Author: Alexei V. Chvetsov, Sunan Cui, Sunyoung Jang, Robert D. Stewart 👨🔬
Affiliation: University of Washington and Fred Hutchinson Cancer Center, Penn State College of Medicine, University of Washington 🌍
Purpose: To evaluate spatial variation of relative biological effectiveness (RBE) in COMS (Collaborative Ocular Melanoma Study) eye plaques with low energy 125I and 103Pd photon sources as a function of dose rate.
Methods: Eye plaque brachytherapy with low energy photon sources is characterized by strong spatial nonuniformity of dose rate by the superficial location of radiation source and different time intervals of protracted irradiation (3-8 days). Different radionuclides may be characterized by different RBE. Assuming that cellular survival is described by the liner-quadratic (LQ) model, the RBE correction is applied using the theory of dual radiation action that predicts that the higher LET radiation increases the linear component (α) of radiation damage, while the quadratic component (β) remains unchanged. Based on the theory of dual radiation action, the RBE approaches 1.0 when the dose rate approaches infinity or its maximum value when the dose rate approaches zero. Measurements show that isotopes 125I and 103Pd have an RBE approximately 1.4 and 1.9 respectively that were obtained at low dose rates approximately 0.07 Gy/h. The cell line Mel 202 with the parameters of the LQ survival curve: aX=0.274 Gy-1, βX=0.035 Gy-2 has been selected the repair constant of sublethal damage μ=0.46 h-1 was selected that corresponds to the repair half-life value of 1.5 h.
Results: Central axis dose rates have been calculated for COMS eye plaques with diameters 12-20 mm using MIM Symphony (MIM Software Inc., Cleveland, OH, USA). Using radiobiological parameters and computed dose rates, we show that the RBE approaches it maximum value at the tumor apex and its minimum value of 1.0 at the tumor base for both 125I and 103Pd photon sources.
Conclusion: Base-to-apex ratio of biological effect in eye plaque brachytherapy with low energy photons can be reduced using interplay of relative biological effectiveness and dose rate.