**Core Concept**
The patient is diagnosed with a gastrointestinal stromal tumor (GIST), a type of tumor that arises from the interstitial cells of Cajal or their precursors in the gastrointestinal tract. GISTs are characterized by the expression of specific markers and mutations, including DOG-1 and SDH-1.

**Why the Correct Answer is Right**
The diagnosis of GIST is confirmed by the presence of DOG-1 marker positivity and SDH-1 gene mutation. The treatment of choice for GIST is targeted therapy with tyrosine kinase inhibitors (TKIs), such as imatinib. Imatinib works by inhibiting the KIT and PDGFRA tyrosine kinases, which are often mutated in GISTs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the treatment of GIST. Chemotherapy is not the first-line treatment for GIST.
* **Option B:** This option is not the correct answer. While surgery may be indicated in certain cases of GIST, it is not the primary treatment for this condition.
* **Option C:** This option is not the correct answer. Radiotherapy is not typically used as a primary treatment for GIST.

**Clinical Pearl / High-Yield Fact**
GISTs can be classified into two main types: KIT-positive and SDH-deficient. KIT-positive GISTs are typically treated with TKIs, while SDH-deficient GISTs may be resistant to TKI therapy.

**Correct Answer:** C. Imatinib