A 14-year-old girl with a history of unilateral retinoblastoma treated with radiotherapy 8 years ago now presents with severe pain and swelling in the proximal humerus. Imaging reveals a mixed lytic and sclerotic lesion with periosteal new bone formation. Biopsy shows malignant spindle cells with prominent osteoid production and mitotic figures. Which of the following genetic alterations is most likely responsible for the development of this tumour?

Explanation

## Genetic Basis of Radiation-Induced Osteosarcoma in Retinoblastoma Survivors ### Clinical Context **Key Point:** Osteosarcoma developing in a patient with prior retinoblastoma (especially post-radiotherapy) is a secondary malignancy associated with RB1 gene loss. This is a well-established association in NEET PG pathology. ### RB1 Gene and Osteosarcoma Pathogenesis | Aspect | Details | |--------|----------| | **Gene location** | Chromosome 13q14 | | **Normal function** | Tumour suppressor; regulates G1/S checkpoint; inhibits E2F transcription factors | | **Mechanism in osteosarcoma** | Loss of RB1 → uncontrolled cell cycle progression → malignant transformation | | **Association with retinoblastoma** | Germline RB1 mutations predispose to retinoblastoma AND secondary osteosarcoma | | **Risk increase** | 500–1000× higher risk in RB1 mutation carriers | **High-Yield:** Patients with hereditary retinoblastoma (germline RB1 mutation) have a 400–500-fold increased risk of developing osteosarcoma, especially in the field of prior radiotherapy. This is a classic NEET PG association. ### Pathogenesis of Secondary Osteosarcoma 1. **Germline RB1 mutation** → predisposition to retinoblastoma 2. **Radiotherapy for retinoblastoma** → additional somatic mutations (TP53, PTEN, others) 3. **Loss of RB1 checkpoint control** + radiation-induced DNA damage → malignant transformation in osteoblasts 4. **Aggressive phenotype** → high-grade sarcoma with rapid progression **Clinical Pearl:** The latency period between radiotherapy and secondary osteosarcoma is typically 5–20 years. This patient's 8-year interval is consistent with radiation-induced secondary malignancy. ### Comparison of Genetic Alterations in Bone Tumours | Tumour | Primary Genetic Alteration | Gene Function | |--------|---------------------------|----------------| | **Osteosarcoma (sporadic)** | TP53, RB1, PTEN mutations | Cell cycle, apoptosis, growth | | **Osteosarcoma (hereditary RB)** | RB1 loss (germline) | G1/S checkpoint | | **Ewing sarcoma** | t(11;22) EWS-FLI1 fusion | Transcription factor | | **Giant cell tumour** | H3F3A mutation | Histone variant | | **Chondrosarcoma** | IDH1/IDH2 mutations | Epigenetic regulation | **Mnemonic:** **RB-OS** = Retinoblastoma survivors → Osteosarcoma (RB1 loss) ### Why This Is the Answer ```mermaid flowchart TD A[Retinoblastoma survivor]:::outcome --> B[Germline RB1 mutation]:::outcome B --> C[Prior radiotherapy]:::action C --> D[Somatic mutations + RB1 loss]:::outcome D --> E[Osteosarcoma development]:::urgent E --> F[High-grade malignancy]:::urgent F --> G[Aggressive course, early metastases]:::outcome ``` **Key Point:** The combination of retinoblastoma history + radiotherapy + osteosarcoma in a young patient is pathognomonic for RB1-driven secondary malignancy.