A 50-year-old lady presented with a 3-month history of pain in the lower third of the right thigh. There was no local swelling; tenderness was present on deep pressure. Plain X-rays showed an ill-defined intra medullary lesion with blotchy calcification at the lower end of the right femoral diaphysis, possibly enchondroma or chondrosarcoma. Sections showed a cartilaginous tumor. Which of the following histological features (if seen) would be most helpful to differentiate the two tumours?

**Core Concept:** Histological examination of tumors is essential to differentiate between enchondroma and chondrosarcoma, as they have distinct characteristics that can be evaluated under a microscope.

**Why the Correct Answer is Right:** Enchondroma is a benign cartilaginous tumor originating from chondrocytes, usually found in the epiphyses of long bones. It is characterized by a monomorphic appearance, low cellularity, and no mitotic activity. On the other hand, chondrosarcoma is a malignant cartilaginous tumor originating from chondrocytes. It shows high cellularity, pleomorphism, and increased mitotic activity.

**Why Each Wrong Option is Incorrect:**

A. **Mitotic activity:** Enchondroma exhibits minimal mitotic activity, while chondrosarcoma demonstrates significant mitotic activity, making this feature less helpful in differentiating the two tumors.

B. **Cellularity:** While both tumors can exhibit varying degrees of cellularity, chondrosarcoma generally has a higher degree of cellularity than enchondroma due to its malignant nature.

C. **Mitotic index:** Mitotic index refers to the number of mitotic figures per unit area. Enchondroma has a low mitotic index, while chondrosarcoma has a higher mitotic index, making this feature less helpful in differentiating the two tumors.

D. **Microscopic appearance:** Both enchondroma and chondrosarcoma can exhibit similar microscopic appearances due to their cartilaginous origin. The focus should be on histological features beyond microscopic appearance.

**Clinical Pearl:** Histological examination is crucial in distinguishing between enchondroma and chondrosarcoma, as the presence of pleomorphism, mitotic activity, and cellularity help differentiate these two entities.

**Correct Answer:** **D. Pleomorphism:** Pleomorphism refers to the presence of cells with diverse shapes and sizes within a tumor. Enchondroma typically demonstrates uniform cells, while chondrosarcoma exhibits pleomorphic cells, making this feature more helpful in differentiating the two tumors.

✓ Correct Answer: B. Tumor permeation between bone trabeculae at periphery