**Core Concept**
The AJCC 8th edition provides a standardized framework for staging pancreatic cancer, which is crucial for determining treatment options and prognosis. The staging system takes into account the tumor's size, location, and involvement of nearby structures, such as blood vessels.

**Why the Correct Answer is Right**
The AJCC 8th edition classifies pancreatic cancer into several stages based on the tumor-node-metastasis (TNM) system. For a 2 cm pancreatic tumor that involves the portal vein, we need to determine the correct stage. Involvement of the portal vein is a critical factor in staging pancreatic cancer, as it indicates potential for lymphatic spread. According to the AJCC 8th edition, a tumor of any size that involves a major blood vessel, such as the portal vein, is classified as T3. This is because T3 tumors have a higher risk of lymph node metastasis and poorer prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a valid AJCC 8th edition staging classification.
**Option B:** This option is incorrect because involvement of the portal vein is a critical factor in staging pancreatic cancer, but it does not change the tumor size classification.
**Option C:** This option is incorrect because it does not accurately reflect the AJCC 8th edition staging classification for a 2 cm pancreatic tumor with portal vein involvement.

**Clinical Pearl / High-Yield Fact**
When staging pancreatic cancer, it's essential to remember that involvement of major blood vessels, such as the portal vein, significantly impacts the tumor's classification and prognosis.

**Correct Answer:** C.