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

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 F defined as [EDR(tum)/ERD(organ)](HDR)/[ERD(tum)/ERD(organ)](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 α/β ratio, the tumor and normal tissue repair time constants (μ(t) and μ(l)), number (N) of HDR fractions, the dose per fraction (d), and the fractional tumor and organ dose (f(t) and f(l)) relative to the dose at the reference point. We have studied the variation of Γ with d, μ(t), μ(l), f(t), and f(l) and have derived specific conditions for which Γ can have a value equal to or greater than unity leading to a therapeutic gain in HDR. It is found that Γ is less than unity for commonly used parameter values, specifically when μ(t) is assumed to be equal to μ(l). However, if μ(t) is greater than μ(l), Γ can have a value greater than unity for many possible values of f(t) and f(l).