**Core Concept**
Radiotherapy plays a crucial role in the treatment of Wilms' tumor, a type of childhood kidney cancer. The optimal timing of radiotherapy is essential to minimize long-term sequelae and maximize treatment efficacy. The decision to start radiotherapy after surgery is influenced by various factors, including the histological grade of the tumor and the presence of any residual disease.

**Why the Correct Answer is Right**
The ideal timing for starting radiotherapy in Wilms' tumor after surgery is generally considered to be 3-4 weeks after surgical resection. This delay allows for the recovery of the patient's bone marrow and other tissues, which are often compromised by the surgical procedure. Additionally, a short period of chemotherapy may be administered during this time to control any residual disease and reduce the risk of recurrence. The timing of radiotherapy is critical to minimize the risk of radiation-induced complications, such as secondary malignancies and growth impairment.

**Why Each Wrong Option is Incorrect**
**Option A:** Immediate radiotherapy is not recommended, as it may exacerbate surgical complications and increase the risk of radiation-induced toxicity.
**Option B:** Waiting more than 6 weeks after surgery may lead to an increased risk of tumor recurrence and decreased treatment efficacy.
**Option C:** There is no evidence to support the use of radiotherapy within the first week after surgery, as the patient's tissues are still recovering from the surgical procedure.

**Clinical Pearl / High-Yield Fact**
In the treatment of Wilms' tumor, radiotherapy is typically administered in conjunction with chemotherapy, and the timing of these therapies is critical to minimize long-term sequelae and maximize treatment efficacy.

**Correct Answer:** C.