Thyroid collision tumors are an exceedingly rare phenomenon. Bulte and colleagues explained, “A collision tumor is a neoplastic lesion comprised of two or more distinct cell populations that maintain distinct borders. Collision tumors, which are rare but well documented, can be composed of two benign tumors, a benign and malignant tumor, and two malignant tumors.” 

Bojoga and colleagues, in their work on thyroid collision tumors, wrote, “Collision tumors are rare entities, occurring from two distinct histologic neoplasms, found in the same organ, but they are separated by normal tissue and have no histological admixture.” 

In summary, collision tumors are independent of each other and exist simultaneously. They have been described in various organs, including the skin, liver, gastrointestinal tract, and ovary. 

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Features of Collision Tumors

Needless to say, developing two tumors in the same organ is highly unusual and highly undesirable—so what are the predisposing factors, if any? Unfortunately, we still do not know. There are, however, theories on its pathogenesis. Bulte et al wrote, “The most widely accepted theory is that collision tumors have neoplastic heterogeneity, meaning that they result from two different clones of neoplastic cells.” 

Collision tumors can be metastases from other organs or arise (seemingly) spontaneously. However, in the case of thyroid collision tumors, metastases are rare. Studies indicate that only 2% to 3% of thyroid malignancies are a result of metastases. This has surprised scientists since the vascularity of the thyroid is rich and intricate, but some suggest that the rapid blood flow in the thyroid creates an environment hostile to the seeding of metastases. 

Let’s discuss briefly how collision tumors differ from mixed tumors and composite tumors. Mixed tumors differ from collision tumors in that they have a common cellular origin. Bojoga et al wrote, “An example of mixed tumor of the thyroid is mixed medullary and follicular tumor of thyroid whereby there is morphological and immunohistochemical evidence of the both follicular and parafollicular tumor cells in the same lesion.” Composite tumors, meanwhile, arise from “a common driver mutation that induces different histological cellular populations in the same tumor.“  

What are the most common types of thyroid collision tumors? Apparently, medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC). A study found the MTC/PTC pairing in 13.8% of 196 patients with thyroid collision tumors, with the rest being micro PTC. Collision tumors of PTC and follicular thyroid carcinoma (FTC) also exist but are rarer still. 

Read more about MTC etiology 

With regards to treatment options, Bojoga and colleagues wrote, “Due to the paucity of reported literature on collision thyroid tumors, treatment guidelines are poorly defined.“

“Management of thyroid collision tumors is challenging, as collision tumors usually contain components with different aggressiveness, treatments options, and prognosis,“ they wrote. “These tumors ought to be managed in a multidisciplinary framework and need to be individualized, considering biological aggressiveness and the stage of the tumor.” 

However, standard thyroid cancer treatments generally apply: adjuvant radioactive iodine therapy, surgical resection, and palliative care for thyroid metastases.

A Collision Tumor Amid a Pandemic 

Vlad and colleagues reported on a patient diagnosed with a collision tumor during the COVID-19 pandemic. The 59-year-old female presented with a painless left latero-cervical swelling that had been rapidly increasing in size over the past 4 weeks, along with dysphasia, inspiratory dyspnea, and fatigue. She was diagnosed in 2007 with a left nodular goiter and was recommended for surgery but declined. In 2020, she was diagnosed with chronic autoimmune thyroiditis. 

A physical neck exam revealed a palpable painless nodule of approximately 4 cm with hard consistency and adherence to the skin and a left latero-cervical adenopathy of approximately 5 cm that was fixed to underlying tissues. Thyroid ultrasound revealed a markedly hypoechoic small solid nodule in the right lobe that measured 6/7.5/7 mm, and a solid, markedly hypoechoic nodule in the left lobe measuring 27.7/42.6/26.6 mm with ill-defined margins and microcalcifications. In addition, the left latero-cervical lymphadenopathy was round, hypoechoic, and nonhomogenous, had mild internal vascularity and loss of hilar architecture, and measured 51/45.7/40 mm. 

Read more about MTC treatment

Thoracic computed tomography (CT) demonstrated a normal-sized right thyroid lobe with an infracentimetric nodule and a left thyroid lobe with increased dimensions of 45/50 mm that displaced the sternocleidomastoid muscle anteriorly. The latero-cervical left adenopathy measured 54/52/50 mm, displaced the left jugular vein anteriorly, and was in close contact with the left thyroid lobe. In addition, a solitary infracentimetric pulmonary nodule was detected in the apex of the right lung which was inconclusively described as a metastasis. 

Blood tests revealed a slightly raised white cell count and a moderately elevated erythrocyte sedimentation rate. Thyroid function tests revealed euthyroid status, with an increase in antithyroid peroxidase and antithyroglobulin antibody levels. Calcitonin levels were normal. The patient was negative for COVID-19. 

Pathology results confirmed that the patient had “a massive lymph node metastasis from an anaplastic carcinoma, most probably located within the thyroid gland.“ Doctors performed a total thyroidectomy. Postoperative pathological results revealed anaplastic thyroid cancer in the left thyroid lobe, papillary microcarcinoma, and coexisting chronic autoimmune thyroiditis. Vlad et al wrote, “From a pathological point of view, the concurrence of two different tumors, split by normal tissue architecture and appearing in the same organ, is the definition for collision tumor, in our case, of the thyroid.” 

Although collision tumors are rare and there is still much we do not know, present clinical studies indicate they carry a poor prognosis. The authors of the case study likewise concluded, “A collision tumor of the thyroid, consisting of papillary thyroid microcarcinoma and anaplastic thyroid cancer implies a bad prognosis, due to the presence of the anaplastic tumor portion and should be evaluated and treated by surgery without delay, even in pandemic conditions.” 


Bulte CA, Hoegler KM, Khachemoune A. Collision tumors: a review of their types, pathogenesis, and diagnostic challenges. Dermatol Ther. 2020;33(6):e14236. doi:10.1111/dth.14236

Bojoga A, Stănescu L, Badiu C. Collision tumors of the thyroid. A special clinical and pathological entity. Arch Clin Cases. Published online December 29, 2021. doi:10.22551/2021.33.0804.10191

Vlad M, Corlan A, Balas M, et al. Collision tumor of the thyroid – a challenge during the COVID-19 pandemic. Arch Clin Cases. 2021;8(4):64-71. doi:10.22551/2021.33.0804.10189