Tc-99m MDP Radiochemical Purity Falls Below Specification After Dosing
It is 14:40 on a Friday. A 6.5 GBq Tc-99m generator, calibrated five days earlier and last eluted 26 hours ago, yields 7.1 GBq in 8.0 mL. Molybdenum-99 breakthrough is 0.06 kBq per MBq of Tc-99m (limit 0.15) and alumina is under 10 micrograms per mL. At 13:50 a technologist reconstitutes a Tc-99m MDP kit with 5.5 GBq of eluate in 6.0 mL and labels doses for three patients. All three are injected between 14:10 and 14:20 for oncologic staging: a 71-year-old man with prostate cancer, a 58-year-old woman with breast cancer, and a 66-year-old man with newly diagnosed multiple myeloma. At 14:30 the radiochemical purity by instant thin-layer chromatography in methyl ethyl ketone reads 86.4 percent; a second strip prepared by the same technologist reads 87.1 percent. The kit insert and your SOP require at least 90 percent. The three patients are in the waiting room and the first has started imaging. The technologist asks whether 87 percent is "close enough" given that the doses are already in.
1. Is 87 percent acceptable? Explain what the number means biologically and what you would do before deciding the fate of the remaining kit.
Read the worked answer
No. The 90% figure is a release specification, not an aspiration, and the number is not a formality: the impurity here is free pertechnetate, which does not bind bone. It goes to the thyroid, salivary glands, stomach, and bowel, raising background and reducing lesion contrast, and the patient absorbs a thyroid dose for no diagnostic benefit.
But do not discard the kit on the basis of a test that has not been verified. False-low ITLC results are common and usually technique-related:
- Over-spotting (more than about 1-2 microlitres), which broadens the origin and drags activity up the strip.
- Strip not dried before development, or the chamber not equilibrated with MEK vapour.
- Solvent beyond its shelf life, contaminated, or the wrong solvent used.
- Eluate left standing too long or oxidised, or the kit used past its expiry or after partial reconstitution.
Repeat once with a fresh strip, correct spotting volume, a dried strip, and a properly equilibrated chamber; run a positive control if available. Confirm with a second, orthogonal method if your lab has one. Only then accept or reject. Record both results and the investigation.
2. The three patients have already been injected. What are your obligations to them, to the referring physicians, and to the records, and should any of them be rescanned?
Read the worked answer
Do not decide reportability alone - escalate to the authorised user and RSO the same day. The administration matched the prescribed pharmaceutical and activity, so this is most likely a documented quality deviation rather than a reportable medical event, but the determination depends on your regulator and facility policy and should be made by the AU, in writing.
Clinical handling, patient by patient:
- Have the interpreting physician review images as they are acquired rather than after the list finishes. ~13% free pertechnetate mainly degrades contrast and adds thyroid, gastric, and renal-pelvic activity that can mimic or mask disease.
- The myeloma patient is the highest risk: small lytic lesions and rib disease are exactly what a contrast-poor bone scan misses. If his images are degraded, repeat with a fresh QC-passed preparation on another day.
- The prostate and breast patients are usually interpretable if bone uptake is adequate, but the decision belongs to the reporting physician, not the technologist and not you alone.
- Tell each patient, in plain terms, that a quality check failed and what it means for them; do not leave them to find out from a repeat appointment. Inform the referring physicians in writing.
Record the failed RCP, the administered activity and time, the images, the reading physician's assessment, and the disclosure.
3. Design the corrective action and the change to the release procedure that would have prevented this.
Read the worked answer
Root cause is twofold and both parts need fixing: the product failed, and the workflow allowed administration before the result existed.
Product side. Audit the specific batch: kit expiry, eluate age since last elution, volume and activity added, any air introduced, and the technologist's ITLC technique observed directly. Retain the failed vials and strips. If a second technologist using correct technique reproduces 87%, the defect is real and the kit or eluate is the cause; if the repeat is above 90%, the defect is the test.
Workflow side - the real fix. Change the release procedure so that radiochemical purity is completed and documented before the first patient is dosed. Where patient scheduling makes this impossible, prepare earlier in the day, not later. Add:
- A second-person verification for kit expiry, eluate age, and volume at reconstitution.
- A written RCP log with results trended over time, so that a run of 91% results is recognised as a warning rather than a pass.
- Competency assessment for ITLC technique at induction and annually, including spotting volume and chamber equilibration.
- A rule that no dose leaves the radiopharmacy without the QC result attached.
Finally, record the deviation, its patient impact, and the corrective action in the quality system, and review it at the next QA meeting with the AU present.