Dispensing of 99mTc-MDP Doses
Once the multidose preparation of 99mTc-medronate (99mTc-methylene diphosphonate, or 99mTc-MDP) is ready, each individual patient dose must be drawn up for administration to patients scheduled at particular times during the day. The preparation of the individual patient doses is normally done in the hospital, more specifically in the radiopharmacy or in the nuclear medicine department, except in cases where individual patient doses are being delivered to the nuclear medicine department by a central radiopharmacy. Unlike the situation in the US, the number of central radiopharmacies in Europe is limited, although the situation is slowly changing.
The procedure can vary from department to department. In some places, the individual doses are withdrawn immediately before administration to the patients. This means that the doses are withdrawn from the 99mTc-MDP vial at different times during the day, depending on the scheduling of the respective patients (in this case step one in the procedure described below is omitted). In other places, all the doses are withdrawn immediately after preparation of the 99mTc-MDP solution and stored in shielded boxes until the moment of injection into a patient.
Step 1: Calculating Individual Patient Doses
In order to ensure that sufficient radioactivity was added to the labelling kit in the form of 99mTc-pertechnetate, the activity of each dose should have already been calculated (see radioactive calculations for details). The volume of preparation required for each of the patient doses should then be calculated if they are all to be withdrawn immediately after preparation. Find out how to perform these individual dose calculations.
Step 2: Checking Radioactivity Concentration of 99mTc-MDP solution
Before withdrawing these doses from the vial, the radioactivity concentration of the 99mTc-MDP preparation, calculated previously (see example calculation), should be checked. This is to ensure that the doses will be of the expected activity at the time of administration to the patient. This check is performed by measuring the activity of the vial using an isotope calibrator and dividing the measured activity by the known volume of solution in the vial. If, exceptionally, the radioactivity and thus also the radioactivity concentration deviates by more than 10% from the expected value, the measurement should be repeated, making sure that the correct radionuclide has been selected on the isotope calibrator. If the second reading is also not as expected, then
- if the activity is too high, the volume to be withdrawn for the individual doses should be recalculated, or;
- if the activity is too low, a new or additional preparation has to be made.
Example of an isotope calibrator
Step 3: Withdrawing Dose and Checking its Radioactivity Concentration
The volume for each dose can then be withdrawn from the 99mTc-MDP preparation into a shielded syringe. After the dose has been withdrawn from the vial, its radioactivity concentration should be measured using the isotope calibrator and adjustments should be made (removal or addition of 99mTc-MDP solution) if the measured activity does not correspond with the calculated activity required.
Step 4: Preparing Dose for Distribution
When the activity reading is correct, the dose should be prepared for distribution to the patient. The procedure for this may be slightly different from department to department. Examples include replacing the needle with a new needle of suitable gauge for injection, or with a blind hub, leaving the person administering the injection to fit a new needle (of suitable gauge for injection) or butterfly needle at the time of the injection. This prevents needles that may have been blunted (by passing through the septum during the withdrawal of doses) from being used for injecting patients.
The syringe then has to be labelled with relevant information to ensure that the correct radiopharmaceutical with the correct activity will be administered to the correct patient, usually:
- patient details
- the name of the radiopharmaceutical
- the measured activity
- the date
- the time
- the time of injection
Lead-lined transport box with labelled syringe in syringe shield
A duplicate label should be prepared and kept for the purposes of documentation. The syringe should then be placed in a lead-lined transport box, to adhere to radiation protection standards, and another label (giving the same information as that shown on the syringe) should be attached to the box. The transport box with the syringe is then transferred to the injection room of the nuclear medicine department or to the room where the imaging will be performed if the injection is to be done while the patient is already positioned under or in front of the gamma camera to allow immediate acquisition of data and images.
This four stage procedure should be repeated for all subsequent doses.
This dispensing process is shown in detail in the interactive slideshow.