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Brachytherapy is an internal radiation therapy technique where sealed radioactive sources are placed inside or very close to the target tissue. Dose falloff is rapid due to inverse square law, allowing high tumor dose with limited surrounding tissue exposure. Common isotopes include Ir-192, I-125, and Cs-137.
Worked example 1 (dose falloff concept): If dose rate is 200 cGy/hr at 1 cm from source, at 2 cm: I2 = 200 × (1/2)^2 = 200 × 1/4 = 50 cGy/hr. This steep falloff protects nearby normal tissue.
Worked example 2 (LDR vs HDR concept): Low Dose Rate (LDR) may deliver 0.5–2 Gy/hr continuously. High Dose Rate (HDR) may deliver >12 Gy/hr in short fractions with remote afterloading.
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