Comparison of 123I and 131I for Whole-Body Imaging in Thyroid Cancer ===================================================================== * Salil D. Sarkar * Tomy P. Kalapparambath * Christopher J. Palestro ## Abstract We compared the diagnostic sensitivities of 123I and 131I whole-body imaging in differentiated thyroid cancer. **Methods:** Twelve thyroidectomized patients (3 previously treated with 131I) were studied. After a period of thyroid hormone withdrawal, whole-body imaging was performed approximately 24 and 72–96 h after administration of 74–185 MBq (2–5 mCi) 123I and 111–185 MBq (3–5 mCi) 131I, respectively. **Results:** Both 123I and 131I revealed residual thyroid tissue, present in 9 patients. 131I detected metastases in 5 studies of 4 patients. In 4 of 5 studies, 123I missed metastases shown by 131I in 8 body regions including the neck, mediastinum, lungs, and bone and detected 3 other sites of metastasis only in retrospect. No lesion was better seen with 123I than with 131I. **Conclusion:** Although 123I is adequate for imaging residual thyroid tissue, it appears to be less sensitive than 131I for imaging thyroid cancer metastases. * 123I * 131I * thyroid cancer Several medical centers are now using 123I for evaluation of thyroid cancer, in part because of concern about stunning of thyroid tissue by 131I. However, the efficacy of 123I for identifying thyroid cancer metastases, particularly those distant from the neck, remains uncertain. Here, we compare 123I and 131I whole-body imaging in patients with and without metastases. ## MATERIALS AND METHODS Patients with differentiated thyroid cancer were referred for pretherapy radioiodine imaging. A total of 12 patients underwent 13 sets of 123I and 131I studies. All patients had prior thyroidectomy; 3 had prior 131I treatment, and endogenous serum thyroid-stimulating hormone levels were generally 50 μU/mL or greater at the time of imaging. Eleven of 12 patients received 131I treatment, which was given within 1 wk after imaging. The 123I study was performed approximately 24 h after oral administration of 74–130 MBq (2–3.5 mCi) tracer in solution (1 patient received 185 MBq [5 mCi]). Immediately after the 123I study, 111–185 MBq (3–5 mCi) 131I were administered in capsule form. Imaging was performed routinely at 72–96 h, and in a single instance, at 24 h as well. Imaging was repeated approximately 1 wk after ablative 131I therapy. 123I and 131I images were acquired using low-energy, all-purpose collimators and high-energy, high-resolution collimators, respectively. Routinely, composite whole-body images (scan speed, 5–6 cm/min) were obtained together with 10-min head-to-pelvis spot views. Two experienced readers compared 123I and 131I images for ease of lesion detection. Findings were confirmed by posttherapy imaging, CT, 18F-FDG PET, or follow-up/previous radioiodine imaging when available. ## RESULTS No metastases were found in 6 of the 12 patients. None of these 6 patients had prior 131I treatment, and scintigraphy showed residual cervical thyroid tissue in all. This tissue was seen equally well with 123I and 131I. Metastases were found in the remaining 6 patients. In 2 of these, metastases (pulmonary) were seen only at posttherapy imaging, and the pretherapy 123I and 131I images were concordant, showing only residual cervical thyroid tissue. The remaining 4 patients had 5 sets of 123I and 131I studies. In 2 patients, 131I images showed diffuse pulmonary and cervical nodal metastases not seen on 123I images (Figs. 1 and 2A). In one of these patients, the 123I image was concordant with the 131I image at 24 h but discordant with the image at 96 h (Fig. 1). In a third patient, 131I imaging showed multiple skeletal lesions and diffuse pulmonary metastases, which were not seen or were seen only in retrospect on 123I imaging (Fig. 2B). At follow-up 1 y later, 123I and 131I images were comparable. The fourth patient had uptake of 131I in a tumor mass in the right axilla and faint uptake in mediastinal metastases, not clearly seen on 123I imaging. ![FIGURE 1.](http://jnm.snmjournals.org/https://jnm.snmjournals.org/content/jnumed/43/5/632/F1.medium.gif) [FIGURE 1.](http://jnm.snmjournals.org/content/43/5/632/F1) **FIGURE 1.** An 81-y-old man with papillary thyroid cancer presented with radiographic evidence of pulmonary metastases. Posterior whole-body images at 24 h with 123I (left) and 131I (middle) are unremarkable (thyroid bed uptake was noted with both tracers on anterior images, not shown). Stomach and bowel activities are noted in abdomen. Posterior 131I image at 96 h (right) shows uptake in pulmonary and left cervical metastases (arrows). ![FIGURE 2.](http://jnm.snmjournals.org/https://jnm.snmjournals.org/content/jnumed/43/5/632/F2.medium.gif) [FIGURE 2.](http://jnm.snmjournals.org/content/43/5/632/F2) **FIGURE 2.** (A) A 63-y-old man with Hürthle cell thyroid cancer had distant metastases and persistent disease despite 2 prior treatments with 131I. Anterior whole-body 123I image (left) is negative for metastases. Midline chest activity is in esophagus. Corresponding 131I image (right) shows diffuse pulmonary and right cervical nodal metastases (arrows). This patient received larger amount of 123I (185 MBq) than of 131I (148 MBq). (B) A 53-y-old woman with follicular thyroid cancer and distant metastases received 4 prior 131I treatments for persistent disease. Anterior whole-body 123I image (left) shows metastases in left hip, right femur, and left proximal humerus. These foci are better seen on 131I image (right), which additionally shows diffuse uptake in lungs and focal lesions in skull and right iliac bone (arrows). Midline chest activity is in esophagus. ## DISCUSSION Whole-body radioiodine imaging helps assess residual thyroid tissue and detect recurrent or metastatic thyroid cancer (1). Traditionally, imaging has been performed using 131I in amounts of 74–185 MBq (2–5 mCi). But such amounts may be associated with stunning of thyroid tissue, so that uptake of a subsequent ablative dose of radioiodine is decreased (2). While the issue of stunning continues to be debated, several medical centers have started using 123I instead of 131I for whole-body imaging. Our study showed little difference between 123I and 131I for evaluation of the residual thyroid tissue present in 9 patients. This finding is consistent with a recent report that 123I is at least as accurate as 131I for evaluation of residual thyroid tissue (3). More important, our data indicate that 131I is superior to 123I for identification of metastases. In 4 patients undergoing 5 sets of studies, metastases in a total of 8 body regions including cervical nodes, bone, lung, and mediastinum were not detected with 123I, and 3 other lesions (in bone and right axilla, third and fourth patients) were appreciated only in retrospect. In no instance was a lesion better seen with 123I than with 131I. The discordance in diagnostic sensitivities of the 2 tracers is probably related to differences in the intervals between tracer administration and imaging. The longer physical half-life of 131I permits later imaging, when target-to-background ratios are higher. For the patient shown in Figure 1, the 24-h 131I (and 123I) images were negative for metastases, but the 96-h 131I study clearly identified tumor in the neck and lungs. Differences in tracer amounts were unlikely to account for our results. The generally higher 131I dose was largely offset by the use of a high-energy, high-resolution collimator with an inherently lower counting efficiency. In fact, for the patient shown in Figure 2A, the 123I study missed metastases despite a relatively higher tracer amount. For the patient shown in Figure 1, 24-h 131I images with higher count rates were not as accurate as lower-count 96-h images. Our report does not address the potential for improved efficacy by the use of substantially larger amounts of 123I, perhaps with later imaging. A recent study comparing 123I studies using 185–555 MBq (5–15 mCi) tracer with posttherapy 131I imaging showed 94% and 82% concordance among patients undergoing their first and second 131I treatments, respectively (4). Use of larger amounts of 123I, therefore, deserves consideration, but there are limitations. 123I is very expensive (3- to 4-fold more costly than 131I), requires administration of a large number of capsules (each capsule contains only 3.7–7.4 MBq [100–200 μCi]), and is not readily available in liquid form from commercial vendors. Consequently, the routine use of large amounts of 123I for whole-body imaging is beyond the scope of many institutions. ## CONCLUSION When administered amounts of tracer are 185 MBq (5 mCi) or less, 123I is comparable with 131I for imaging thyroid remnants. However, 123I appears less sensitive than 131I for imaging thyroid cancer metastases, missing lesions in bone, lungs, and lymph nodes. Our experience, albeit limited, questions the practice of routinely substituting 123I for 131I for whole-body imaging. ## Footnotes * Received Jul. 25, 2001; revision accepted Jan. 16, 2002. For correspondence or reprints contact: Salil D. 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