Joint Program in Nuclear Medicine
Thyroid Nodules
Richard Kuno, MD
J. Anthony Parker, MD, PhD
October 29, 1996
Presentation
A 29 year old female was referred for thyroid scan and uptake after a small
thyroid nodule was detected by her physician in the lower right aspect
of her gland. Patient was asymptomatic and without any significant medical
history.
Imaging Technique
Imaging was performed 24 hours following the oral ingestion of 322 uCi
of I-123 sodium iodide. Images were obtained using a pinhole collimator
in the anterior, LAO, and RAO positions. A marker view was also obtained
by placing cobalt disc markers at the sternal notch and 10 cm above the
notch. An anterior view was repeated with a Co-57 string marker placed
around the palpable nodule. The patient was then imaged in the anterior
position. Following the imaging portion of the exam, thyroid uptake was
measured.
Imaging Findings
Thyroid scintigraphy (RAO, Ant, LAO; 10 cm marker,
string marker on nodule) showed decreased tracer uptake in the right lower
pole (arrows show nodule; arrowheads show
10 cm marker). The other portions of the thyroid gland had a normal appearance.
The cobalt marker image demonstrated that the area of decreased uptake
corresponded to the palpable abnormality. 24 hr thyroid uptake of radioiodine
was 13% (normal range 5-35%).
Results
Because the palpable abnormality had decreased uptake on thyroid scintigraphy,
the nodule was biopsied using fine needle aspiration, and a diagnosis of
papillary carcinoma was made. The patient then underwent subtotal thyroidectomy
and the diagnosis of carcinoma was confirmed. Lymph node sampling revealed
no evidence of distant tumor. Future plans include a metastatic survey
with I-131.
Discussion
Thyroid nodules have a high incidence in the general population. By autopsy
series, there is up to a 50% incidence of single or multiple nodules. In
unselected populations, there is up to a 4% incidence by palpation.
In general, work-up of a nodule may include the following general categories:
-
history and physical exam,
-
thyroid imaging,
-
thyroid function tests, and
-
biopsy.
Features in the history which increase the likelihood that a given nodule
represents carcinoma include:
-
young age,
-
male sex,
-
family history of medullary carcinoma, and
-
history of radiation exposure to the neck.
Physical findings suggesting carcinoma include:
-
hardness,
-
non-mobility,
-
nodule > 2cm,
-
tracheal deviation, and
-
enlarged cervical lymph nodes.
Thyroid imaging
A variety of imaging modalities can be used to image thyroid nodules including
thyroid scintigraphy, ultrasound, CT, and MRI. Practically speaking, scintigraphy
and ultrasound have been most widely used to help differentiate benign
versus malignant disease.
The three agents commonly used in thyroid scintigraphy include Tc-99m,
I-123, and I-131. I-123 is often used because it is physiologic (both transported
and organified) and it gives a reasonably low total body dose. However,
I-123 exams usually require that the patient returns 24 hours following
the administration of the tracer making this a two day procedure. Radiation
dose with Tc-99m is also reasonably low and there is the advantage that
images can be obtained the same day. However, there is the occasional problem
of discordant nodules when using Tc-99m (tumors may be hot on Tc-99m, cold
on I-123). The possibility of discordant nodules arises because Tc-99m
is only transported and not organified, and some tumors can transport Tc-99m.
I-131 is not used for normal thyroid imaging because of the high radiation
burden and poor imaging characteristics. Types of nodules and the most
common I-123 imaging findings are given below:
| Type of Nodule |
I-123 features |
| Functioning Adenoma |
Increased |
| Non-functioning Adenoma |
Decreased |
| Multinodular goiter |
Increased and Decreased |
| Colloid Nodule |
Decreased |
| Cyst |
Decreased |
| Malignant Tumor |
Decreased |
| Local Thyroiditis |
Increased or Decreased |
Scintigraphic findings in thyroid nodules are non-specific. Focal areas
of decreased uptake are often called cold nodules. These cold regions are
of concern because they can potentially represent malignant disease; however,
the likelihood of carcinoma for any given cold nodule is generally considered
to be less than 20% with more recent literature showing an incidence of
only 4% (1). On the other hand, finding multiple cold areas interspersed
between regions of increased activity can indicate a multinodular gland
where there is a low incidence of associated malignancy.
A focal area of increased uptake is called a hot nodule. These hot nodules
virtually never represent malignant disease but instead usually represent
either autonomous or hypertrophic adenomas. Thyroid function tests and
suppression scans can play a role in working up these hot lesions.
As with scintigraphy, ultrasound findings in the work-up of nodules
are often non-specific. Occasionally, a simple cyst can be found and in
this situation no further work-up is usually required. However, simple
cysts are rare and any nodule found usually requires more investigation.
Some clinicians use ultrasound to guide biopsies or to follow the size
of nodules.
Fine Needle Aspiration
Fine needle aspiration (FNA) is widely regarded as the procedure of choice
in evaluating nodules. Numerous studies since the 1970’s have shown the
usefulness of this procedure. Reported sensitivities in detecting carcinoma
by FNA range from 65%-99% with specificities ranging from 72%-100% (2).
As might be expected, experience plays a critical role in attaining the
highest sensitivities and specificities. The use of FNA has reduced the
numbers of thyroidectomies by 25% and has increased the yield of carcinoma
at surgery to 30% from 15%. Problems with FNA include difficulty differentiating
follicular adenoma from carcinoma, false positives in Hashimoto’s thyroiditis,
and false negatives due to inadequate sampling.
Conclusions:
The work-up of thyroid nodules remains somewhat controversial. A good history,
physical exam, and thyroid function tests often provide valuable clues
in reaching a correct diagnosis but are non-specific. Despite difficulties
with FNA, it is usually the initial procedure of choice. However, thyroid
scintigraphy and ultrasound can both play useful roles in the evaluation
of a nodule. Scintigraphy can be particularly useful in diagnosing a hot
nodule where the risk of malignancy is extremely low. US can be used to
guide biopsies, follow the size of a nodule, and differentiate cystic from
solid lesions.
References
1. Kusic Z, Becker DV, Saenger EL. Comparison of Tc-99m and I-123 imaging
of thyroid nodules. J Nuc Med 1990;31:393-99.
2. Mazzaferri EL. Management of a solitary thyroid nodule. NEJM 1993;8:553-9
3. Society of Nuclear Medicine. Procedure Guideline for Thyroid Scintigraphy.
Version 1. June 24, 96.
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J. Anthony Parker, MD PhD, Tony_Parker@bidmc.harvard.edu