Anaplastic thyroid cancer (ATC) is a rare and aggressive form of thyroid malignancy that poses significant diagnostic and treatment challenges. Its rapid progression makes it one of the most aggressive human malignancies. Diagnostic imaging, particularly ultrasound, plays a role in its detection and characterization.
Understanding Anaplastic Thyroid Cancer
Anaplastic thyroid cancer originates from the thyroid gland’s follicular cells. It differs significantly from other thyroid cancers, such as papillary or follicular types, which are generally well-differentiated and have a much better prognosis. Unlike these, ATC cells are “undifferentiated,” bearing little resemblance to normal thyroid cells. This rapid growth can lead to symptoms like a fast-growing, hard, and painful neck lump, hoarseness, difficulty swallowing, or trouble breathing.
ATC is among the fastest-growing cancers, capable of doubling its volume in a short period. This rapid growth often leads to spread (metastasis) to other body parts, with about 50% of individuals having distant metastasis at diagnosis, commonly in the lungs, bones, and brain. The prognosis for ATC is poor, with a mean survival of about six months after diagnosis, making early and accurate identification important. While it accounts for a small percentage (1-2%) of all thyroid cancers, it contributes to a significant portion (14-50%) of thyroid cancer-related deaths.
Role of Ultrasound in ATC Management
Ultrasound is a primary imaging tool in the diagnosis and management of anaplastic thyroid cancer. It is the initial diagnostic modality used to evaluate a palpable thyroid mass or suspected thyroid nodule, providing a detailed view of the thyroid gland for detection and characterization of suspicious lesions. This imaging helps determine the tumor’s size and extent, although it cannot confirm an ATC diagnosis on its own.
Ultrasound also guides fine needle aspiration (FNA) or core needle biopsies, essential for pathological confirmation of ATC. For large, rapidly growing masses, ultrasound guidance helps ensure the biopsy needle samples solid tumor portions, as these aggressive cancers can outgrow their blood supply and develop necrotic (dead) areas. Ultrasound allows for the assessment of local invasion into surrounding neck structures, such as muscles, the trachea, or the esophagus, and helps identify suspicious regional lymph node involvement, which is present in a majority of ATC cases.
Ultrasound surveillance can also detect recurrent disease in the thyroid bed, remaining contralateral lobe, or lateral neck lymph nodes after surgery. This makes ultrasound useful from initial detection to assessing disease extent and guiding subsequent procedures.
Ultrasound Features of Anaplastic Thyroid Cancer
Anaplastic thyroid cancer presents with specific sonographic characteristics that aid identification. ATC appears as a large, ill-defined, and infiltrative mass that can involve an entire lobe or the entire thyroid gland. These tumors display heterogeneous echogenicity and are hypoechoic, appearing darker than the surrounding normal thyroid tissue.
Rapid growth is a hallmark of ATC. The margins of an anaplastic tumor are irregular and ill-defined, reflecting its invasive nature. While microcalcifications are often associated with other thyroid cancers like papillary thyroid cancer, ATC can also present with internal calcifications, particularly dense and amorphous types, observed in about 58% of cases.
Extrathyroidal extension, or invasion into surrounding neck structures like the trachea, esophagus, or perithyroid soft tissues, is a finding with ATC. Suspicious lymphadenopathy, found in approximately 76.4% of cases, is also associated with anaplastic thyroid cancer. Color Doppler ultrasound may reveal prominent, small, and chaotic intratumoral vessels, indicating the tumor’s increased vascularity.
Beyond Ultrasound: Complementary Imaging and Diagnosis
While ultrasound is a primary tool for initial evaluation and guiding biopsies in anaplastic thyroid cancer, comprehensive diagnosis and staging require additional imaging modalities. Computed Tomography (CT) scans are utilized to assess the full extent of the tumor, particularly for evaluating local invasion into adjacent structures like the trachea, larynx, and mediastinum, which ultrasound may not fully visualize. CT also plays a role in identifying regional lymph node involvement and distant metastases, which are common at the time of ATC diagnosis.
Positron Emission Tomography/Computed Tomography (PET/CT) scans using 18F-fluorodeoxyglucose (FDG) are recommended for initial ATC staging, helping evaluate both tumor resectability and the presence of distant metastases. ATC cells exhibit high glucose metabolism, leading to significant FDG uptake on PET/CT, which can help differentiate ATC metastases from other thyroid cancer types. PET/CT is also used in follow-up evaluations to detect residual, recurrent, or metastatic disease after initial therapy.
Pathological confirmation through a biopsy remains the definitive diagnostic step for anaplastic thyroid cancer. While ultrasound guides the biopsy, tissue samples are examined under a microscope to confirm the diagnosis and distinguish ATC from other thyroid conditions or other aggressive malignancies like lymphoma or metastatic cancers to the thyroid. This multi-modal imaging approach, combined with biopsy results, provides a comprehensive understanding for accurate diagnosis and treatment planning.