Evaluation of Bystander Effects In Spatially Fractionated Radiotherapy Using In Vitro Growth Assays with Co-Registered Dosimetry
Abstract
Purpose
Currently, there is no standard geometry identified to maximize therapeutic benefits of spatially fractionated radiotherapy (SFRT). This work aims to assess bystander effects in vitro after SFRT while identifying which SFRT geometric parameters influence different biologic outcomes.
Methods
Two cell culture experiments using the 4T1 murine mammary cell line were conducted: a modified colony-forming assay (CFA) and a growth inhibition assay. For both assays, radiotherapy was delivered as broad beam or SFRT using three collimators with different geometric configurations. For the modified CFA, radiochromic film was aligned to the treatment area using a fixture designed and fabricated in-house. The alignment fixture was used to scan the stained plates with the film, thereby associating dosimetric regions with cell cultures. Expected outcomes without bystander effects for each assay were calculated using broad beam and non-treated control data scaled to peak and valley areas.
Results
Bystander effects were identified as a decrease in overall cell growth compared to the expected growth. GRID 3 (105 peaks of 0.8-mm width and 1.25-mm center-to-center spacing) did not produce bystander effects in either assay. GRID 2 (67 peaks of 1-mm width and 1.73-mm center-to-center spacing) showed evidence of bystander effects at 8 Gy in both assays and at 2 Gy in the growth inhibition assay only. GRID 1 (single 8-mm peak) showed evidence of bystander effects at 2 Gy with the modified CFA and 8 Gy for growth inhibition.
Conclusion
The GRID 2 collimator is the best geometry for eliciting bystander effects based on these assays. Dosimetry was successfully associated with the stained cell cultures to compare growth in the peak and valley regions to observe bystander effects. This methodology can be used to further compare the bystander effects of different SFRT geometries and to investigate the optimal spatial arrangement for a given biological outcome.