus-phosphor scintillator stacks optimized for MV photons. Vendor-specific brands include the Varian aS1200 / aS1000 / aS500, Elekta iViewGT, and Siemens / Accuray equivalents.

For radiation-therapy fleet operations, the EPID is the second-most-replaced major component behind the klystron / magnetron RF-source on most platforms — drop damage during mechanical retract / extend cycles and panel-aging-driven dead-pixel growth are routine end-of-service drivers.

Fits

EPID panels are platform-specific:

  • Varian Clinac iX / Trilogy / TrueBeam — aS500 (legacy), aS1000, aS1200 across generations.
  • Elekta Synergy / Infinity / Versa HD — iViewGT and successor panels.
  • Accuray TomoTherapy / Radixact — proprietary detector array integrated into the rotating gantry; not a conventional EPID architecture.
  • MR-linac platforms (Elekta Unity, ViewRay MRIdian) — MV imaging is replaced by integrated MR for guidance; no EPID in the conventional sense.

Distinctive technology

  • Amorphous-silicon TFT panel with metal-plate scintillator stack tuned for MV-photon detection.
  • Retract / extend mechanism — the panel is mounted on a motorized arm that extends to imaging position and retracts during high-dose-rate delivery (the panel is not rated for direct beam exposure during full-dose treatment).
  • Pre-treatment imaging mode — kV imaging using the on-board imager, MV imaging using the EPID.
  • Cine-mode acquisition — frame-rate imaging during VMAT delivery for in-vivo dosimetry / motion verification.

Failure modes

  • Dead-pixel growth — analogous to diagnostic DR detectors but with MV-energy-specific aging characteristics. See Dead-pixel growth.
  • Retract / extend mechanism wear — motors, bearings, cable management on the support arm fail before the panel itself. The arm retracts / extends multiple times per treatment fraction; cumulative cycle count is high.
  • Panel impact damage — collisions during retract / extend or during patient setup. The panel is positioned close to patient anatomy on some setups and impact incidents occur.
  • Cable / connector wear at the panel-to-system interface.
  • Calibration drift outside tolerance.

Diagnosis

  • Daily QA imaging acquisition — TG-142 standard.
  • Bad-pixel-map trending in the service log.
  • Retract / extend motion log review.
  • Visual inspection for impact damage at PM intervals.

Replacement path

  • Panel-level swap for panel failures.
  • Arm / mechanism service for retract / extend issues.
  • Calibration suite post-swap.

Field notes

  • Retract-arm wear is often the limiting factor — the panel can outlast the support mechanism on high-volume linacs.
  • Dosimetry-program continuity matters — sites running EPID-based in-vivo dosimetry need calibration continuity across panel swaps; baseline dosimetric calibration must be re-established post-replacement.
  • Refurb linac due-diligence — EPID panel age + bad-pixel count + retract-arm cycle history.

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