**Core Concept:** Familial Retinoblastoma is a genetic disorder caused by mutations in the RB1 gene, which leads to a higher risk of developing retinoblastoma, a rare form of childhood eye cancer. Additionally, these patients have an increased risk of developing second malignancies, including certain types of adult cancers due to the underlying genetic defect.

**Why the Correct Answer is Right:** The correct answer is lung cancer. In patients with familial retinoblastoma, the RB1 gene mutation also increases the risk of developing other cancers, including lung cancer. This is because the RB1 gene mutation affects the cell cycle regulation, leading to uncontrolled cell growth and forming tumors.

**Why Each Wrong Option is Incorrect:**

A. Melanoma: Although patients with RB1 gene mutations have a higher risk of melanoma, it is not the most common second malignancy following retinoblastoma.

B. Leukemia: Patients with RB1 gene mutations are more likely to develop solid tumors, not blood cancers like leukemia.

C. Soft tissue sarcoma: While soft tissue sarcoma can occur in these patients, it is less common than lung cancer as the second malignancy.

D. Lymphoma: Similar to leukemia, lymphomas are blood-related cancers and less common than lung cancer in this context.

**Clinical Pearl:** The understanding of genetic predispositions like RB1 mutations highlights the importance of regular cancer surveillance and early detection for patients with inherited cancer predisposition syndromes. This can significantly improve cancer management and patient outcomes.