Radiopharmacy

 
Every morning, starting at about 7.30am, two of the radiographers go into the radiopharmacy (which is opposite the main camera control room in St Vincent’s) to make up the injections for all the patients who will attend the department that day.  The radiopharmacy is performed in a purpose-designed cabinet which is in a “clean” room maintained at positive air pressure to reduce ingress of any particles or pathogens.
This room does not have a door onto the corridor, but is to the left of the room with the “Radiopharmacy” sign on the door.  It is entered via an interlocked set of doors, which also helps to reduce ingress of any particles.  The operator changes into a surgical gown and sterile gloves, exactly as for an operation.  This ensures that the final product which will be injected into the patient is of extremely high quality.
 

A useful radiopharmaceutical has two components - a radioactive isotope that will emit gamma rays which can be detected by the gamma camera and a pharmaceutical or tracer that is processed in a particular physiological or pathophysiological process by the body.  In most cases, the two components are initially separate and have to be combined in the radiopharmacy.  An example of this is Tc-99m HMDP, the agent we use for bone scintigraphy.  Once injected, the HMDP (hydroxymethylene diphosphomate) is extracted from blood and taken up in areas of osteoblastic activity in the skeleton.  The gamma rays that will form the image of osteoblastic activity in the body are produced by radioactive decay of Tc-99m, the workhorse isotope of medical radionuclide imaging.


Sometimes both components of a radiopharmaceutical are provided by the same substance, for example I-123, which is taken up by the thyroid in the same way as the non-radioactive isotope of iodine and also emits gamma rays, which can be used to form an image of the thyroid.

The radioactive component of most radiopharmaceuticals is technetium-99m.  This isotope has a half-life of 6 hours 15 minutes, which allows plenty of time for imaging, but avoids unnecessarily long exposure to radiation after the imaging test is over.  The emitted gamma rays have an energy of 140keV, which is similar to the energy of X-rays used for medical imaging, and which is ideally suited to detection by a gamma camera.


Once a week we receive a delivery of a technetium generator to the radiopharmacy.  This contains molybdenum-99, which decays with a half-life of 66 hours to produce Tc-99m.  The generator produces high levels of activity early in the week, with less Tc-99m available late in the week, as the parent isotope (Mo-99) decays. Tc-99m is eluted each morning from the generator (a bit like milking a cow) and this is then combined with the pharmaceutical.  Some radiopharmaceuticals are easy to make, just by injecting Tc-99m into a vial and gently agitating it.  Others such as radiolabelled white blood cells, which are used to image infection, require a much more time-consuming process.


Once the radiopharmaceutical has been prepared, the correct dose of radioactivity (measured in megaBecquerels, MBq) is drawn up for each patient, allowing for radioactive decay that will occur prior to the intended time of injection.  The syringe is then placed in a lead syringe shield, which in turn is placed in a lead carrier box and the box and syringe are labelled with information about the patient and the radiopharmacetical that has been prepared for them.


 

Vials containing pharmaceuticals prior to the addition of Tc-99m to make radiopharmaceuticals

To measure how much radioactivity has been drawn up in each injection, the syringe is lowered into the white cylinder pictured to the left, which is called a well counter (because it is like lowering a bucket into a well).  The screen pictured to the right shows how much activity the syrimge contains.

Once the correct dose has been drawn up, the syringe is placed inside a syringe shield and this in turn is put into a lead lined box, as pictured here.  The box is labelled with the name of the radiopharmaceutical and also with a detailed docket that carries information about the radiopharmaceutical, the dose of radioactivity at the intended time of injection and the patient’s details.  All of these details are carefully checked at the time of injection, to ensure that the correct patient receives the correct radiopharmaceutical.