Medullary thyroid carcinoma is not the common papillary variety that dominates thyroid cancer statistics and fuels much of the active surveillance debate. It arises from the parafollicular C cells of the thyroid, the cells that produce calcitonin, and it behaves differently from its more indolent cousins. It does not respond to radioiodine therapy, it secretes measurable biomarkers that track its burden, and it can metastasize to cervical lymph nodes early in its course. When such a tumor is caught while still under one centimeter in diameter, it earns the designation of microcarcinoma, and it is precisely in this small, seemingly harmless form that the disease increasingly presents. As ultrasound technology improves and thyroid imaging becomes more common, more of these minute tumors are being found, and clinicians are left with a growing population of patients for whom the natural history is poorly charted.

The research team, led by Yuanmeng Li, Naishi Li, Weigang Zhao, and Xiaolan Lian, combed through their institutional records and identified 75 cases of medullary thyroid microcarcinoma. Within that group, they focused on a subset of 20 patients whose diagnosis and surgical treatment had been delayed and for whom baseline ultrasound imaging of adequate quality existed. For these patients, the observational period before surgery exceeded six months, with a median duration of 27 months. That is a substantial stretch of time for a tumor to do whatever it is going to do, and it gave the investigators a rare longitudinal window into the behavior of an entity that is usually excised quickly and studied only in retrospect.

What the ultrasound scans revealed was striking. Seventy-five percent of the delayed-surgery patients, 15 of the 20, showed sonographic evidence that their microcarcinoma nodules had grown during the observation period, a proportion with a 95 percent confidence interval spanning roughly 53 to 89 percent. When the researchers compared the final nodule volumes with the initial measurements, the difference was statistically robust, with p values below 0.001. In plain terms, these were not static lesions idling in the thyroid gland. During a median of more than two years without intervention, the majority of these tumors expanded, and the volume change was unmistakable rather than a borderline statistical artifact.

Growth alone, however, does not tell a clinician whether a tumor has become more dangerous. To address that question, the team turned to the pathological and biochemical signatures that medullary thyroid carcinoma leaves behind. They stratified their patients using a 5 millimeter threshold, a cut point that divides the microcarcinoma range into its smallest and largest halves. The results were consistent across every measure they examined. Tumors larger than 5 millimeters were associated with higher preoperative levels of calcitonin and carcinoembryonic antigen, the two biomarkers that serve as the workhorses of medullary thyroid cancer surveillance. They also carried a greater incidence of lymph node metastasis, more advanced TNM staging, and a lower rate of biochemical cure after surgery, meaning a lower likelihood that postoperative biomarker levels fell to undetectable territory. Every one of these comparisons reached statistical significance at p values below 0.01.

Correlation analysis reinforced the same story along a continuous gradient rather than a binary divide. Tumor size in this cohort was positively associated with preoperative calcitonin levels, with a correlation coefficient of 0.357 and a p value of 0.022, and with carcinoembryonic antigen levels, which showed an even stronger association at r equals 0.428 and p equals 0.003. Size also correlated with lymph node metastasis at r equals 0.388, with TNM stage at r equals 0.416, and with worse overall outcome at r equals 0.330, the latter with a p value of 0.006. None of these correlations is overwhelming in isolation, and the study’s own authors are careful to note that tumor size and observation time are intertwined in this setting, making it impossible to fully disentangle whether the tumors that grew were intrinsically more aggressive or whether the passage of time itself allowed indolent lesions to progress. But the direction of every association points the same way: bigger tumors, in this disease, mean more disease.

The biological logic behind these findings is worth unpacking. Calcitonin secretion is a function of the number of malignant C cells present, so a growing tumor should, and does, produce a rising biomarker signal. Cervical lymph node metastasis in medullary thyroid carcinoma is known to correlate with primary tumor size, and the 5 millimeter threshold identified here aligns with a broader literature suggesting that risk in this disease accrues continuously rather than in discrete jumps. What makes the new study notable is not that it overturns established biology but that it documents the temporal dimension directly. Most of what is known about medullary thyroid microcarcinoma comes from surgical specimens examined after the fact, snapshots of tumors at the moment of removal. Watching these tumors across a median of 27 untreated months provides something those snapshots cannot: evidence that the clock matters.

The clinical context amplifies the significance. Active surveillance, the strategy of monitoring small thyroid nodules with serial ultrasound rather than operating immediately, has gained genuine traction for papillary thyroid microcarcinoma, where large Japanese cohort studies have shown that the majority of these lesions never grow to a clinically meaningful size. That evidence base has created a certain cultural momentum, a sense among some patients and clinicians that small thyroid cancers in general can be watched safely. The new findings are a pointed reminder that medullary thyroid carcinoma is a different biological entity. There is no equivalent evidence base supporting surveillance for medullary microcarcinoma, and this study now provides the first direct, if retrospective and limited, evidence that waiting carries measurable risk. The authors stop short of claiming that every microcarcinoma will progress, and indeed a quarter of their delayed-surgery cohort showed no growth, but the burden of proof has shifted.

Several caveats deserve honest acknowledgment. The study is retrospective and comes from a single center, which means selection bias is a real possibility; the very fact that these patients experienced delayed diagnosis and surgery suggests their tumors may have been clinically silent or initially mischaracterized, a circumstance that may not generalize to all microcarcinomas. Twenty patients is a small sample, and the confidence intervals around the 75 percent growth figure are correspondingly wide. The authors themselves note that the clinical implications of their findings require confirmation in future studies, ideally prospective ones with standardized imaging intervals and larger, multi-center cohorts. The study was approved by the Ethics Committee of Peking Union Medical College Hospital and conducted in accordance with the Declaration of Helsinki, and it was supported by funding from the Chinese Academy of Medical Sciences, the National Natural Science Foundation of China, and related institutional programs, lending it institutional credibility even as its scale remains modest.

Even with those limitations, the message for clinical practice is difficult to ignore. Medullary thyroid microcarcinoma should not be presumed to remain stable during an untreated interval, the authors conclude, and once the diagnosis is suspected, prompt workup and timely surgery are the prudent default. For a disease that announces itself through calcitonin and CEA, biomarkers that can be measured with a simple blood draw, and that can be imaged with the ultrasound machines already present in every endocrinology clinic, the barriers to early action are low. What this study adds is the urgency: in the roughly two years that separated baseline imaging from surgery for these patients, three quarters of their tumors grew, and the ones that grew larger arrived at the operating room with higher biomarker levels, more nodal disease, more advanced staging, and poorer chances of biochemical cure. Small, in medullary thyroid cancer, does not mean stationary, and the new data suggest that the window in which these tumors are truly micro may be one worth closing deliberately rather than watching drift by.

Subject of Research: Tumor growth and outcomes in medullary thyroid microcarcinoma during delayed surgery

Article Title: Tumor growth in medullary thyroid microcarcinoma during delayed surgery: a single-center study

Article References: Li, Y., Chai, X., Liu, H., Zhang, H., Li, X., Li, B., Chen, X., Zhou, J., Li, N., Zhao, W., & Lian, X. (2026). Tumor growth in medullary thyroid microcarcinoma during delayed surgery: a single-center study. BMC Endocrine Disorders. https://doi.org/10.1186/s12902-026-02606-2

Image Credits: AI Generated

DOI: 10.1186/s12902-026-02606-2

Keywords: medullary thyroid microcarcinoma, thyroid cancer, tumor growth, calcitonin, carcinoembryonic antigen, lymph node metastasis, delayed surgery, active surveillance, ultrasound, TNM staging, biochemical cure, endocrine disorders