Understanding Your MTC Pathology Report

Your pathology report is the definitive “technical manual” for your cancer. While your surgeon sees the tumor during the operation, the pathologist is the one who looks at the cells under a microscope to determine how the cancer is likely to behave in the future.

In recent years, the way Medullary Thyroid Carcinoma (MTC) is reported has changed significantly. You should audit your report to ensure it contains the most current and relevant information.

The International MTC Grading System (IMTCGS)

The 2022 World Health Organization guidelines introduced a new, standardized way to “grade” MTC. This system, known as the IMTCGS, categorizes your tumor as either Low-Grade or High-Grade based on how fast the cells are dividing and whether they are dying off ^[1].

A tumor is classified as High-Grade if it meets at least one of the following criteria ^[2][3]:

• Mitotic Count: There are 5 or more “mitoses” (cells in the act of dividing) seen in a specific 2mm² area.
• Ki-67 Index: This is a special stain that measures the percentage of cells actively growing. A Ki-67 index of 5% or higher is considered high-grade ^[2].
• Tumor Necrosis: This refers to areas of “coagulative necrosis,” where cancer cells have died off because the tumor is outgrowing its blood supply. This is a strong indicator of aggressive behavior ^[3].

Note on Ki-67 ≥20%: If your report shows a Ki-67 index of 20% or higher, it identifies a specific subgroup that may require more intensive monitoring or aggressive treatment, as this level is associated with the highest risk of recurrence ^[2][4].

Key Prognostic Variables

Beyond the grade, several other factors on your report determine your “stage” and long-term outlook:

• TNM Staging: This stands for Tumor size, Node involvement, and Metastasis (spread). A higher “pT” (pathologic tumor) or “pN” (pathologic node) stage generally correlates with a higher risk of distant spread ^[4].
• Lymph Node Ratio (LNR): This is one of the most important numbers on your report ^[5]. It is calculated by dividing the number of positive lymph nodes by the total number of nodes removed. For example, if 10 nodes were removed and 5 were positive, your LNR is 0.50. A higher LNR is an independent predictor that the cancer may be more likely to return or spread ^[6].
• Vascular Invasion: This tells you if cancer cells have begun to “tunnel” into the blood vessels within or around the thyroid.
• Extrathyroidal Extension (ETE): This indicates if the tumor has grown beyond the “capsule” or outer edge of the thyroid and into the surrounding tissues.

Pathology Report Completeness Checklist

According to professional standards, a complete and high-quality report should use a “synoptic” (checklist) format and MUST include ^[1]:

• [ ] Tumor Size: The maximum diameter of the primary tumor.
• [ ] Margins: Whether the edges of the removed tissue are clear of cancer.
• [ ] Mitotic Count: The number of cells dividing per 2mm².
• [ ] Necrosis: Explicitly stating if necrosis is present or absent.
• [ ] Ki-67 Index: The percentage of cells staining positive for growth.
• [ ] Lymph Node Status: The total number of nodes found and the number that contain cancer.
• [ ] Vascular Invasion: Present or absent.
• [ ] Extrathyroidal Extension: Present or absent.