The term "collision tumor" refers to coexistent but independent tumors that are histologically distinct . Collision tumors can occur within the same organ or adjacent organs or in conjunction with a systemic malignancy or as a metastatic phenomenon . Various mechanisms have been proposed for collision tumors. The first, is a "chance accidental meeting" of two primary tumors. Another hypothesis suggests that the presence of the first tumor alters the microenvironment facilitating the development of the second primary tumor or seeding of metastatic tumor cells. The third theory suggests a common stem cell of origin for the two tumors . Multicentricity of thyroid carcinoma especially papillary carcinoma is not a rare event, occurring in a range of 18% to 22% of thyroid neoplasms. However, the presence of morphologically and histogenetically dissimilar primary neoplasia within the thyroid gland is very unusual . Collision tumors of the thyroid must be differentiated from mixed and composite tumors which show parafollicular and follicular derived cellular elements. Mixed tumors have a common cell of origin; tumor cells show expression of both thyroglobulin and calcitonin. Composite tumors on the other hand, have two discrete cellular populations – thyroglobulin positive and calcitonin positive . These definitions automatically rule out the presence of a mixed or composite tumor in our case which has a co-existent follicular variant of a papillary carcinoma and a squamous carcinoma.
The origin of squamous cells within the thyroid gland has many theories. These can be found as a result of persistence of thyroglossal duct or from a branchial pouch. They may also arise from a squamous metaplasia in a papillary carcinoma, anaplastic carcinoma, Hashimoto's thyroiditis or other conditions [4, 5]. There was no evidence of squamous metaplasia or transformation from one tumour to the other. The histological features of both tumours were distinct and remained consistent in our examination. The de novo occurrence of primary squamous cancer from follicular cells has also been advocated. However, such primary squamous cancers of the thyroid are extremely rare and account for less than 1% of all thyroid neoplasms. Moreover the tumour cells stain positively with thyroglobulin. Lack of thyroglobulin positivity in squamous carcinoma cells ruled out the possibility of follicular epithelial origin in our case.
Primary squamous cancers of the thyroid gland are innately aggressive tumors and typically present with a high incidence of pressure symptoms (dysphagia and dyspnea), infiltration of the surrounding soft tissue and history of recent onset of the symptoms, usually within weeks or months . Moreover, squamous cell carcinomas are the commonest metastatic disease in the head and neck . Hence, it is important to rule out infiltration of the thyroid gland from an adjacent organ and metastasis from a distant organ before labeling a squamous cell carcinoma as a primary thyroid cancer. In our case, the squamous cancer was restricted to the thyroid gland. Absence of symptoms, a negative physical examination and a negative review of systems helped to exclude a metastatic squamous carcinoma. Although metastatic squamous cell cancers most commonly present themselves in the cervical nodes , a true exclusion of a metastatic disease from secondary sites would warrant an autopsy. In retrospect, a histopathological examination of the scalp lesion could have helped. Given the fact that skin metastasis especially to the scalp from a follicular variant of papillary carcinoma are extremely rare ; a finding of a pure thyroid metastasis would have confirmed the findings in this case and also would have excluded a possibility of a primary cutaneous squamous cell carcinoma of the scalp with metastasis to the thyroid.
Thyroglossal duct remnants, most typically thyroglossal cysts, harbor carcinoma in less than 1% of cases. 95% of these cancers are papillary carcinomas and only 5% are squamous cancers, the latter having a worse prognosis . Literature has described only two cases of concurrent papillary and squamous carcinomas in the thyroglossal cyst as yet, reflecting the rarity of this histopathological combination . However, localization of a carcinoma to a clearly demonstrable thyroglossal duct and a normal thyroid gland are a prerequisite to diagnose a primary thyroglossal cyst carcinoma . These criteria rule out the contributions of a thyroglossal duct cyst to the development of the collision tumor in our case, as both tumors clearly arose within the thyroid gland and remnants of thyroglossal cyst were not seen either on preoperative sonography or after extensive sampling of the specimen.
Metastasis to the thyroid gland is not as unusual as previously believed. The incidence of thyroid gland involvement in autopsy studies ranges from 1.25% – 24.2%. The primary tumor can usually be identified in 95% of the cases. The presence of metastasis to thyroid indicates a disseminated disease and reflects a very poor prognosis, with average survival from diagnosis to death of 2 months. However, a previous history of a malignancy is essential to make this diagnosis . Another source of a squamous cell carcinoma in the thyroid could be an invasion from a carcinoma of the larynx, tongue base or esophagus; which commonly invade the thyroid gland . A normal head and neck examination, panendoscopy and barium swallow ruled out the possibility of infiltration of the thyroid gland by a primary in an adjacent organ.
These observations validate our presentation of a collision tumor in the thyroid composed of a squamous carcinoma and a follicular variant of papillary carcinoma i.e. morphologically different primary malignant tumors.
Even though metastatic disease of the thyroid have a poor prognosis in general, Shaha et al, in the study of 44 patients followed for an average of 20 years, have shown a long term survival of 43% in patients who had distant metastasis at presentation . Our patient was offered surgery followed by radioiodine treatment. However, the presence of a squamous carcinoma infiltrating the skeletal muscles in this collision tumor prompted the addition of external beam radiotherapy because squamous cell carcinomas of the thyroid are not known to take up radio iodine . It is well acknowledged that primary squamous carcinoma of the thyroid tends to have a poor prognosis and majority of the patients die within 1 year of presentation in spite of using combination therapy including surgery, radiotherapy and chemotherapy . However on the other hand, Cook et al have shown that long-term survival may be possible if the disease is diagnosed early, resected completely and treated with surgery followed by radical radiotherapy . In either case, we feel that there is insufficient literature to extrapolate this to predict how squamous carcinoma will behave as a component of a collision tumor. The absence of squamous carcinoma in the node metastasis and distant metastatic sites suggests an overall predominance of a differentiated thyroid cancer.