Author: Jonathan J. Chen, Alexei V. Chvetsov, Yulun He, Ramesh Rengan, Andrew Stacey, Kent Wallner 👨🔬
Affiliation: University of Washington and Fred Hutchinson Cancer Center, University of Washington 🌍
Purpose: To show that the RBE of low-energy photon sources used in COMS (Collaborative Ocular Melanoma Study) eye plaques compensates for protracted irradiation and attenuation of low-energy photons in heterogeneous eye plaque compositions to produce the required cell killing effect.
Methods: Biological effectiveness of COMS eye plaques with 125I and 103Pd sources is evaluated using the equivalent uniform RBE-weighted dose (EUDRBE), defined as the uniform dose distribution with fixed RBE=1 that produces the same cell survival as a nonuniform dose distribution with variable RBE. The EUDRBE concept can be applied to both fractionated external-beam radiotherapy and continuous irradiation in brachytherapy. EUDRBE is computed in 1D model of COMS eye plaques, in water-equivalent medium with heterogeneity correction, and in 5 instantaneous fractions using the LQ cell survival model that is corrected for the effects of protracted irradiation and RBE.
Results: In the simulations with a hypothetical RBE=1, EUDRBE for eye plaques with both 125I and 103Pd sources is less than the selected reference dose of 50 Gy(RBE) for the most common implant durations and tumor heights. The total dose of 50 Gy(RBE) is the lowest dose usually used in hypofractionated proton therapy for ocular melanoma. For RBE=1.4 and 1.9, the EUDRBE for eye plaques becomes larger than 50 Gy(RBE). The range of EUDRBE for 125I is within 50-60 Gy(RBE) depending on the implant duration and tumor height; therefore, the smallest EUDRBE is comparable to the dose used in hypofractionated proton therapy. The range of EUDRBE for 103Pd is within 57-67 Gy(RBE).
Conclusion: RBE effects for COMS eye plaques with 125I and 103Pd sources should be considered to explain the clinically observed >80% tumor control probability. The EUDRBE for 103Pd sources is larger than the EUDRBE for both the 125I sources and hypofractionated proton therapy; therefore, the dose de-escalation may be considered.