Volume Determination
Table 1 shows a comparison of thyroid remnants volumes between the measured surgical specimens and the values calculated using SPECT images with 131I. Table 2 gives the values calculated using the SCMS program.
Table 1. Thyroid remnant volumes estimated by SPECT and surgical
specimens in patients undergoing thyroidectomy.
|
Patient
|
Volume of Surgical Specimen
(mL)
|
Volume Obtained via SPECT
(mL)
|
Total Volume
(mL)
|
|
A
|
13.3 ± 0.9
|
1.2 ± 0.2
|
14.5 ± 0.9
|
|
B
|
6.4 ± 0.5
|
4.3 ± 0.6
|
10.7 ± 0.8
|
|
C
|
17.7 ± 2.4
|
2.0 ± 0.3
|
19.7 ± 2.4
|
|
D
|
18.6 ± 1.2
|
3.5 ± 0.5
|
22.1 ± 1.3
|
|
E
|
14.7 ± 1.3
|
6.0 ± 0.8
|
21.2 ± 1.5
|
|
F
|
11.9 ± 0.9
|
10.1 ± 1.4
|
22.0 ± 1.7
|
|
G
|
17.4 ± 1.8
|
2.2 ± 0.3
|
19.6 ± 1.8
|
|
H
|
19.4 ± 1.7
|
1.8 ± 0.2
|
21.1 ± 1.7
|
|
I*
|
7.5 < V < 10.0
|
7.8
|
15.3 < V < 17.8
|
*Exact dimensions of the pathological sample were not known, as only a lobectomy was performed. Value deduced by consideration of the remnant in the other lobe.Volver>>
Table 2. Comparison of thyroid remnant volume between the value estimated by SPECT and the SCMS method.
|
Patient
|
Volume calculated by SPECT
(mL)
|
Volume calculated by SCMS
(mL)
|
Percent Relative Difference
|
|
E
|
6.0
|
5.2
|
15.4%
|
|
G
|
2.2
|
2.5
|
12.0%
|
|
H
|
1.8
|
1.6
|
12.5%
|
|
I
|
7.8
|
8.8
|
11.3%
|
Volver>>
Activity Concentration
Table 3 gives the concentrations of activity observed at various times post administration in the patient population studied. The concentrations varied between 0.02 MBq/mL and 7.23 MBq/mL. Using these measurements and knowing the remnant volume in each case, the iodine activity in the remnant tissue may be calculated at each time. Figure 1 shows the results for one subject after administration of 148 MBq (4 mCi) of 131I. These data are typical of iodine retention after thyroidectomy.
Table 3. 131I activity concentration in thyroid remnants of patients with CDT.
|
Time after administration of 131I (h)
|
Activity Concentration (MBq/mL)
|
|
2-8
|
0.03 - 5.72
|
|
24-26
|
0.03 - 7.23
|
|
48-51
|
0.04 - 6.83
|
|
63-75
|
0.02 - 5.82
|
|
97
|
5.45
|
 |
| Figure 1. Kinetic model for 131I in thyroid remnant tissue for one subject, after administration of 148 MBq (4 mCi). Volver>> |
Using the retention curve for each subject, we calculated the maximum thyroid activity values, which varied between 0.06 and 20.7 MBq (1.6 - 560 ěCi), corresponding to maximum percent uptake values between 0.04% and 13.8%, with a mean value of 3.8%. These values agree with the findings of others in similar studies 8, 9, 10.
Therapeutic Activity
The best therapeutic activity for each subject was calculated using the data above, and assuming a desired therapeutic absorbed dose of 300 Gy, as suggested by various authors 11, 12, 13. These results are shown in Table 4.
Table 4. Therapeutic activities suggested for the subjects in this study, assuming a desired dose of 300 Gy to the thyroid remnant tissue. Volver
|
Patient
|
Calculated Activity
|
| |
(GBq)
|
(mCi)
|
|
A
|
0.8
|
20.4
|
|
B
|
1.1
|
28.9
|
|
C
|
1.1
|
30.3
|
|
D
|
3.2
|
87.1
|
|
E
|
3.1
|
84.9
|
|
F
|
1.8
|
47.3
|
|
G
|
30.2
|
815.7
|
|
H
|
77.6
|
2096.1
|
|
I
|
2.3
|
62.2
|
The PlanDose Program
The program requires as input the results of the study of the individual subject’s biokinetics, i.e. the activity measured at various times after administration of the tracer quantity of 131I. The user must also provide the mass of the thyroid remnant tissue and the amount of 131I administered in the tracer study. The user may also enter some limited data for patient identification, as is shown in Figure 2. The program uses units for input and output as are commonly used in most nuclear medicine centers currently. The program fits the entered data to the functional form of Eq. (2), and calculates the maximum thyroid uptake and therapeutic activity recommended for the subject. The program also shows the figure of merit observed for the data (Figure 3).
 |
| Figure 2. Form for the PlanDose Program, showing the input data for one thyroidectomy patient |
 |
| Figure 3. Form for the PlanDose Program, showing the results of the curve fit to the entered data and the calculation of suggested therapeutic activity |
