TH-CD-209-12: Spatial Mapping of Scanned Proton Biologic Effect Using the High-Throughput Technique, Continued.
PURPOSE
To investigate the biologic effects of scanned protons by evenly sampling dose-averaged LET (LETd) values.


METHODS
Our previous high-throughput clonogenic study demonstrated a distinct relationship between RBE and LETd. However, our initial experimental design resulted in over-sampling the low LETd values in the plateau region of the Bragg curve while under-sampling in the region proximal to the Bragg peak as well as the high LETd values in the distal edge of the Bragg curve. To further examine the relationship between RBE and LETd, we refined the experimental design to more evenly sample proton LETd values from 1 to 20 keV/µm by optimizing the thicknesses of the irradiation jig steps. We used the clonogenic survival as the biological endpoint for the H460 lung cancer cell line cultured in 96-well plates (12 columns by 8 rows). In the irradiation, the 8 wells in each column received a uniform dose-LETd pair. The dose-LETd pairs of the 12 different columns were sampled along the Bragg curve of 81.4 MeV scanned protons. Five peak dose levels from 1.5 Gy to 7.5 Gy were delivered with an increment of 1.5 Gy in the preliminary test. Two 96-well plates were irradiated simultaneously to decrease the statistical uncertainties.


RESULTS
In the proximal region, for LETd = 5 keV/µm and 8 keV/µm, we did not observe any distinct differential biologic effects between the survival curves. At the Bragg peak (LETd = 9.5 keV/µm) and in the distal edge, irradiation with increasing LET values resulted in decreasing cell survival.


CONCLUSION
The survival curves from the new experimental design support our previous findings that below 10 keV/µm, the LET effect in cell kill is obscured, but above 10 keV/µm, the biologic effects increase with LETd. Funding Support: U19 CA021239-35 and R21 CA187484-01.