**Core Concept**
Hypercellular fibroadenomas, also known as cystosarcomas phyllodes, are a rare type of breast tumor characterized by an overgrowth of fibroblastic and epithelial cells. These tumors can range from benign to malignant and are often diagnosed incidentally during breast biopsies.

**Why the Correct Answer is Right**
The management of hypercellular fibroadenomas depends on their histological grade and margin status. For low-grade lesions with negative margins, simple excision may be sufficient. However, for high-grade lesions or those with positive margins, a more aggressive approach is warranted. This may involve wide local excision with clear margins, possibly in conjunction with radiation therapy. The decision to perform lymph node dissection is typically not necessary unless there is evidence of metastasis.

**Why Each Wrong Option is Incorrect**
**Option A:** Mastectomy is generally not the first-line treatment for hypercellular fibroadenomas, as it may be overly aggressive for benign or low-grade lesions.
**Option B:** Sentinel lymph node biopsy is usually not necessary unless there is evidence of metastasis, making this option incorrect.
**Option C:** Radiation therapy may be used in conjunction with wide local excision for high-grade lesions, but it is not a standalone treatment.

**Clinical Pearl / High-Yield Fact**
For breast tumors with uncertain or suspicious margins, it is essential to obtain a second opinion from a breast pathologist to ensure accurate diagnosis and guide management decisions.

**Correct Answer: C.**