## **Core Concept**
Postoperative radiotherapy in breast cancer is a treatment approach used to reduce the risk of local recurrence. It involves the use of ionizing radiation to target any remaining cancer cells in the breast, chest wall, or lymph nodes after surgical removal of the tumor. This treatment is based on the principle that radiation can effectively kill microscopic cancer cells that may remain after surgery.

## **Why the Correct Answer is Right**
The correct answer, **Tumor size > 4 cm or positive margins**, is right because tumors larger than 4 cm in size or those with positive margins (cancer cells found at the edge of the tissue removed) have a higher risk of local recurrence. Postoperative radiotherapy is recommended in these cases to eliminate any residual cancer cells that might be present in the breast or the surrounding tissues, thereby reducing the risk of recurrence.

## **Why Each Wrong Option is Incorrect**
- **Option A: Only for patients with lymph node-negative disease** - This option is incorrect because postoperative radiotherapy is not limited to patients with lymph node-negative disease. Patients with lymph node-positive disease also benefit from radiotherapy, especially if they have other high-risk features.
- **Option B: Only for patients undergoing mastectomy** - This option is incorrect because postoperative radiotherapy can be given to patients undergoing breast-conserving surgery (lumpectomy) as well. In fact, radiotherapy is a standard component of breast-conserving therapy.
- **Option C: All patients undergoing breast surgery** - This option is incorrect because not all patients undergoing breast surgery require postoperative radiotherapy. The decision to use radiotherapy is based on specific factors, including tumor size, margin status, lymph node involvement, and the type of surgery performed.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the use of postoperative radiotherapy in breast cancer is guided by the assessment of risk factors for local recurrence, including tumor size, margin status, and lymph node status. The **St. Gallen Consensus** and other guidelines provide frameworks for making these decisions, emphasizing the importance of multidisciplinary assessment in determining the need for radiotherapy.

## **Correct Answer:** D. Tumor size > 4 cm or positive margins.