**Core Concept**
Leiomyosarcoma of the breast is a rare type of soft tissue sarcoma that originates from the smooth muscle cells of the breast. It is a malignant tumor that can cause significant morbidity and mortality if not treated promptly. The key characteristic of leiomyosarcoma is its potential to invade adjacent tissues and metastasize to distant sites.

**Why the Correct Answer is Right**
Leiomyosarcoma of the breast is often described as a well-encapsulated tumor, meaning that it is surrounded by a distinct border of fibrous tissue. This characteristic can facilitate surgical resection and improve the chances of a successful outcome. The encapsulation of leiomyosarcoma can also make it easier to distinguish from other types of breast tumors, such as phyllodes tumors or carcinomas. The encapsulation of leiomyosarcoma is a critical factor in determining the optimal treatment approach.

**Why Each Wrong Option is Incorrect**
**Option A:** Axillary lymph node dissection is not mandatory for leiomyosarcoma of the breast, as the tumor is less likely to metastasize to the lymph nodes compared to other types of breast cancer. However, lymph node status may still be relevant in determining the prognosis and guiding further treatment.

**Option C:** Follow-up is indeed required for patients with leiomyosarcoma of the breast, as the tumor can recur locally or metastasize to distant sites. Regular imaging studies and clinical evaluations are necessary to monitor for any signs of recurrence or metastasis.

**Option D:** Mastectomy is not the mainstay treatment for leiomyosarcoma of the breast. Instead, surgical resection of the tumor with wide margins is often the primary treatment approach. Radiation therapy and systemic chemotherapy may also be used in certain cases to improve outcomes.

**Clinical Pearl / High-Yield Fact**
Leiomyosarcoma of the breast is a rare but aggressive tumor that requires prompt recognition and treatment. A high index of suspicion is necessary to distinguish it from other types of breast tumors, and a multidisciplinary approach is often required to optimize outcomes.

**✓ Correct Answer: B. Well encapsulated**