Gigabecquerel to Millicurie

The patient swallows a capsule containing 1–7 GBq of I-131. The thyroid gland concentrates iodine from the bloodstream — it cannot tell radioactive iodine from stable iodine — so the isotope accumulates right where you want it. I-131 emits beta particles with a range of about 2 mm in tissue, which destroy thyroid cells from the inside while sparing nearby structures. The gamma rays it also emits are used for imaging to verify uptake. Within weeks the targeted tissue is dead, no scalpel required.

At 3–7 GBq, a freshly treated thyroid cancer patient is a walking radiation source. They emit gamma rays and excrete I-131 in sweat, saliva, and urine for days. Regulations typically require isolation until the retained activity drops below 1.1 GBq or the dose rate at 1 meter falls below 25 µSv/hr. That usually means 2–5 days of sleeping alone, using a dedicated bathroom, and avoiding prolonged close contact — especially with children and pregnant women, who are more radiation-sensitive.

Brachytherapy places a sealed radioactive source directly inside or next to a tumor — "brachy" is Greek for "short distance." High-dose-rate (HDR) sources of iridium-192 at 100–370 GBq deliver an intense, highly localized dose in minutes. The inverse-square law means tissue just centimeters away receives dramatically less radiation. This precision is why brachytherapy can treat cervical, prostate, and breast cancers with fewer side effects than external beam radiation alone.

Industrial radiography sources (1–20 GBq of Ir-192 or Se-75) live inside heavy shielded containers called "cameras" or "projectors" made of depleted uranium or tungsten. The source is only pushed out through a guide tube during an exposure, and the area is roped off with radiation monitors. Strict transport regulations, tamper-proof locks, and regular inventory audits apply. When sources decay below useful activity, they are returned to the manufacturer. The IAEA maintains a database of lost or orphaned sources — the ones that slip through the system occasionally cause severe accidents.

Diagnostic procedures use just enough activity to produce a readable image — typically 50–800 MBq (0.05–0.8 GBq). The goal is information, not tissue destruction. Therapeutic procedures aim to kill cells, so they use 10 to 100 times more: 1–7 GBq for thyroid ablation, 100–370 GBq for HDR brachytherapy sources. The line between them is roughly 1 GBq. Below that, you are taking a picture; above it, you are prescribing a lethal dose to a very specific target.

Yes, and it creates real problems. If a patient who received therapeutic I-131 (30–200 mCi) dies within days, the body can trigger radiation alarms at funeral homes and crematoria. Cremation is the bigger concern — burning the body aerosolises the isotope, contaminating the crematorium and potentially exposing workers. Most radiation safety programs require a waiting period before cremation, or direct burial with notification to the funeral director. In 2019, an Arizona crematorium unknowingly cremated a patient with residual lutetium-177, contaminating the facility. Hospitals are supposed to flag these cases, but the system is imperfect.

The radiopharmacist draws the Tc-99m solution into a syringe, places it in a dose calibrator (a pressurized argon ionisation chamber), and reads the activity in mCi or MBq. Because the isotope is decaying constantly — Tc-99m loses half its activity every 6 hours — the calibrator reading must be decay-corrected to the planned injection time. If the scan is at 2pm and the dose is drawn at 10am, the pharmacist dispenses more than the prescribed 20 mCi, knowing it will decay to exactly 20 mCi by injection. Timing is everything.

The Tc-99m bone scan, with about 20–25 mCi (740–925 MBq) injected intravenously. Technetium-99m accumulates in areas of high bone turnover — fractures, infections, metastases — and emits 140 keV gamma rays that a gamma camera images. The scan itself takes 2–3 hours (allowing time for the tracer to distribute), and the patient's radioactivity drops to negligible levels within 24–48 hours. Over 30 million Tc-99m procedures are performed worldwide each year, making it by far the most-used medical radioisotope.

Technically yes, but radiation detectors at airports, borders, and government buildings may alarm for days after certain scans. A patient who received 10 mCi of I-131 can trigger a portal monitor for up to 3 months. Most nuclear medicine departments provide a wallet card explaining the procedure, isotope, and date — TSA and customs agents are trained to recognize these. The actual radiation risk to fellow passengers is negligible; the issue is entirely about security system sensitivity, not safety.

Hyperthyroidism treatment aims to kill just enough thyroid tissue to normalize hormone production — typically 5–15 mCi (185–555 MBq) of I-131. Thyroid cancer ablation aims to destroy every remaining thyroid cell after surgery and kill any metastases — that takes 30–200 mCi (1.1–7.4 GBq). The higher doses require inpatient isolation and more aggressive radiation safety precautions. Some oncologists are exploring whether lower ablation doses (30 mCi) work as well as high ones (100+ mCi) for low-risk cancers — the evidence is surprisingly close.