Impact of Post-Thyroidectomy I-123 Versus I-131 Whole Body Scan on the Outcome of Radioactive Iodine (I-131) Ablation for Differentiated Thyroid Carcinoma

Objectives: I-131 is a widely used
radiopharmaceutical for the
management of patients with
differentiated thyroid carcinoma (DTC).
It is the main agent for the ablation of
residual thyroid tissue and the treatment
of recurrent or metastatic DTC.
However, diagnostic doses of I-131 may
cause some form of cell damage
“stunning” which reduce subsequent
uptake of the therapeutic dose of I-131.
In contrast, I-123 is considered an ideal
isotope for diagnostic studies; it emits
only gamma radiation and thus lacks the
stunning effect. The aim of the current
study was to assess the impact of postthyroidectomy
I-123 versus I-131
Whole Body Scan (WBS) on the
outcome of I-131 ablation for DTC.
Methods: 70 consecutive DTC patients
(60 papillary thyroid Ca., and 10
Follicular thyroid Ca.), underwent total
thyroidectomy, followed by WBS, neck
ultrasound (US), and subsequent I-131
ablation after 40 days without thyroid
hormone replacement (TSH > 30
μIU/ml) were retrospectively enrolled in
current study. Scans were acquired using
the Forte dual head gamma camera
(Phillips medical systems) equipped
with a low energy parallel hole
collimator, 24 hours after an oral
administration of 37-111 MBq (1-3
mCi) of I-123, in 41 patients (Group 1),
and equipped with a high energy parallel
hole collimator, 48 hours after an oral
administration of 74-185 MBq (2-5mCi)
of I-131 in 29 patients (Group 2). 6
months later follow up I-131 WBS, neck
US, serum thyroglobulin (Tg) and Tg
antibodies (TgAb) were performed
following suspension of L-thyroxin for
one month (TSH > 30 μIU/ml) in 61
patients and following rhTSH
stimulation in 9 patients. Results: No
significant difference regarding age,
gender and histopathology between both
groups. 32/41, (78%) patients of group
(1) and 16/29 (55%) of group (2) had
negative follow up WBS, however, 5
patients of group (1), and 4 of group (2)
with negative follow up WBS, had Tg >
5 ng/ml, with US findings suggestive of
residual disease in the neck. The overall
successful ablation rate were 27/41, 66%
in group (1) versus 12/29, 41.4% in group
(2) (P = 0.042). Conclusions: We found
a significantly higher successful I-131
ablation rate among patients studied
with I-123 WBS compared to those
studied with I-131 WBS before
radioiodine ablation, and recommend
the use of I-123 instead of I-131 for
WBS before radioiodine ablation.