Point-source irradiation with eccentric rotation causes inhomogeneous dose distribution.
Radioactive point sources are regularly used for irradiating cell culture and other biological materials. Eccentric rotation is often used to minimize dose disparities that arise from irradiating samples that span a distance from the point source. Rotation provides a great improvement in dose homogeneity compared to inert irradiation yet still presents an obvious shortcoming for exposures in which the sample completes only partial rotation or fractional rotation. In such cases, certain areas of the sample have a closer average distance to the radiation source than other areas within the same sample. This obstacle can be partially overcome by adjusting rotation speed so the sample traverses a full rotation (or multiple thereof) throughout the total irradiation time. Here we investigate the effects of irradiation with eccentric rotation on dose homogeneity. We show that due to the inverse square law that governs dose, even exposures with full rotation result in inhomogeneous dose distributions. This dose inhomogeneity can be substantial, especially for large samples and small source- sample distances. We observed a 33% difference in survival across 100-mm dishes and a 400% difference for 150-mm dishes. The dose inhomogeneity inherent to eccentric rotation increases the actual average dose delivered across the sample compared to that delivered at sample center. We offer a table of correction factors that account for this dose increase and correct the dose delivered at center to the actual average dose delivered across the entire sample.
Hafer
Kurt
K
Departments of Pathology, UCLA School of Medicine and Public Health, Los Angeles, California, USA. khafer@mednet.ucla.edu
Scuric
Zorica
Z
Iwamoto
Keisuke S
KS
Schiestl
Robert H
RH