Anaplasia: Definition
Anaplasia is a term pathologists use to describe cancer cells that have lost the appearance and organization of the normal cells they came from. In healthy tissue, cells are differentiated — meaning they have developed specialized shapes and functions suited to the organ they belong to. Anaplastic cells have shed this specialization entirely. They look abnormal, disorganized, and bear little resemblance to normal tissue. Finding anaplasia in a tumor is a sign of high-grade, aggressive behavior and indicates that the cancer is likely to grow and spread more quickly than tumors without this feature.
What do anaplastic cells look like under the microscope?
When a pathologist examines anaplastic tissue, several abnormal features stand out:
- Loss of normal structure — the cells no longer form the organized patterns typical of their tissue of origin. The tissue looks disordered and chaotic.
- Abnormal cell and nuclear size — anaplastic cells are often much larger than normal, and their nuclei — the control centers of the cells — are enlarged, irregular in shape, and may contain an abnormal amount of DNA, making them appear darker than normal under the microscope.
- Pleomorphism — cells and nuclei vary greatly in size and shape from one cell to the next, a feature called pleomorphism.
- Increased and abnormal cell division — anaplastic tumors contain many dividing cells (mitotic figures), and some of these divisions are abnormal, producing cells with the wrong number of chromosomes.
- Giant cells — some anaplastic tumors contain very large cells with multiple nuclei, called tumor giant cells.
How does anaplasia relate to grade and differentiation?
Anaplasia is closely related to the concept of grade, the system pathologists use to describe how abnormal tumor cells look compared to normal tissue. Grade is based on the degree of differentiation:
- Well differentiated (low grade) — tumor cells still closely resemble the normal tissue they came from. Little or no anaplasia.
- Poorly differentiated (high grade) — tumor cells look very different from normal tissue. Significant anaplasia is present.
- Undifferentiated (high grade) — tumor cells bear almost no resemblance to normal tissue. Marked anaplasia throughout.
Anaplasia is therefore a defining feature of high-grade tumors. The more anaplastic a tumor is, the higher its grade tends to be, and the more aggressively it is likely to behave.
Which tumors are commonly described as anaplastic?
The word “anaplastic” appears in the names of several specific cancer types, where it signals a particularly aggressive form of that cancer:
- Anaplastic thyroid carcinoma — the most aggressive form of thyroid cancer, with rapid growth and a poor prognosis.
- Anaplastic large cell lymphoma (ALCL) — a type of non-Hodgkin lymphoma in which the lymphoma cells are markedly abnormal in appearance. ALCL is divided into ALK-positive and ALK-negative subtypes with different outlooks.
- Anaplastic astrocytoma — a high-grade brain tumor arising from astrocytes, more aggressive than lower-grade astrocytomas.
- Glioblastoma — the most aggressive primary brain tumor, characterized by marked anaplasia throughout.
Anaplasia can also be found as a feature of many other high-grade cancers, even when the word “anaplastic” does not appear in the tumor’s name.
What does anaplasia mean for my diagnosis?
Finding anaplasia in a tumor is a significant pathological finding. It indicates that the cancer cells have lost their normal identity and organization, which is generally associated with more aggressive behavior — faster growth, a higher likelihood of spread, and a greater risk of recurrence after treatment. Anaplasia is almost exclusively seen in malignant (cancerous) tumors; finding it in a benign tumor is exceedingly rare.
However, anaplasia is only one part of the overall picture. Prognosis and treatment decisions are based on the full pathology report — including the tumor type, stage, and any biomarker results — not on any single feature. Aggressive tumors may require more intensive treatment, but many high-grade cancers respond well to chemotherapy, radiation, immunotherapy, or targeted therapies. Your oncologist will interpret the finding of anaplasia in the context of your complete diagnosis.
Questions to ask your doctor
- What grade is my tumor, and does anaplasia affect its classification?
- How does the presence of anaplasia influence my treatment plan?
- What does anaplasia mean for my prognosis, given my specific cancer type and stage?