**5. Diagnosis**

ATC is a very aggressive tumor with a very short doubling time, and hence disease burden should be assessed accurately and promptly.

Laboratory investigations should include a complete blood count, a baseline evaluation of liver and renal function, thyroid function tests, albumin levels, and serum electrolytes levels. Nutritional assessment is also required as most of them will have dysphagia and reduced food intake.

Fine needle aspiration cytology from the thyroid mass can yield an early diagnosis. Morphologic patterns of ATC include spindle cells, pleomorphic giant cells, and squamoid histologies [16]. Most of them will have a mixed morphology of two or all the three patterns and they show extensive necrosis, atypical mitoses, and numerous mitotic figures. However, for immunostaining and molecular studies, a core biopsy is preferred. Routinely they will not stain positive for TTF1, PAX8, and thyroglobulin. Patterns of IHC in various thyroid malignancies are summarized in **Table 2**.

Poorly differentiated carcinoma, large cell lymphoma, and extension of laryngeal carcinoma are the most common differential diagnosis.

Radiological evaluation should be done immediately without any delay. Highresolution ultrasound can be done as it is convenient, rapid, and easy in assessing tumor extent, neck nodes, and adjacent structure involvement.


#### **Table 2.**

*Patterns of IHC in various thyroid malignancies [25].*

Other imaging modalities include CT/MRI of the neck and chest to assess tumor extent and infiltration to adjacent structures. FDG-PET scan has recently gained a role in imaging ATC as it helps in the accurate diagnosis of distant metastasis compared to other imaging modalities [15]. If trachea or esophagus infiltration is suspected, upper GI endoscopy or bronchoscopy is indicated [20, 25].

Molecular testing can be done if available; however, its clinical utility is not well established. Next-generation sequencing should be performed for targetable mutations under the context of a clinical trial. Rapid testing of BRAF V600E mutation is most commonly done if available in FNA or core needle biopsy specimens.
