**Core Concept**
Radiotherapy is generally contraindicated in benign conditions due to risk of radiation-induced malignancy and tissue damage. However, in rare cases, especially when surgical intervention is impossible or highly risky, radiotherapy may be considered as a last resort.

**Why the Correct Answer is Right**
Extensive pigmented villonodular synovitis (PVNS) is a benign but destructive synovial lesion that can lead to severe joint deformity and functional impairment. In cases where the disease is extensive and involves major joints (e.g., knee), and surgical removal is not feasible due to anatomical or structural constraints, radiotherapy may be used to control growth and reduce symptoms. Though rare, it is the only benign condition in orthopaedics where radiotherapy is approved as a therapeutic option due to its aggressive nature and surgical inaccessibility.

**Why Each Wrong Option is Incorrect**
Option A: Chondromyxoid fibroma is a benign cartilage-forming tumor, but it is typically curable with surgery. Radiotherapy is not indicated due to low risk of malignancy and high surgical success rate.
Option C: Benign fibrous histiocytoma is usually managed surgically with excellent outcomes; radiotherapy is not recommended due to risk of transformation.
Option D: Desmoplastic fibroma is a benign bone lesion, but it is typically excisable and often curable with surgery. Radiotherapy is not indicated due to excellent surgical outcomes and low malignancy risk.

**Clinical Pearl / High-Yield Fact**
PVNS is the only benign orthopaedic condition where radiotherapy is considered due to its inoperability and aggressive growth—always consider surgical options first, but know this exception for joint-threatening lesions.

✓ Correct Answer: B. Extensive pigmented villonodular synovitis