The dose distribution for a 125I source is not isotropic due to what reason?

The dose distribution for a 125I source is not isotropic primarily due to the excessive attenuation along the axis from source encapsulation welding. In brachytherapy, where 125I seeds are often used to deliver localized radiation treatment, the geometry and materials involved can significantly influence how radiation is emitted and interacts with surrounding tissues. The encapsulation of the source, which protects the radioactive material, can introduce differences in the radiation dose distribution due to the directionality of the emitted photons.

When the photons travel through the encapsulating material, there can be more attenuation along the axis of the seed than in other directions, causing a disparity in the dose that is delivered to tissues at varying angles from the source. This attenuation negatively impacts the uniformity of the dose distribution, making it less isotropic (i.e., not equal in all directions). The anisotropic nature of the dose distribution is a critical factor to consider for treatment planning and for ensuring effective and safe radiation therapy outcomes.

The other factors, such as radioactive decay rate, size of the seed, or temperature variation, do not contribute significantly to the non-isotropic nature of the dose distribution compared to the effect of encapsulation-related attenuation. The decay rate determines how quickly the radioactive material will emit radiation over