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 cailaginous tumor. Which of the following histological features(if seen) would be most helpful to differentiate the two tumours?

**Core Concept**
The question requires the differentiation between enchondroma and chondrosarcoma, two cartilaginous tumors that can present with similar radiological features. Enchondromas are benign tumors that arise from the cartilage of the bone, while chondrosarcomas are malignant tumors that can arise from the cartilage or from the bone itself.

**Why the Correct Answer is Right**
Chondrosarcomas are characterized by the presence of atypical cartilage cells with hyperchromatic nuclei and prominent nucleoli, which are not typically seen in enchondromas. In contrast, enchondromas typically have well-differentiated cartilage cells with bland nuclei. The presence of binucleated or multinucleated cartilage cells with a large number of mitoses would be highly suggestive of chondrosarcoma. Furthermore, the presence of a cartilaginous matrix with a characteristic "chicken wire" or "ring and arc" pattern, which is often seen in enchondromas, would be less typical of chondrosarcoma.

**Why Each Wrong Option is Incorrect**
**Option A:**
* The presence of a high cellularity index alone is not sufficient to differentiate enchondroma from chondrosarcoma, as both tumors can have a variable degree of cellularity.

**Option B:**
* The presence of a high mitotic rate alone is not sufficient to differentiate enchondroma from chondrosarcoma, as enchondromas can have a low but variable mitotic rate.

**Option C:**
* The presence of a cartilaginous matrix alone is not sufficient to differentiate enchondroma from chondrosarcoma, as both tumors can have a similar cartilaginous matrix.

**Clinical Pearl / High-Yield Fact**
The "chicken wire" or "ring and arc" pattern seen in enchondromas is a classic radiological feature that can help differentiate these tumors from chondrosarcomas. However, it's essential to note that the final diagnosis should always be made based on a combination of clinical, radiological, and histological features.

**Correct Answer:**
C.

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