Indium-111
A radioactive metal used, attached to octreotide, as the original scan for neuroendocrine tumours (Octreoscan) before gallium-68 PET took over, and still used to label white blood cells for infection imaging.
Overview
Indium-111 decays by electron capture to stable cadmium-111 with a 2.805-day half-life, emitting gamma photons suited to gamma-camera and SPECT imaging. For two decades it was the standard somatostatin-receptor imaging agent as 111In-pentetreotide (Octreoscan), used to select neuroendocrine tumour patients before peptide receptor radionuclide therapy, first with 90Y-labelled and later with 177Lu-labelled somatostatin analogues; SPECT-based Octreoscan has since been largely replaced by 68Ga- or 64Cu-DOTATATE PET, which has better resolution and quantification, but 111In-pentetreotide remains on label and in use where PET access is limited.
It is produced on a cyclotron by proton bombardment of an enriched cadmium target, either 112Cd(p,2n)111In or 111Cd(p,n)111In, the latter giving higher radionuclidic purity. Its 2.8-day half-life is long enough to ship internationally and to track slower-clearing tracers such as labelled leukocytes and antibody fragments, which is why it is also used for infection and inflammation imaging (111In-labelled white cells) and in some antibody biodistribution and dosimetry studies.
- γ 171 keV (90%)
- γ 245 keV (94%)
- electron capture
- DTPA
Theranostic partners
- Yttrium-90 · A radioactive metal that fires high-energy particles a short distance into surrounding tissue. Millions of microscopic beads carrying it are injected into the artery feeding a liver tumour, where they lodge and irradiate the tumour from inside.
- Lutetium-177 · A radioactive metal that gives off short-range radiation. Attached to a molecule that sticks to a tumour, it irradiates the tumour from the inside while sparing most healthy tissue.
An antibody engineered to grab onto two markers found together on some solid tumours, but not on healthy cells, carries a powerful short-range radioactive payload into the tumour to try to destroy it.
The original nuclear medicine scan for finding neuroendocrine tumours, used before PET scans like Ga-68 DOTATATE became available. It is still FDA-approved but has largely been superseded because PET gives sharper pictures with less radiation.
A version of the PSMA prostate cancer scan that uses the same cheap, widely available radioactive tag as a standard bone scan instead of the PET tracers. Surgeons can also carry a small radiation detector into the operating room to help find and remove cancerous lymph nodes. It has not been through formal drug approval; it is used as an academic, in-house preparation.
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