**Core Concept**
Small cell lung cancer (SCLC) is a highly aggressive and malignant type of lung cancer characterized by its rapid growth and early metastasis. It accounts for approximately 10-15% of all lung cancer cases and is often associated with smoking.

**Why the Correct Answer is Right**
Cranial irradiation is a common treatment approach in SCLC, especially in cases with extensive disease or brain metastases. This is because SCLC has a high propensity to spread to the brain, and radiation therapy helps to control or eliminate these metastatic lesions. The role of cranial irradiation is to prevent or treat central nervous system (CNS) involvement and improve survival outcomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Non-small cell lung cancer (NSCLC) is a more common type of lung cancer, but it does not typically require cranial irradiation as part of its treatment regimen, except in rare cases with brain metastases.
* **Option B:** Adenocarcinoma is a subtype of NSCLC and does not usually necessitate cranial irradiation as a standard treatment.
* **Option D:** Squamous cell carcinoma is another subtype of NSCLC and is not typically associated with cranial irradiation in its treatment.

**Clinical Pearl / High-Yield Fact**
It is essential to differentiate between SCLC and NSCLC, as their treatment approaches and prognoses differ significantly. SCLC tends to have a more aggressive course and requires more aggressive treatment, including cranial irradiation in some cases.

**Correct Answer: C. Small cell lung cancer. Small cell lung cancer.**