♦ The probability of local tumour control increases with increasing radiation dose, but so does the probability of normal tissue damage. Brachytherapy allows the delivery of a localized high radiation dose to a small tumour volume, increasing the chance of local control. There is a sharp fall-off of radiation dose in the surrounding normal tissue, therefore the risks of complication are reduced.

♦ The overall duration of brachytherapy is short, generally between 2-7 days. The constant low-dose irradiation takes advantage of the different rates of repair and re-population of normal and malignant tissue to produce differential cell killing, enhancing the therapeutic ratio.

♦ Hypoxic cells are relatively resistant to radiation treatment. Re-oxygenation may occur during low-dose-rate radiotherapy, with initially resistant hypoxic cells becoming well aerated and sensitive.

♦ The dose distribution within the tumour volume is often not homogeneous. Treatment is often prescribed to the minimum dose received around the periphery of the treated volume. Areas close to the radiation sources in the centre of the tumour volume often receive up to twice this dose. Hypoxic cells are situated in avascular, sometimes necrotic areas in the centre of tumours and the higher doses received in the centre help to compensate for the relative radio-resistance of these hypoxic cells.

♦ Irregular-shaped tumours can be treated by judicious positioning of radiation sources and critical surrounding normal tissues can be avoided.

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