Background Counts

The background count is obtained from a scan with no radioactive source. This should be obtained following the count of the calibrated source and just prior to obtaining the patient count.

If abnormally high background counts are measured, the source should be identified and, if possible, removed. If abnormally low background counts are measured, the camera energy window setting and collimator should be verified before repeating the background counts.

The counts per μCi are obtained by dividing the background-corrected source count by the calibrated activity for that day. For a specific camera and collimator, the counts per μCi should be relatively constant. When values vary more than 10% from the established ratio, the reason for the discrepancy should be ascertained and corrected and the source count repeated.

Patient Total Body Counts

The source and background counts are obtained first and the camera sensitivity (i.e., constant counting efficiency) is established prior to obtaining the patient count. The same rectangular region of interest (ROI) must be used for the whole body counts, the quality control counts of the radioactive source, and the background counts.

Acquire anterior and posterior whole body images for gamma camera counts. For any particular patient, the same gamma camera must be used for all scans. To obtain proper counts, extremities must be included in the images, and arms should not cross over the body. The scans should be centered on the midline of the patient. Record the time of the start of the radiolabeled dosimetric infusion and the time of the start of each count acquisition.

Gamma camera counts will be obtained at the three imaging timepoints:

Count 1: Within an hour of end of the infusion of the Iodine I 131 Tositumomab dosimetric dose prior to patient voiding.
Count 2: Two to 4 days after administration of the Iodine I 131 Tositumomab dosimetric dose and immediately following patient voiding.
Count 3: Six to 7 days after the administration of the Iodine I 131 Tositumomab dosimetric dose and immediately following patient voiding.
Assessment of Biodistribution of Iodine I 131 Tositumomab

The biodistribution of Iodine I 131 Tositumomab should be assessed by determination of total body residence time and by visual examination of whole body camera images from the first image taken at the time of Count 1 (within an hour of the end of the infusion) and from the second image taken at the time of Count 2 (at 2 to 4 days after administration). To resolve ambiguities, an eva luation of the third image at the time of Count 3 (6 to 7 days after administration) may be necessary. If either of these methods indicates that the biodistribution is altered, the Iodine I 131 Tositumomab therapeutic dose should not be administered.

Expected Biodistribution

On the first imaging timepoint: Most of the activity is in the blood pool (heart and major blood vessels) and the uptake in normal liver and spleen is less than in the heart.
On the second and third imaging timepoints: The activity in the blood pool decreases significantly and there is decreased accumulation of activity in normal liver and spleen. Images may show uptake by thyroid, kidney, and urinary bladder and minimal uptake in the lungs. Tumor uptake in soft tissues and i