Analysis of factors affecting the therapeutic gain in high dose rate gynecological implants relative to low dose rate...

The radiobiological efficacy of high dose rate (HDR) intracavitary implant relative to that of low dose rate (LDR) implants has been represented quantitatively by the therapeutic factor lamba defined as [EDRtum/ERDorgan]HDR/[ERDtum/ERDorgan]LDR where ERD is the extrapolated response dose. The ERDs for the tumor and a critical organ are calculated using the linear-quadratic model and depend on the values of alpha/beta ratio, the tumor and normal tissue repair time constants (mu 1 and mu 1), number (N) of HDR fractions, the dose per fraction (d), and the fractional tumor and organ dose (ft and fl) relative to the dose at the reference point. We have studied the variation of gamma with d, mu t, mu l, ft, and ft and have derived specific conditions for which lamba can have a value equal to or greater than unity leading to a therapeutic gain in HDR. It is found that lamba is less than unity for commonly used parameter values, specifically when mu t is assumed to be equal to mu l. However, if mu t is greater than mu l, lamba can have a value greater than unity for many possible values of ft and fl.