**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children. It arises from the neural retinal cells and is characterized by the loss of function of the RB1 tumor suppressor gene, which regulates the cell cycle.

**Why the Correct Answer is Right**
Retinoblastoma can occur in both hereditary and non-hereditary forms. In the hereditary form, there is a 50% chance of passing the mutated RB1 gene to offspring. The tumor can be unilateral or bilateral, with bilateral cases often associated with a family history of retinoblastoma. Treatment options include chemotherapy, radiation therapy, and enucleation.

**Why Each Wrong Option is Incorrect**
**Option A:** Retinoblastoma is most commonly diagnosed in children under the age of 5 years. This statement is true, as retinoblastoma typically presents in early childhood, often within the first two years of life.

**Option B:** Retinoblastoma is associated with a high risk of secondary cancers. This statement is also true, as individuals with retinoblastoma have an increased risk of developing other cancers, such as osteosarcoma and melanoma, due to the loss of tumor suppressor function.

**Option C:** Retinoblastoma can occur in adults. This statement is incorrect, as retinoblastoma is primarily a pediatric tumor, with the majority of cases diagnosed in children under the age of 5 years.

**Option D:** The RB1 gene is a tumor suppressor gene. This statement is true, as the RB1 gene plays a crucial role in regulating the cell cycle and preventing the uncontrolled growth of cells that can lead to cancer.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma is often associated with a characteristic "cat's eye" appearance of the tumor, which can be seen on imaging studies such as ultrasound or CT scans. This appearance is due to the central location of the tumor within the retina.

**Correct Answer: C. Retinoblastoma can occur in adults.**