Novel bioerodable eluting-spacers for radiotherapy applications with in situ dose painting

Objective: To investigate feasibility of using bioerodable/bioerodible spacers (BES) over biodegradable spacers (BDS) loaded with gold nanoparticles for radiotherapy applications with in situ dose-painting, and to explore dosimetric impact on dose enhancement ratio of different radioisotopes. Methods: Analytical models proposed were based on experimentally reported erosion rate constant (k0 = 5. 5E-7 kgm^-2s^-1 ) for bioerodible polymeric matrix. An in vivo determined diffusion coefficient (2.2E-8 cm²/s) of 10 nm gold nanoparticles (AuNP) of concentration 7 mg/g was used to estimate diffusion coefficient of other AuNP sizes (2, 5, 14 nm) using the Stoke-Einstein diffusion equation. The corresponding dose enhancement factors (DEF) were used to study dosimetric feasibility of employing AuNP-eluting BPS for radiotherapy applications. Results: The results showed AuNP release period from BES was significantly shorter (116 h) compared to BDS (more than a month) reported previously. The results also agree with reported Hopfenberg equation for a cylindrical matrix undergoing surface erosion. The DEF at tumour distance 5 mm for Cs-131 (DEF >2) greater than that of I-125 (DEF > 2) and Pd-103 (DEF ≥2) could be achieved for AuNP sizes (2, 5, 10, and 14 nm) respectively. Conclusion: Our findings suggested that BES could be used for short-lived radioisotopes like Pd-103 and Cs-131 in comparison to eluting BDS which is feasible for longlived radioisotopes like I-125. Advances in knowledge: The study provides scientific basis for development of new generation eluting spacers viable for enhancing localized tumour dose. It concludes that BES gives higher DEF for Cs-131, and good candidate for replacing conventional fiducials/spacers.