9.2 Dose Calibrator Quality Control

9.2 Dose Calibrator Quality Control

Quick Answer: Constancy daily, accuracy annually (NIST-traceable), linearity quarterly, geometry at install/repair. Assays read mCi or MBq. Teaching tolerances often ±5–10%—know your SOP. Out-of-range → stop clinical use, investigate, document.

The dose calibrator (radionuclide calibrator) is a pressurized gas ionization chamber operated in current mode. Activity in a vial or syringe in the well produces ionization current converted to activity using isotope-specific calibration factors. It does not produce a gamma spectrum like a well counter; correct nuclide button/setting, geometry, and QC determine accuracy.

Units and Display

Always confirm units before drawing or documenting a dose. Misreading MBq as mCi (or the reverse) is a high-severity medication error.

The Four Core Tests

Frequencies and numeric limits are set by license, manufacturer, and department SOP. CNMT items use the classic teaching schedule above.

Constancy (Daily)

Constancy uses one or more long-lived sealed sources (commonly Cs-137, sometimes Co-57, Ba-133, Co-60) measured on designated settings.

Procedure concept:

1. Zero/background as required by the device.
2. Place the constancy source in the standard holder/position.
3. Assay on the assigned setting(s).
4. Compare reading to the expected activity (decay-corrected certificate activity or established baseline).
5. Record result and pass/fail vs departmental limit.

Interpretation: A reading within the accepted band (commonly taught as about ±5% to ±10% of expected, depending on SOP) means the chamber/electronics are stable enough for that day. A sudden step change suggests wrong source, wrong setting, geometry change, HV/electronics fault, or contamination in the well.

Worked example: Certificate activity of Cs-137 constancy source, decay-corrected to today, is 180 µCi. Department limit is ±5%. Acceptable range ≈ 171–189 µCi. A reading of 165 µCi fails constancy—do not assay patient doses until the problem is fixed or an alternate calibrator is validated.

Accuracy (Installation / Annual)

Accuracy compares the calibrator reading to NIST-traceable (or equivalent nationally traceable) reference sources of known activity. Often two or more photon energies/sources are used so that calibration factors spanning clinical radionuclides are checked.

Accuracy answers “Is the number absolutely right?” Constancy answers “Did it stay the same since yesterday?” Both are required; neither replaces the other.

Linearity (Quarterly)

Linearity verifies that indicated activity is proportional to true activity from the highest clinical activities down to the lowest activities assayed (including residual syringe assays when relevant).

Decay Method

Assay a high-activity short-lived source (commonly Tc-99m) repeatedly over time as it decays, or assay aliquots, and plot indicated vs expected decay-corrected activity. Deviation outside limits at high or low ends indicates nonlinearity (saturation, wrong calibration curve, electronics).

Shield (Sleeve / Calicheck-type) Method

Assay the same source through a series of calibrated attenuating sleeves. Each sleeve simulates a lower activity. Compare readings to expected transmission factors. Faster than multi-day decay but requires proper sleeve kit and technique.

Worked interpretation: Linearity data show agreement within 5% from 300 mCi to 1 mCi but a +18% overestimate at 50 µCi residual assays. High-dose assays may still look fine while low-activity residual measurements are unsafe for regulatory residual limits—repair/recalibrate before relying on low-end readings.

Geometry (Installation / After Repair)

Geometry dependence means the same activity can read differently in a syringe vs vial, different volumes, or different positions in the well because of self-absorption and solid-angle effects.

Geometry test concept: assay known activity in representative clinical containers and volumes (e.g., 1 mL vs 5 mL syringe; 10 mL vial). Establish correction factors or confirm manufacturer factors if readings differ beyond limits. Repeat after repair, chamber replacement, or relocation.

Trap: Assaying a dose in a glass vial when the correction was established for plastic syringes (or ignoring volume) produces systematic over/under-dosing.

Acceptance Criteria — Teaching Ranges

Many textbooks and programs teach that results should agree with expected values within about ±5% for some tests and up to ±10% for others, depending on device, test, and license. Always apply the numeric limit written in your facility’s SOP and RAM license. Exam stems often use ±5% or ±10% as the decision threshold—read the stem carefully.

Operational Discipline

• Select the correct radionuclide setting (including Tc-99m vs Mo-99 on eluate assays when required).
• Use the same holder and position as during QC.
• Assay before injection; record activity, time, and calibrator ID.
• Keep the well clean—contamination mimics elevated activity.
• Mo-99 breakthrough and other RP QC assays still depend on a valid calibrator.

Memory aid: Constancy Daily, Accuracy Annually, Linearity Quarterly, Geometry at Go-live/repair → “CAL-G”.