The radiobiology and physics of brachytherapy.

The principles of radiobiology are the same for both EBRT and brachytherapy. Strict spatial limitation of the delivered dose, a function of the inverse square law, is one reason that brachytherapy is attractive. The biologic rationale of LDR implants is maximization of normal tissue repair to enhance the therapeutic ratio. Fractionated HDR treatment schedules have been formulated that provide good biologic approximation of the well-established LDR practices. The HDR approach improves both patient comfort and radiation safety; also, the brief treatment times can permit geometric improvements by transiently displacing sensitive structures.