Synchronous Medullary Carcinoma and Primary Follicular Lymphoma in the Same Thyroid Lobe: A Case Report and Literature Review

The human thyroid is a small, butterfly-shaped gland in the neck that acts as the body's thermostat, regulating how fast we burn energy. Sometimes, this gland develops lumps called nodules. While most are harmless, some grow into cancer. Doctors generally expect a thyroid nodule to be one type of disease, such as a carcinoma that starts in the hormone-producing cells or a lymphoma that begins in the immune cells. However, the thyroid is also a common site for a chronic condition called autoimmune thyroiditis, where the body's immune system mistakenly attacks the gland, causing inflammation and scarring over many years. This long-term irritation is a known risk factor for developing lymphoma, a cancer of the immune system. The medical world usually treats these possibilities as separate paths, assuming that if a patient has one type of cancer, they do not have another distinct type growing right next to it in the same spot.

A team of researchers from Russia recently documented a case that defies this standard expectation, revealing a rare biological coincidence. They describe a fifty-three-year-old woman who had lived with autoimmune thyroiditis for years and was admitted for surgery to remove a suspicious lump in her right thyroid lobe. When the surgeons removed the tissue and pathologists examined it under a microscope, they found something extraordinary: two completely different cancers growing side-by-side in the same small area. One was medullary thyroid carcinoma, a rare cancer that starts in the hormone-sensing cells of the thyroid. The other was a primary follicular lymphoma, a cancer of the immune system that had started directly within the thyroid tissue. This simultaneous occurrence, known as a collision tumor, is exceptionally uncommon, especially because the lymphoma was a specific, high-grade type that rarely appears in the thyroid on its own.

The patient's journey began years before the surgery. In 2020, routine checks found small nodules, and she was diagnosed with an underactive thyroid caused by the autoimmune condition. By 2022, blood tests showed elevated levels of a protein called calcitonin, which is a specific marker for medullary thyroid carcinoma. In 2025, an ultrasound revealed a larger, dark-looking area in the right lobe of her thyroid, measuring 21 by 17 by 13 millimeters, along with a smaller nodule. Blood work confirmed the high calcitonin levels, and a genetic test ruled out a common inherited cause for the cancer. A needle biopsy of the larger nodule suggested it was malignant, while the smaller one appeared benign. The doctors proceeded with surgery, removing the right half of the thyroid and nearby lymph nodes to ensure the cancer was gone.

When the removed tissue was analyzed in the laboratory, the true nature of the tumor became clear. The large nodule was not a single mass but a mixture of two distinct populations of cells. The majority of the tumor consisted of a dense collection of immune cells, specifically large, irregular cells forming clumps with areas of dying tissue. This pattern indicated a high-grade lymphoma. Interspersed within this immune mass were smaller islands of different cells: polygonal shapes with a specific cytoplasm that formed solid and rope-like structures. This second, smaller component was the medullary carcinoma. To confirm exactly what these cells were, the researchers used a technique called immunohistochemistry, which uses special stains to highlight specific proteins on the surface or inside cells.

The staining results provided the final proof. The smaller islands of cells tested positive for calcitonin and chromogranin A, proteins that confirm they were medullary thyroid carcinoma. These cells did not show signs of rapid division. The larger mass of immune cells, however, tested positive for markers that identify them as B-cells, a type of white blood cell. Crucially, these cells lacked a specific protein called BCL2, which is usually present in this type of lymphoma. The absence of this protein, combined with the high rate of cell division seen in the tissue, identified the lymphoma as a specific, localized subtype of follicular lymphoma, grade 3b. This finding was significant because most follicular lymphomas found elsewhere in the body carry the BCL2 protein, but this specific thyroid variant does not. The surgery was successful in removing the tumors, and no cancer cells were found in the sixteen lymph nodes that were also removed.

This case highlights a critical lesson for medical practice: the presence of one type of tumor does not guarantee that it is the only one present. The researchers emphasize that when a patient has a long history of autoimmune thyroiditis and a rapidly growing nodule, doctors must be prepared for the possibility of multiple, distinct cancers. Standard needle biopsies can sometimes miss the second cancer because the cells of lymphoma can look very similar to the inflammation caused by the autoimmune disease. In this instance, the diagnosis was only confirmed after the entire tumor was removed and examined with a wide range of special stains. The study concludes that careful, comprehensive examination of thyroid nodules is essential to ensure that patients receive the correct treatment for every type of cancer they might have, rather than just the most obvious one.