Dose from radioactive seeds

Dose calculation geometry

The calculation of radiation dose from radioactive seeds is crucial in the planning and administration of brachytherapy treatments. Most modern calculation are done using the formalism published by the American Association of Physicists in Medicine.^[1] For the geometry in figure 1, this formalism uses five parameters.

• Strength of the source: How much radiation is being emitted by the seed, expressed as air kerma strength and denoted by $S_k$.
• Dose rate of the source: How much dose the seed will deliver to the reference point over a certain period of time, denoted by $\Lambda$.
• Geometry factor: How the shape of the seed will affect the dose at points away from the reference point, denoted by $G(r,\theta)$.
• Anisotropy function: How the much radiation will be stopped before passing out of the seed, denoted by $g(r)$.
• Radial dose function: How the radiation will interact with the material surrounding the seed, denoted by $F(r,\theta)$

The equation which links these parameters is, $D(r,\theta)=S_k \Lambda \frac{G(r,\theta)}{G(r_0,\theta_0)}g(r)F(r,\theta)t$

References

1. Nath, R.; Anderson, L. L.; Luxton, G.; Weaver, K. A.; Williamson, J. F.; Meigooni, A. S. (1995). "Dosimetry of interstitial brachytherapy sources: Recommendations of the AAPM Radiation Therapy Committee Task Group N. 43.". Med Phys. 22: 209–234.