TH‐C‐230A‐04: Characterizing a Monochromatic X‐Ray Beam From a 1.3 GeV Synchrotron for Auger Electron Radiotherapy and Dosimetry Studies

Abstract

Purpose: Auger electron radiotherapy and dosimetry methods are being studied in preparation for future small animal irradiations using monochromatic x‐ray beams and IUdR. Aims of the present study are: (1) establish methods for characterizing the LSU CAMD synchrotron monochromatic x‐ray beam and (2) validate MCNP dose calculations in polymethylmethacrylate (PMMA). Method and Materials: The synchrotron's tunable (6–40 keV), monochromatic beam was set to 15 keV and collimated to ≈2.5‐mm wide. Beam energy was determined from photons Compton scattered by a 56 mg⋅cm−2 aluminum target, whose energy was measured using a thin window 1‐mm thick × 2.54‐cm diameter NaI(Tl) scintillation detector. Beam cross section and divergence were measured using radiochromic film digitized with an Epson 1680 scanner. Depth‐dose measurements within a PMMA phantom were made using a 0.23 cm3 air‐ionization chamber. Ionization was converted to dose in air and PMMA at each depth and a percent depth‐dose curve generated. Results of MCNP5 Monte Carlo dose calculations simulating measured conditions were compared with measured data. Results: Measurements indicated the nominal 15 keV beam had energy of 15.5 keV, horizontal width of 3.1 cm, Gaussian distribution vertically with FWHM = 0.1 cm, and beam divergence <0.004 horizontally and vertically. A dose rate of 69 cGy⋅s−1, measured at 0.58‐cm depth in PMMA with 92 mA beam current, corresponds to 3.4×1011 photons⋅cm−2⋅s−1. Measured percent depth‐dose curve agreed with MCNP5 simulated curve, yielding a PMMA mass attenuation coefficient of 1.1 cm2 g−1, approximately equal to the NIST value. Conclusion: The LSU CAMD 15‐keV monochromatic beam has been characterized demonstrating utility of the measurement methods for future studies at energies suitable for iodine k‐edge capture (>33.2 keV). MCNP5 Monte Carlo calculations have been shown to predict depth dose in PMMA validating its use and showing its potential for future treatment planning dose calculations in small animals.