Brachytherapy Source Activity Check
A therapist is preparing a high-dose-rate (HDR) brachytherapy afterloader for a cervical cancer implant using iridium-192. The source was calibrated 75 days ago at 10 Ci. The treatment plan was generated assuming an air-kerma strength consistent with the calibration date, but the physicist is delayed. The therapist notices the console displays a source strength lower than the plan's reference value. The patient is already in the treatment room with the applicator in place, and the schedule is tight because the operating room slot is fixed. The department's written directive requires independent verification of source strength before each treatment day.
1. Should the therapist proceed with treatment using the displayed source strength? Explain the decay reasoning.
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No. Ir-192 has a half-life of about 73.8 days, so after 75 days the activity is roughly 10 Ci × 0.5^(75/73.8) ≈ 4.9 Ci - about half the calibration value. Treatment planning systems correct for decay, but only if the correct source strength and calibration date are entered and independently verified. The displayed value being lower than the plan reference is expected from decay, not necessarily an error; however, the therapist cannot assume the plan was updated. The written directive requires independent verification before treatment. The therapist should stop, contact the physicist or authorized medical physicist to verify the source strength with the well chamber and confirm the plan's decay correction, and only then proceed. Proceeding on assumption risks a significant dose error.
2. What documentation and communication steps are required before and after the delay?
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Document the time of the pause, the displayed and independently measured source strength, the calibration date, and the physicist's verification signature. Communicate the delay to the patient, the operating room team, and the radiation oncologist, and explain the reason in plain language. After verification, confirm the treatment plan reflects the correct decayed source strength and that the planned dwell times produce the intended dose. Record any schedule changes and the final delivered parameters. If the delay exceeds the applicator's safe dwell time or the patient's tolerance, the oncologist must decide whether to proceed, adjust, or reschedule, and the decision should be documented. The therapist's role is to safeguard accuracy, not to meet the schedule at the expense of safety.