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Brachytherapy, often referred to as internal radiation therapy, is a technique in radiation oncology where a sealed radioactive source is temporarily or permanently placed directly into or adjacent to the area requiring treatment, allowing for the delivery of a very high dose of radiation precisely to the tumor volume while minimizing exposure to surrounding healthy tissues.

Its fundamental advantage lies in the principle of the inverse square law, where the radiation dose falls off rapidly with distance from the source, creating a highly localized, conformal dose distribution that is particularly effective for treating cancers in accessible sites such as the prostate, cervix, breast, and skin.

Brachytherapy is typically categorized into Low Dose Rate ($\text{LDR}$), where sources remain implanted for a prolonged period (e.g., permanent seeds for prostate cancer), and High Dose Rate ($\text{HDR}$), where a single, high-activity source is delivered via a computer-controlled afterloader for short, highly precise treatment fractions through temporary applicators. The entire procedure is guided by sophisticated imaging (ultrasound, CT, or MRI) to ensure accurate placement of the catheters or applicators. Treatment planning and dosimetry are computationally intensive processes, involving the precise calculation of dose distribution based on the position of the source and the anatomy of the target and nearby organs-at-risk ($\text{OARs}$). $\text{HDR}$ brachytherapy, in particular, allows for the precise, millimeter-level movement of the source within the applicator, creating a highly customized dose map. Clinically, brachytherapy is often used as monotherapy for early-stage disease or as a boost following external beam radiation therapy ($\text{EBRT}$) to escalate the total dose delivered to the highest-risk portion of the tumor, significantly enhancing local control rates while maintaining functional preservation in sensitive organs.