Journal of Head Neck & Spine Surgery
Abstract
Introduction: Thyroid “incidentalomas” are
increasing in frequency, thought mainly to be a result of the enhanced
sensitivity of ultrasonography and the increasing use of this imaging
technique. We present two cases of thyroid nodules in paediatric
patients which were found to be intrathyroidal thymic tissue.
Case report: The first case displayed
ultrasonographic features that were highly suspicious for papillary
thyroid cancer with microcalcification, however following diagnostic
hemithyroidectomy the lesion was diagnosed as intra-thyroidal ectopic
thymic tissue. Histologically, calcified Hassall’s corpuscles would
account for the calcified appearance of the nodule on ultrasound scan.
We describe a further case of intra-thyroidal ectopic thymic tissue in
another paediatric patient who presented with a thyroid mass.
Discussion:It is important to consider thymic remnant tissue as a differential diagnosis for incidental thyroid nodules.
Keywords: Paediatric; Otolaryngology; Incidental finding; Endocrine gland neoplasms; Thyroid malignancy
Introduction
Thyroid nodules are less commonly found in children,
affecting 0.2-1.5%, when compared to adult patients [1]. Paediatric
thyroid lesions are however more likely to be malignant and must be
carefully investigated [1,2]. There are sporadic reports of
intra-thyroidal thymic tissue in the literature, however increasing
ultrasound scan use and enhanced sensitivity of ultrasonographic images
has resulted in a rise in incidentally found thyroid lesions, which is
likely to increase further [3,4]. This has created a diagnostic dilemma
for clinicians and lead to an increase in diagnostic thyroid surgeries
and an increase in detection of microcarcinomas. Thymic tissue has a
distinctive appearance on ultrasound with features that may be
misinterpreted as a malignant thyroid nodule [3].
We report two cases of intra-thyroidal ectopic thymic
tissue in paediatric patients, both presenting with a thyroid mass. One
patient displayed radiological features that were suspicious of
malignancy. Both patients underwent diagnostic hemi-thyroidectomy.
Case Report
Case 1
A 4-year-old girl with no co-morbidities presented
with cervical lymphadenopathy following an upper respiratory tract
infection. An ultrasound noted cervical lymphadenopathy and a solitary
thyroid nodule with appearances suspicious of papillary thyroid cancer.
There was no family history or other risk factors for thyroid
malignancy. Blood tests revealed normal thyroid function, calcitonin and
thyroid auto-antibodies.
A repeat ultrasound scan of the neck revealed a
hypo-echoic nodule within the left lobe of the thyroid, displaying
micro-calcification and was taller than wide leading to a U5 (malignant)
diagnostic ultrasound grading (Figure 1). Following diagnostic
hemi-thyroidectomy, the nodule was histologically found to be ectopic
thymic tissue within the thyroid lobe, with calcification of Hassall’s
corpuscles, which would account for the calcified appearance on
ultrasound scan.
Case 2
A 12-year-old boy with no co-morbidities presented with
a midline neck swelling that had appeared 6 weeks previously.
Clinically he was found to have a 4.0 x 5.0cm, non-tender neck
mass in the midline with no movement on tongue protrusion.
Thyroid function, calcitonin and thyroid auto-antibodies were
normal.
Ultrasound revealed a 4.0cm hyper-echoic thyroid lesion
arising from the isthmus. Fine needle aspiration cytology (FNAC)
was reassuring with morphological features consistent with a
benign colloid nodule and classified as Thy 2, however due to the
size of the mass the child underwent hemi-thyroidectomy which
revealed nodular hyperplasia; ectopic intrathyroidal thymic
tissue was also identified throughout the sample.
Discussion
Intra-thyroidal thymic tissue is an uncommon cause of neck
swellings and has rarely been reported in the literature with
published data from a small number of case reports [3,5]. The
thymus is involved in adaptive immunity and T-cell function;
it proliferates in the neonatal period and first decade of life
and is vital for development of a mature immune system [6].
Embryologically, the thymus is derived from the endoderm of
the third and fourth pharyngeal pouch during the 6th gestational
week and as the thymo-pharyngeal duct elongates during the 7th
week the thymus migrates inferiorly and medially towards the
superior mediastinum [4,6]. Ectopic thyroid tissue can therefore
be found anywhere from the angle of mouth to the superior
mediastinum [5], meaning ectopic thymic tissue can easily be
misinterpreted as a pathological neck lump, particularly in the
paediatric population.
There are various reports of ectopic thymic tissue. Most
commonly it is found as aberrant tissue within the neck, known
as ectopic cervical thymic tissue [5]. There are only sporadic
reports of in thyroidal thymic tissue, however it has previously
been misdiagnosed as papillary thyroid cancer on ultrasound,
similar to the case presented [3,5]. The exact epidemiology of
intra-thyroidal thymic tissue remains unclear, however one
study from Japan suggests that it is present in 1% of children
[7]. Similar numbers were found in a study of perinatal thyroid
glands [8]. With the increasing use and enhanced sensitivity of
ultrasound, higher numbers of ectopic thymic tissue located
within the thyroid gland may be detected and potentially
misdiagnosed as malignancy, leading to diagnostic surgery, as in
the two cases presented.
We present the cases of two children with intra-thyroidal
thymic tissue who were referred to our paediatric head and neck
tertiary referral service with thyroid masses. One child had a
thyroid lesion that was radiologically suspicious for a papillary
thyroid cancer with micro-calcifications and a mass that was
taller than wide. Micro-calcifications are considered a highly
suspicious feature on ultrasound, suggestive of papillary thyroid
cancer and would result in a U5 grading on ultrasound according
to the British Thyroid Association guidelines [9,10]. Hyperechoic
foci within ectopic thymic tissue may be misinterpreted
as micro-calcifications, which could mimic a malignant thyroid
lesion. The other child had incidental thymic tissue identified
throughout the thyroid gland in association with nodular
hyperplasia.
The cases presented highlight the complexities of preoperative
diagnosis of intra-thyroidal thymic tissue. However,
with increasing usage of ultrasound imaging it is important to
consider thymic remnant tissue as a differential diagnosis for
thyroid nodules, especially as the imaging findings may mimic
a malignant thyroid lesion. Unfortunately, due to the suspicious
ultrasound features diagnostic thyroid surgery may still be
inevitable.
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