**Core Concept:** Post-operative radiotherapy is a treatment modality used to reduce the risk of cancer recurrence after surgical resection of tumors. Radiotherapy involves the use of ionizing radiation to damage the DNA of cancerous cells, preventing their growth and division. In this context, we will discuss when radiotherapy is minimally used.

**Why the Correct Answer is Right:** Post-operative radiotherapy is minimally used in cases where the surgical margins are close (R1 or R2) or when there is microscopic cancerous tissue left behind after surgery. This approach aims to reduce the risk of local recurrence by targeting the remaining cancer cells.

**Why Each Wrong Option is Incorrect:**

A. Rarely used in breast cancer: While radiotherapy is often prescribed after breast-conserving surgery for early-stage breast cancer, the focus here is on minimally used cases.
B. Typically used in prostate cancer: Post-operative radiotherapy is more commonly used in prostate cancer, not minimally used.
C. Often used in rectal cancer: Similar to option A, radiotherapy is frequently used in rectal cancer, not minimally used.
D. Indicated in early-stage lung cancer: In early-stage lung cancer, radiotherapy is often used instead of minimally used.

**Clinical Pearl:** In cases where the surgical margins are close or the tumor is not completely resected, minimally using post-operative radiotherapy can significantly reduce the risk of local recurrence, demonstrating the importance of considering individual patient factors when determining treatment strategies.