What percent of retinoblastomas are Bilateral?

**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children, and is associated with genetic mutations, particularly in the RB1 gene. Bilateral retinoblastoma refers to the occurrence of this cancer in both eyes.

**Why the Correct Answer is Right**
Approximately 30-40% of retinoblastoma cases are bilateral, indicating that the cancer has developed in both eyes simultaneously. This increased likelihood of bilateral involvement is often linked to inherited mutations in the RB1 gene, which can be inherited in an autosomal dominant pattern. The presence of bilateral retinoblastoma necessitates a comprehensive evaluation for potential genetic predisposition and family history.

**Why Each Wrong Option is Incorrect**
**Option A:** 10% - This percentage is too low, as bilateral retinoblastoma is more common than this estimate.
**Option B:** 50% - This percentage is too high, as the majority of retinoblastoma cases are unilateral, affecting only one eye.
**Option C:** 80% - This percentage is excessively high, as the actual incidence of bilateral retinoblastoma is significantly lower.

**Clinical Pearl / High-Yield Fact**
When encountering a child with retinoblastoma, it's essential to consider the possibility of bilateral involvement and assess for any family history of the disease, as this can impact treatment decisions and genetic counseling.

**Correct Answer: C. 35%**