A1.4 Brachytherapy

Topic

Brachytherapy places a sealed radioactive source directly inside or immediately adjacent to the tumor, rather than directing radiation from outside the body; this proximity between the radioactive source and the tumor forms the physical basis of the technique, as it determines how the dose is distributed around the source.

This dose is not distributed evenly around the radioactive source but instead follows the inverse-square law: as the distance from the source increases, the dose decreases in inverse proportion to the square of that distance, resulting in a much sharper drop-off than if the decrease were simply proportional to the distance. Consequently, the dose is extremely high just a few millimeters from the source, whereas it becomes minimal a short distance away.

This combination of a very high dose near the source and a minimal dose at a distance enables brachytherapy to deliver dose levels to the tumor that external photon radiotherapy cannot achieve; with external photon radiotherapy, increasing the dose in such a manner would cause toxicity in the healthy tissues surrounding the tumor, as that modality cannot concentrate the dose with the same degree of proximity to the source that brachytherapy offers.

Among the sealed radioactive sources used in brachytherapy, each delivers the dose at a different rate depending on its constituent material: iridium-192 is used for high-dose-rate delivery; iodine-125 and palladium-103 are used for low-dose-rate delivery, typically in permanent prostate implants; and cesium-131 ​​is another sealed radioactive source used for the same purpose of concentrating the dose near the tumor. To determine the exact dose delivered by each of these sealed radioactive sources to any given point within the tumor, brachytherapy dosimetry relies on a standardized calculation formalism that combines three factors: the geometry of the radioactive source, the reference dose rate specific to that source, and additional geometric factors that adjust the calculation based on the precise position within the tumor. Through this combination of the inverse-square law and precise dosimetric calculation, the sealed radioactive source achieves the very objective of brachytherapy: delivering the highest possible dose to the tumor while minimizing the dose received by the healthy tissues surrounding it.

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