**Core Concept**
Cystosarcoma phylloides, also known as phyllodes tumor, is a rare type of breast tumor that can be benign, borderline, or malignant. It arises from the stromal component of the breast tissue, unlike the more common epithelial tumors.

**Why the Correct Answer is Right**
The treatment of phyllodes tumor depends on its histological characteristics, with a focus on surgical management. For benign phyllodes tumors, simple excision is usually sufficient. However, for borderline or malignant tumors, wide local excision with clear margins is recommended, often accompanied by a negative margin of 1-2 cm. This approach helps to prevent local recurrence and minimize the risk of metastasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Mastectomy is not the primary treatment for phyllodes tumors, as it is usually reserved for cases with extensive local recurrence or metastasis.

**Option B:** Radiation therapy is not a standard treatment for phyllodes tumors, although it may be considered in cases with positive margins or high-grade malignancy.

**Option C:** Chemotherapy is not typically used for the treatment of phyllodes tumors, as they are generally less aggressive than other types of breast cancer.

**Clinical Pearl / High-Yield Fact**
It's essential to note that phyllodes tumors can exhibit a range of histological features, making accurate diagnosis and grading crucial for determining the optimal treatment approach.

**Correct Answer:** D. Wide local excision with clear margins.