Zhonglu Wang

Calculation of effective dose from measurements of secondary neutron spectra and scattered photon dose from dynamic MLC IMRT for 6 MV, 15 MV, and 18 MV beam energies

Effective doses were calculated from the delivery of 6 MV, 15 MV, and 18 MV conventional and intensity-modulated radiation therapy (IMRT) prostate treatment plans. ICRP-60 tissue weighting factors were used for the calculations. Photon doses were measured in phantom for all beam energies. Neutron spectra were measured for 15 MV and 18 MV and ICRP-74 quality conversion factors used to calculate ambient dose equivalents. The ambient dose equivalents were corrected for each tissue using neutron depth dose data from the literature. The depth corrected neutron doses were then used as a measure of the neutron component of the ICRP protection quantity, organ equivalent dose. IMRT resulted in an increased photon dose to many organs. However, the IMRT treatments resulted in an overall decrease in effective dose compared to conventional radiotherapy. This decrease correlates to the ability of an intensity-modulated field to minimize dose to critical normal structures in close proximity to the treatment volume. In a comparison of the three beam energies used for the IMRT treatments, 6 MV resulted in the lowest effective dose, while 18 MV resulted in the highest effective dose. This is attributed to the large neutron contribution for 18 MV compared to no neutron contribution for 6 MV.

Characterization of a Gold-and-Indium Dual-Activation-Foil-Based Bonner Sphere System

Abstract A gold-and-indium dual-activation-foil-based Bonner sphere system (BSS) for neutron spectral measurements was investigated. A customized polyethylene holder was designed to simultaneously accommodate one indium foil and one gold foil placed orthogonally to each other. The holder, which fits into the center of a Bonner sphere, resembles a standard 6LiI(Eu) detector. The response of the dual-activation-foil-based system was calculated using the MCNPX code with different activation cross-section libraries (ACTL and ENDF/VI for gold foils and ACTL and 532DOS2 for indium foils). To verify and correct the calculated response functions, measurements were performed using 252Cf and AmBe neutron sources of known strengths. Working response functions were determined for neutrons with energies ranging from thermal to 20 MeV by applying correction factors from measurements in well-characterized beams to the Monte Carlo–calculated response functions. Measured data were successfully unfolded with the corrected response functions and found to be in good agreement with the known spectra.

MO-D-T-617-03: Secondary Neutron Spectra From High Energy IMRT and Conventional Treatment Plans Using Bonner Spheres and Au-197 Activation Foils

Purpose: The purpose of this work is to measure and compare secondary neutron spectra both in‐field and out‐of‐field from the delivery of high energyIMRT and conventional radiation therapy. Direct measurement of neutron spectra in phantom for IMRT has received little attention. Studies are needed to demonstrate the strength of the assumption that the spectra are the same for IMRT and conventional radiation therapy.Method and Materials: 197 Au activation foils are placed on the surface of a special holder which is inserted into different Bonner Spheres. Seven levels of moderation were used: the bare foil holder and the holder inside 2″, 3″, 5″, 8″, 10″, and 12″ spheres. Net counts of the 411keV gamma peak were measured for each foil using a high purity Germanium detector with spectral analysis capabilities. The activity at the end of irradiation, and the production rate per unit mass of target (Bqs−1g−1) were calculated. Data were unfolded with MXD_FC33 algorithm, PTB (Germany) with a response matrix specifically calculated for this measurement system using MCNPX. The response matrix was verified by unfolding 197 Au ‐Foil Bonner Sphere system data from irradiations with 252 Cf . Results: The unfolded neutron spectra and neutron flux are evaluated. Ambient Dose Equivalent, H*(10) is calculated from spectral data. The data show an increase in the secondary neutron flux and H*(10) for IMRT compared to conventional radiotherapy. The IMRT spectra show a small shift to lower energies. Ratios of in‐field to out‐of‐field flux for 18MV fall off approximately with the inverse square. The ratios of 18MV to 15MV IMRT and conventional out of field flux are 2.13 and 2.6, respectively. Conclusion: 197 Au activation foils inside Bonner spheres technique for the measurement of neutron spectra inside the treatment room is an effective tool for the determination of neutron spectra from IMRT and conventional radiation delivery.

FERMILAB NEUTRON THERAPY FACILITY / NEUTRON SPECTRUM DETERMINATION BY THRESHOLD FOILS

The Fermilab Neutron Therapy Facility produces neutrons by bombarding a 2.21-cm-thick beryllium target with 66-MeV protons. The neutron beam is collimated to produce a desired field size and is monitored using dual parallel-plate ionization chambers. The neutron spectrum of this beam was determined by unfolding bare and moderated foil activation data and an MCNPX calculation of the spectrum was also performed. Aluminium and copper activation foils were selected for the neutron fluence measurement. The activation products in the foils were measured for various levels of moderation using polymethylmethachrylate (PMMA) slabs. Five reactions were considered in the experiment: 27 Al(n,γ) 28 Al, 27 Al(n,p) 27 Mg, 27 Al (n,α) 24 Na, 65 Cu(n,γ) 66 Cu and 63 Cu(n,2n) 62 Cu. The activities of these end products after the irradiation period were used to compute their production rates during irradiation. Responses as a function of neutron energy were generated for the production of 28 Al, 27 Mg, 24 Na, 66 Cu and 62 Cu using MCNPX and the LA150N library. The resulting response matrix was binned into 49 energy groups from 10 -10 MeV to 70 MeV and includes responses both for bare foils and moderated foils. The PTB code MXD_FC33 code was used to unfold the activation data. The resulting spectrum is compared to both the MCNPX-computed spectrum as well as a previous measurement by Cupps and Elwyn