Nesterov D.V., Titova
L.S., Titova T.S., Nesterov
V.G.
THE INFLUENCE OF 131I DIAGNOSTIC DOSES
ON THYROID UPTAKE
Introduction
Differentiated thyroid carcinoma(DTC) is the commonest endocrine cancer, with 80%
of patients aged less than 60 years. Early intervention with surgery and 131I
ablation enhances remission and has the potential to reduce mortality
(Schlumberger et al., 1986). Detection and treatment of local recurrence or metastatic disease relies upon the initial abolition of
functioning thyroid tissue by total thyroidectomy and
radioiodine ablation of any remnant. Thyroid remnants are generally present
after thyroidectomy, can be detected by radioiodine
scanning. Subsequent management generally utilizes uptake of diagnostic doses
of 131I by the tumour which, if present, can
guides subsequent treatment with therapy doses of 131I. Of course remnant
destruction has been shown to reduce the recurrence rate and improve survival [1],
but the dose of radioactive iodine required to destroy remnants remains
controversial [2,3].
The radiation dose
delivered by 131I concentrated in a tissue depends on two factors:
first, the radioactive concentration and second, the effective half- life [4]. Thus
the dose delivered by 131I to thyroid tissue is proportional to
thyroid uptake and to thyroid ability retain radioiodine.
So the results of the
diagnostic scan may be used to calculate the appropriate dose. But it is a less
of quantitative data in domestic literature about using 131I as predictor
of thyroid uptake.
In order to try to shed
some light on this problem, we studied the changes in thyroid uptake induced by
a diagnostic dose of 131I.
Material and Methods
Twenty-two patients who
had undergone recent surgery for thyroid cancer were investigated using 131I.
There were 8 females and 14 males aged between 38 and 76 years (median 48
years). In those patients who had required thyroid hormone supplement, this was
discontinued 4 weeks before the investigation.
131I uptake was measured 4 weeks after surgery. The
diagnostic activity of 131I (185 MBq) was
administered orally. Uptake in the thyroid remnant measured 3 days later using
a gamma camera (Ohio Nuclear 110) operating in the whole-body scanning mode
(scan time 30 min). A high-energy parallel-hole collimator was employed. An
aliquot of the administered activity, placed in a neck phantom of
Following acquisition of
the images, appropriate thyroid and background regions of interest were drawn
to allow measurement of uptake. These regions were stored and employed for the
post-ablation study.
The ablation activity of
131I (4,000 MBq) was administered 3–38
days after the diagnostic activity and uptake in the thyroid remnant measured
using the gamma camera. None of the patients had received thyroid hormone
supplement in the interim. However, on this occasion, the gamma camera was
operated in high-count rate mode and the imaging time reduced to 5 min. All
other operating conditions were exactly the same as used in the diagnostic
study. Uptake of the ablation activity was measured at least once prior to the
patient being discharged from hospital, that is within
3 days of the radioiodine administration. In 6 of the patients there were made
2 measurements on consecutive days.
Statistical analysis. Non-parametric methods were used in
the statistical analysis of the results (Mann-Whitney U test or Wilcoxon test for pair differences).
Results
Reduced uptake of the
therapeutic radioiodine was observed in all 22 patients. The median of 131I
diagnostic doses uptake was 7,8 %
(range 0,4-17,3%). The
uptake of therapeutic doses had median 2,8%
(range 0,1-16,1%). Uptake of the later was
significantly lower (P<0,001) then diagnostic uptake. Also shown is the
uptake of the therapeutic 131I expressed as a percentage of the
uptake of the diagnostic 131I. The therapy/diagnostic uptake ratio
had median value 28,3% ( range 5,1 – 84,8 %).
Discussion
Our study clearly demonstrates that a dose of 131I
commonly advocated for the diagnosis of remnants or metastasis is sufficient to
significantly decrease thyroid iodine uptake. It was observed in all 22
patients. The quantitative observations presented here show that uptake by the
thyroid remnant after ablation lower then the predicted value, but for
individual patient the degree of decreasing is quite variable. Such variability complicate the closely prediction of thyroid
uptake of therapeutic doses.
In those cases where two measurements of uptake
were made within 1-3 days of ablation activity, there was no evidence of a
large rapid loss of the 131I. Thus we can suggest that diagnostic
doses of 131I decreased ability of thyroid tissue to trap rather
than decreased ability retain radioiodine.
We didn’t find any dependency between degree of
decreasing and time delay between two administrations of 131I.
In summary, the results of presented report
indicate the impossibility of closely prediction of thyroid uptake radioiodine
therapeutic doses after 131I diagnostic study.
References:
1.
Mazzaferri EL. Thyroid remnant 131I ablation
for papillary and follicular thyroid carcinoma. Thyroid 1997; 7: 265–271.
2.
Beierwaltes WH. The treatment of thyroid carcinoma with
radioactive iodine. Semin Nucl
Med 1978; 8: 79–94.
3.
Maxon MR,
Smith HS. Radioiodine-
4.
Michele
Klain Radioiodine therapy for papillary and
follicular thyroid carcinoma Eur J Nucl Med (2002) 29 (Suppl. 2):S479–S485