Discussion and Conclusion
Although miliary tuberculosis is the most known cause of miliary
infiltrates on chest imaging, other differential diagnoses including
pneumoconiosis, fungal infections, sarcoidosis, histoplasmosis, primary
lung cancer, and hematogenous spread of non-pulmonary malignancies
[12] can mimic this radiographic finding. Rarely, primary lung
adenocarcinoma could be the etiology of miliary nodules on chest
radiography [13]. While renal cancers are the most likely solid
organ malignancies that can manifest with a miliary pattern on chest
imaging [14], primary cancers of the thyroid, melanoma,
trophoblastic tumors and sarcomas also can display a similar radiologic
feature. Chest CT has the highest sensitivity for detecting lung
metastases in patients with suspected metastatic MTC [3]. The
distribution of miliary nodules on chest CT can be centrilobular,
perilymphatic or random [15]. Random micronodular patterns are seen
in entities such as hematogenous spread of malignancies, in line with
the case reported here. Moreover, micronodules that are seen in
hematogenous spread of malignancies to the lungs mainly have a basal
predominance. This finding was also observed on the chest radiography
and CT scan of this patient. In patients with MTC, lung metastases are
commonly accompanied with mediastinal lymph node metastases [3];
however, no mediastinal lymphadenopathy was evident on chest imaging of
our patient.
Non-specific clinical symptoms such as, fever, chills, cough, and weight
loss can be associated with infectious diseases other thanmycobacterium tuberculosis as well as malignancies. Patients with
MTC may present with systemic symptoms such as diarrhea and flushing due
to the secretion of serum calcitonin, calcitonin gene-related peptide
and other hormones from parafollicular C-cells [16]. In this
specific case, our patient was mistakenly treated with anti-tuberculosis
drugs since her clinical symptoms as well as radiologic findings were
compatible with a diagnosis of miliary pulmonary tuberculosis. Taken
together, establishing a diagnosis of miliary tuberculosis should be
based on the combination of radiologic, histologic and microbiologic
examinations. Nevertheless, it is worthy to note that a negative AFB
sputum smear result should not exclude a diagnosis of miliary
tuberculosis as this test is only positive in a small percentage of
patients with disseminated disease [15].
Thyroid FNA is a useful and reliable tool with a diagnostic yield of 50
to 80% for medullary thyroid carcinoma [17-19]; however, this
percentage is increased with the addition of immunohistochemical
staining for calcitonin [20,21]. Among the secretory products of
C-cells, calcitonin and carcinoembryonic antigen (CEA) are the most
valuable biomarkers in patients with MTC. Studies have shown that the
concentration of these tumor biomarkers in patients’ sera is associated
with the C-cell mass [3]. Unlike calcitonin, CEA does not play a
role in the early diagnosis of MTC; in fact, baseline serum CEA levels
are measured preoperatively for determining the extent of disease after
surgery [22]. Histologically, MTC cells are usually round,
spindle-shaped or polyhedral and are arranged in the shape of sheets or
nests with peripheral palisading. Moreover, MTC cells are typically
discohesive or weakly cohesive on aspiration cytology [2,3]. Our
observations were consistent with these findings.
The significant prognostic value of tumor stage in patients with MTC
[23,24] underlines the importance of early detection of this
aggressive tumor. The 10-year overall survival rate for patients with
stage IV disease is reported to be as low as 21%. Some patients,
however, may survive for longer years [24]. Considering the
increasing number of cases with MTC presenting with miliary nodules on
conventional chest imaging, observation of this radiologic pattern
should prompt a possible diagnosis of metastatic MTC, even in cases
without clinically palpable thyroid nodules. MTCs appearing with a
miliary pattern on radiography could be erroneously diagnosed as miliary
tuberculosis, leading to a delay in the detection of cancer and
ultimately higher stages of disease.
Ethics approval and consent to participate: This study was
approved by the ethics committee of Shahid Beheshti University of
Medical Sciences.
Consent for publication: Written informed consent was obtained
from the patient for publication of this case report, including
accompanying images.
Competing interests: The authors declare that they have no
competing interests.
Funding: This study did not receive funding from any sources.