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Effect of a Diagnostic Dose of 185 MBq ¹³¹I on Postsurgical Thyroid Remnants

Medicina Nuclear, Center de Tecnologia Diagnòstica, Tarrasa, Spain

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Stunning of thyroid remnants after diagnostic scanning with ¹³¹I may limit ¹³¹I therapy. The follow-up scans and serum thyroglobulin levels of such patients have been studied. Methods: Three-hundred seventy-eight patients who underwent thyroidectomy for differentiated thyroid carcinoma were studied. Diagnostic scans were obtained with 185 MBq ¹³¹I; 7.2 wk afterward, the patients received 489 ablative treatments with 4 GBq ¹³¹I. A total of 1575 scans were obtained (pre- and posttreatment scans and 615 follow-up scans). The evolution of serum thyroglobulin levels was determined. Results: In all patients, the thyroid-stimulating hormone level was more than 30 µIU/mL. Posttherapy scans showed less uptake than did diagnostic scans or even showed negative findings in 99 patients (21%). In these patients, the mean time between diagnostic scanning and therapy was 7.9 wk. In 61 of these 99 patients (61.6%), follow-up scans have shown negative findings, serum thyroglobulin was less than 3 ng/mL, and antithyroglobulin antibodies were not present. Twenty-three patients (23.2%) have not undergone follow-up scanning yet. In 8 patients (8.1%), follow-up scans showed negative findings but serum thyroglobulin was more than 3 ng/mL. In 7 patients (7.1%), follow-up scans showed less uptake than posttherapy scans, but the findings were not yet negative and thyroglobulin was less than 3 ng/mL. Conclusion: Our data suggest that a stunning effect does not exist for doses of 185 MBq ¹³¹I for diagnostic scans. However, a therapeutic effect may exist.

Key Words: stunning effect • diagnostic dose • ¹³¹I treatment • differentiated thyroid carcinoma

	INTRODUCTION

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The treatment of differentiated thyroid carcinoma with total or near-total thyroidectomy is well accepted. Even after total thyroidectomy, thyroid remnants are usually present (1). Diagnostic scans are used to determine the appropriate therapeutic dose (2,3).

Several studies concerning the best tracer and the optimal dose of ¹³¹I for diagnostic scans have been published (4–7). Different administered activities yield differences in diagnostic information; with higher activities, more lesions and more advanced stages of disease can be shown (4,5,8,9). However, a subject of debate is whether a high imaging dose of ¹³¹I can decrease the uptake of a subsequent ablative radioiodine dose by normal thyroid remnants and thereby reduce therapeutic efficacy (10–12). This effect has been called thyroid stunning. A possibility is that those higher tracer doses are capable of ablating or affecting thyroid remnants, because these diagnostic activities are commonly used therapeutically for thyrotoxicosis.

Knowing the follow-up findings of patients in whom the stunning effect exists would help in ascertaining whether thyroid stunning is a real phenomenon. Therefore, the aim of this study was to determine whether lesions that show less uptake on posttherapy scans than on diagnostic scans stay the same or become more intense on subsequent follow-up scans.

	MATERIALS AND METHODS

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Subjects
We studied 378 patients (299 females, 79 males; age range, 18–83 y; mean age, 45.9 y) who underwent total or nearly total thyroidectomy for differentiated thyroid carcinoma. Three hundred one patients (79.6%) had papillary carcinoma, 76 (20.1%) had follicular carcinoma, and 1 (0.3%) had sclerosing mucoepidermoid carcinoma with eosinophilia. Seventy (23.3%) of the 301 patients with papillary carcinoma had local lymphatic invasion, and 5 (1.7%) had distant metastases (2 in bone and 3 in lung). Three (3.9%) of the 76 patients with follicular carcinoma had local lymphatic invasion, and 9 (11.8%) had distant metastases (7 in bone and 1 in lung).

¹³¹I Treatment
All patients were referred to our department for radioiodine therapy. They received 489 treatments with ¹³¹I in doses of 1850–7400 MBq (50–200 mCi). The mean dose was 3996 MBq (108 mCi). Two hundred ninety-three patients (77.5%) were treated 1 time with¹³¹I; 69 (18.3%) were treated 2 times, 10 (2.6%) were treated 3 times, 2 (0.5%) were treated 4 times, and 4 (1.1%) were treated 5 times (Table 1). If radioiodine treatment was not administered immediately after the diagnostic scan and, hence, administration of thyroxine hormone was needed, this hormonal treatment was interrupted 3 wk before therapy.

Diagnostic scanning was performed for at least 4 wk after surgery, and the scans were obtained 48 h after administration of 185 MBq ¹³¹I. If thyroxine hormone replacement was need, it was discontinued 3 wk before the diagnostic scan. The mean time between the pretreatment scan and the ¹³¹I therapy was 7.2 wk (range, 1–16 wk). Posttreatment scans were obtained a mean of 6.2 d after the ablative dose of ¹³¹I. Like the pretherapy scans, follow-up scans were obtained 48 h after administration of 185 MBq ¹³¹I. During the previous 3 wk, the patients did not receive thyroid hormone replacement.

All scans were performed with a computerized gamma camera (SP4; Elscint, Haifa, Israel) equipped with a high-energy parallel-hole collimator. Four images were obtained for 10 min each: head and neck, anterior thorax, posterior thorax, and pelvis. The scans were evaluated visually by 2 experienced physicians. Any scan with ¹³¹I uptake out of physiologic locations was considered to show positive findings.

Thyroid-Stimulating Hormone, Thyroglobulin, and Antithyroglobulin Antibodies
A blood sample was obtained just before the ¹³¹I administration for the diagnostic and follow-up scans to determine the levels of serum thyroid-stimulating hormone (TSH) (immunoradiometric assay) and thyroglobulin and to determine whether antithyroglobulin antibodies were present (radioimmunoassay).

Follow-Up
In our protocol, the first follow-up examination was 6 mo after ¹³¹I therapy. The second was 1 y later, and the third, fourth, and fifth were also at 1-y intervals. Afterwards, we obtain a scan at 2-y intervals until 10 y.

At each follow-up examination, we scanned the patient, measured the levels of TSH thyroglobulin, and checked for antithyroglobulin antibodies. A study was classified as negative—that is to say, radioiodine treatment was successful—if the scan findings were negative, the serum thyroglobulin level was less than 3 ng/mL, and antithyroglobulin antibodies were absent. A study was classified as showing decreased uptake if it had fewer foci of pathologic uptake or if it had foci of equal number but less intensity.

	RESULTS

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In all patients, the TSH level was more than 30 µIU/mL. On diagnostic scans, 373 patients (98.7%) showed uptake of ¹³¹I in the thyroid bed and only 5 patients (1.3%) had negative pretreatment scan findings. For 2 of these 5 patients (1 with papillary carcinoma and 1 with follicular carcinoma), activity could be seen in the thyroid bed on the posttherapy scan. For the other 3 patients (2 with papillary carcinoma and 1 with follicular carcinoma), the posttreatment and follow-up scans showed negative findings, serum thyroglobulin was less than 3 ng/mL, and antithyroglobulin antibodies were absent.

For 99 patients (21%), posttherapy scans showed less thyroidal uptake than did diagnostic scans. These patients were the so-called stunned patients. Seventy-seven (77.8%) had papillary carcinoma, and 22 (22.2%) had follicular carcinoma. The proportion of both types of differentiated thyroid carcinoma was similar between the whole group and this subgroup. We saw no variations in the uptake of distant metastases between pre- and posttreatment scanning. In this subgroup of patients, the mean therapeutic dose was 4033 MBq (109 mCi) ¹³¹I, which did not significantly differ from the dose of the whole group (3996 MBq; 108 mCi).

The mean interval between the diagnostic scan and treatment was 7.9 wk, and the mean time between treatment and the posttreatment scan was 5.3 d. Neither of these intervals was significantly different from the whole group (7.2 wk and 6.2 d, respectively).

The follow-up status of these 99 patients is as follows (Table 2): 23 patients (23.2%) have not yet undergone follow-up scanning or are not being followed up. For 61 patients (61.6%), follow-up scan findings became or continued to be negative, the serum thyroglobulin level became or continued to be less than 3 ng/mL, and antithyroglobulin antibodies were absent. These patients had no evidence of disease. For eight patients (8.1%), although follow-up scan findings were negative, the level of serum thyroglobulin was greater than 3 ng/mL. Therefore, we could classify the study as negative. For 7 patients (7.1%), follow-up scans showed less uptake than did the posttherapy scan but were not yet negative, the serum thyroglobulin level was less than 3 ng/mL, and antithyroglobulin antibodies were absent.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

Perhaps the first point to consider is whether the diagnostic scans are needed. Remnants are almost always present after total or near-total thyroidectomy and must be destroyed with a therapeutic dose of ¹³¹I (3,13). Consequently, some authors have abandoned tracer scanning and treat their patients systematically with ¹³¹I after surgery without scanning first (1). We also found that some thyroid tissue usually remains after total thyroidectomy; the pretreatment scan showed negative findings on only 5 occasions (1.3%). But studies have shown that with a high tracer dose, the information obtained is more diagnostic and the staging of disease is more advanced (4,5,8,9). Therefore, a low scanning dose (37–74 MBq) is less useful.

A high dose (185–370 MBq), although producing a beneficial scan, seems also to produce thyroid stunning (1,7,14). Thyroid stunning can be defined as reduced iodine uptake by normal thyroid remnants and tumor after administration of ¹³¹I (15). Stunning may also change the tumor kinetics of radioiodine subsequently given for therapy by reducing its efficacy (16).

A solution to this problem may be to scan with ¹²³I, which appears to give the same diagnostic information as ¹³¹I. ¹²³I is a pure gamma emitter that delivers a low radiation dose to the thyroid and does not decrease the uptake of a subsequent therapeutic dose of ¹³¹I, but ¹²³I has some disadvantages. One is its high cost. Moreover, a study (17) showed that ¹²³I is less sensitive and less accurate than ¹³¹I. Therefore, routine scans with large doses (185–370 MBq) would be too expensive (1).

Another disadvantage is that ¹²³I has a short half-life (13 h). Whole-body imaging requires an interval of at least several hours for the thyroid metastases to take up iodine, so ¹²³I is not ideal for whole-body imaging. ¹²³I presents another drawback. In follow-up of patients who have undergone radioablation, accuracy is less with ¹²³I than with ¹³¹I (17). Consequently, diagnostic scans would be useful with ¹³¹I and with large doses, with the drawback of the supposed stunning effect.

The results concerning the stunning effect seem to differ according to the elapsed time between the diagnostic scan and therapy (Table 4). Park et al. (12) found stunning within days of a diagnostic scan. O'Mara et al. (18) showed no stunning effect from a 185-MBq ¹³¹I diagnostic dose on posttreatment scans when the therapy was given shortly after the diagnostic dose. Muratet et al. (19) found impairment of treatment efficacy after a 111-MBq diagnostic dose of ¹³¹I in comparison with a 37-MBq scanning dose. In their study, the ablative therapy was given 9 d after the diagnostic scan. The results are contradictory. Leger et al. (1) found that 185 MBq ¹³¹I can impair the subsequent uptake of a therapeutic dose. In that study, the delay between the diagnostic dose and the therapeutic dose varied between 15 and 84 d. In our study, this delay has varied between 1 and 16 wk. For that interval, we have seen so-called stunning in 21% of patients, although it is questionable whether the decreased uptake is, in fact, a stunning effect or a decrease of the tissue remnant. Knowing the evolution of these patients—through both scans and serum thyroglobulin levels—can help us determine whether thyroid stunning is a real phenomenon. In light of our results, the stunning effect is not real, because in no patient did the "stunned" lesion become more intense on the follow-up scans or even show the same intensity. All lesions showed less uptake on follow-up scans than on the posttherapy scans or disappeared (Table 2). This finding contrasts with the nonstunned group. As shown in Table 3, only 36.9% of patients in the nonstunned group showed negative findings on the follow-up scans. That the effect is, then, therapeutic is not surprising because the large doses (185–370 MBq) used for diagnostic scans for thyroid carcinoma after thyroidectomy, which leaves only a small mass of thyroid tissue or tumor volume, are usually administered therapeutically for thyrotoxicosis. Although the percentage of the dose taken up is much lower than in thyrotoxicosis, the volume of tissue that receives the dose is also much smaller. Therefore, these activities may affect or even ablate such small remnants (Figs. 1–3).

View larger version (129K): [in this window] [in a new window]   FIGURE 1. Diagnostic scan (185 MBq [5 mCi]) of patient with papillary carcinoma 5 wk after thyroidectomy. TSH level was greater than 30 µIU/mL.

View larger version (201K): [in this window] [in a new window]   FIGURE 3. Follow-up scan (185 MBq [5 mCi]) of patient (same as in Figures 1 and 2) 6 mo after ablative dose (3700 MBq [100 mCi]), without evidence of disease. TSH was greater than 30 µIU/mL and thyroglobulin was less than 3 ng/mL.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
No stunning effect was observed when doses of 185 MBq ¹³¹I were used to stage differentiated thyroid carcinoma on diagnostic scans. The decreased uptake in thyroid remnants seen in 21% of our patients corresponded to a therapeutic effect through which the diagnostic dose was sufficient for total or partial ablation.

View larger version (103K): [in this window] [in a new window]   FIGURE 2. Posttherapy scan of patient (same as in Figure 1) 6 d after therapeutic dose (3700 MBq [100 mCi]). TSH level was greater than 30 µIU/mL.

	FOOTNOTES

For correspondence or reprints contact: M^a Teresa Bajén, MD, Center de Medicine Diagnòstica, Paris, 83-85 sot. 2^a, 08029 Barcelona, Spain.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 

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