Dose distribution outside of a sphere of P-32 chromic phosphorous colloid

Purpose: To evaluate the dose distribution outside of a cyst instilled with phosphorous-32 (P-32, an electron emitter with a short effective range of 2–8 mm and average energy of 0.69 MeV, used to treat cystic craniopharyngioma) as a function of cyst size with and without plating (migration and adhesion of P-32 to the cyst surface). Methods and Materials: A cystic craniopharyngioma treated with instillation of P-32 was approximated by a sphere of uniformly distributed and plated chromic P-32 colloid. The percent depth dose was calculated along a radial position vector exterior to the sphere with a three-dimensional convolution integral and a dose point kernel. Results: The percent depth dose variation of surface or volume source external to a family of spheres was plotted. Complex cyst geometry is amenable to evaluation by approximation with simple spheres. Error estimates are calculated for the dose outside of truncated sphere segments. Plating might occur and raise the dose outside the cyst by more than a factor of 5.0. This has the potential to cause damage to adjacent tissues, including the optic chiasm. Conclusion: Clinicians are faced with a number of treatment options for cystic craniopharyngioma, including intracystic instillation of colloid P-32. Unfortunately, plating might occur and potentially damage adjacent normal tissues. It is recommended that the propensity for a craniopharyngioma to plate be evaluated before full treatment, especially after previous treatment.