Distribution of [32P]-chromic phosphate colloid in cystic brain tumors

To suppress cyst formation in 42 brain tumors, 32P has been stereotactically instilled in doses calculated to deliver 20,000-40,000 rad to the cyst wall, assuming uniform dispersal of the radioisotope. However, samples of cyst fluid obtained at varying intervals after injection showed lower than expected activity levels, suggesting early 'plating' of 32P. To accommodate this phenomenon, a surface-area-dependent dosimetric calculation is compared with a volume-dependent calculation which assumes uniform dispersal. These two approaches represent lower and upper extremes. It appears that in small cysts there is less difference in the required administered dose, but in larger cysts potentially very large differences exist and caution should be exercised if uniform suspension is assumed.