Room: Stars at Night Ballroom 1
Purpose: The relative biological effectiveness (RBE) of protons is highly variable and difficult to quantify. However, RBE is related the local ionization density, which can be related to the physical measurable dose weighted linear energy transfer (LET(D)). This is a first report on the methodology development and results for validation of the first commercially available LET(D) computation in a Monte Carlo dose engine for proton pencil beam treatment planning.
Methods: Raystation (v6R) was used to generate treatment plans in the CIRS-731-HN anthropomorphic phantom for three sites (brain, nasopharynx, neck) with a spherical target (Ã˜=5 cm) with uniform target dose to calculate the LET(D) distribution. Measurements were performed at our proton center (Proteus Plus, IBA) using a u(+)-probe utilizing silicon on insulator microdosimeters capable of detecting lineal energies as low as 0.15 keV/um. Dose averaged mean lineal energy (y(D)) depth-profiles were measured for 70 and 130 MeV spots in water and for the three treatment plans in water and anthropomorphic phantom. Measurements were compared to TPS LET(D) Monte Carlo calculations. D*y(D) was compared to D*LET(D) in terms of a gamma-index with distance-to-agreement criteria according to the positional uncertainty (1 mm for water phantom, 2 mm for anthropomorphic phantom) and increasing dose difference criteria to find the criteria for which a 90% pass rate was accomplished.
Results: Measurements of D*y(D) were in good agreement with calculated D*LET(D) with a 93% gamma pass rate with 1 mm/10% criteria for single spots, a 94% gamma pass rate with 1 mm/10% criteria for plans measured in water and a 92% gamma pass rate with 2 mm/10% criteria for plans measured in the anthropomorphic phantom.
Conclusion: The TPS calculated D*LET(D) distributions are accurate within 10% relative to the experimental value. This work marks an important step towards the use of LET(D) calculations in daily clinical practice.
Funding Support, Disclosures, and Conflict of Interest: Erik Traneus is employed by Raysearch Laboratories.