Table 2 shows that the percent difference between the volumes calculated by SPECT volumetrics and the technique using the SCMS program were on average 12.8 ± 1.8%. Lima et al. ⁴ found a similar accuracy (10.2%) for sources of irregular geometry. The methodology shown here provides a reasonable accuracy (around 10%), given the limits of resolution of the imaging system and other uncertainties in routine dose calculations.

Figure1shows that the maximum thyroid remnant activity occurs at about 8 h post administration of ¹³¹I. This time varied between 8 and 10 h, similar to values observed by Johansson et al (private communication)*. Use of the more realistic two component retention curve (compared to the classical, one compartment model), results in a better estimation of the optimum ¹³¹I activity to be administered. Use of more simplistic methods has been shown to possibly overestimate the activity to be administered by up to a factor of three ^7.

Noting that all of the subjects received therapeutic activities of 3.7 GBq (100 mCi), Table 4 shows that the majority (7 of 9) could have received less activity, thus receiving lower doses to other organs, especially marrow and gonads. The reduction in marrow dose will be due to a lower level of ¹³¹I in the circulating blood, which also contributes to dose in other tissues. The dose to the gonads will be lower, primarily due to lower amounts of activity being eliminated through the urine. Such dose reductions justify the use of methods which optimize the therapy design for individual subjects.

Only two subjects (G and H) would not have had their activity levels reduced. The therapeutic activity levels suggested by the method were not practical; this is most likely due to these subjects showing very low thyroid remnant uptake values (< 1,0%). These levels of concentration were not treated in the study of correlations between count and activity concentrations by Lima et al. According to Mortelmans et al. ¹⁵  (1986) activity levels below a certain threshold cannot be accurately determined due to limitations in image contrast. In such cases, the concentration should be determined using a subtraction method using the maximum counts in the supplied images. It is likely that this method should be used for concentrations less than 0.6 MBq/mL (16 ìCi/mL). In these cases, patients should not receive doses higher than the routing fixed level of 3.7 GBq (100 mCi).

The data here show that for three of the patients, no hospitalization is necessary, as they could have received as little as 1.1 GBq (30 mCi). This would reduce costs and provide the subjects with an improved quality of life, as they can return to their homes immediately after administration of the therapy. Becker et al¹ were able to treat about 40% of their subjects as ambulatory, using reduced activity levels (740-4810 MBq (20-130 mCi)), with therapeutic effectiveness similar to that using fixed activity values. Their methods did not treat the uptake period, but assumed an “instantaneous” uptake value at 24 hours post administration, which led to an overestimate of absorbed dose in the thyroid remnant tissue. Thomas et al¹⁶ found a dose overestimate of only 10%-15%, which they considered acceptable.

The nuclear medicine clinic as well will benefit from lower administered activity values, as they can use less radioactive material over time, which will reduce costs as well as radiation dose to the clinic staff. The PlanDose program facilitates the application of this technique for the clinic staff. Expertise in radiation dosimetry is not required, only the accurate entry of the subject’s biokinetic data. The target radiation dose value of 300 Gy is also an adjustable parameter in the program, so that the physician can vary this as desired. As with any computer program, however, care must be taken in the evaluation of the output, with both a physicist and the attending physician carefully studying the results and making the final recommendation on the desired level of therapeutic activity to be administered.

In our experience, the Grace 5.0 converged rapidly on an optimum fit in all cases. In one case, however, the fit was interrupted as it reached the maximum number of iterations. Computer fitting of measured data must also be reviewed critically by the professional staff in all cases to ensure that reasonable values are used in the final analysis.

Although this work considered only thyroid ablation, the principles could be extended to the treatment of hyperthyroidism. As shown by Traino et al. ^17, patient-specific metabolic data and organ mass data obtained via ultrasound measurements can facilitate the calculation of subject-specific therapeutic activity levels. The thyroid masses can also be measured using SPECT methods, although these methods may be less reliable, considering the resolution limits of current cameras and the appearance of the “functional volume” being larger than the actual tissue volume.

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