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Title: SU-E-T-557: Monte Carlo Modeling of Philips RT-250 Orthovoltage Unit for Beam Spectrum Modulation

Abstract

Purpose: To develop and validate a Monte Carlo (MC) model of a Phillips RT-250 orthovoltage unit to test various beam spectrum modulation strategies for in vitro/vivo studies. A model of this type would enable the production of unconventional beams from a typical orthovoltage unit for novel therapeutic applications such as gold nanoparticle-aided radiotherapy. Methods: The MCNP5 code system was used to create a MC model of the head of RT-250 and a 30 × 30 × 30 cm{sup 3} water phantom. For the x-ray machine head, the current model includes the vacuum region, beryllium window, collimators, inherent filters and exterior steel housing. For increased computational efficiency, the primary x-ray spectrum from the target was calculated from a well-validated analytical software package. Calculated percentage-depth-dose (PDD) values and photon spectra were validated against experimental data from film and Compton-scatter spectrum measurements. Results: The model was validated for three common settings of the machine namely, 250 kVp (0.25 mm Cu), 125 kVp (2 mm Al), and 75 kVp (2 mm Al). The MC results for the PDD curves were compared with film measurements and showed good agreement for all depths with a maximum difference of 4 % around dmax and under 2.5 %more » for all other depths. The primary photon spectra were also measured and compared with the MC results showing reasonable agreement between the two, validating the input spectra and the final spectra as predicted by the current MC model. Conclusion: The current MC model accurately predicted the dosimetric and spectral characteristics of each beam from the RT-250 orthovoltage unit, demonstrating its applicability and reliability for beam spectrum modulation tasks. It accomplished this without the need to model the bremsstrahlung xray production from the target, while significantly improved on computational efficiency by at least two orders of magnitude. Supported by DOD/PCRP grant W81XWH-12-1-0198.« less

Authors:
;  [1]
  1. UT MD Anderson Cancer Center, Houston, TX (United States)
Publication Date:
OSTI Identifier:
22496272
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 42; Journal Issue: 6; Other Information: (c) 2015 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
61 RADIATION PROTECTION AND DOSIMETRY; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; BEAMS; COMPUTER CODES; CURRENTS; DEPTH DOSE DISTRIBUTIONS; IN VITRO; IN VIVO; MODULATION; MONTE CARLO METHOD; PHANTOMS; SIMULATION; X RADIATION; X-RAY SPECTRA

Citation Formats

Reynoso, F, and Cho, S. SU-E-T-557: Monte Carlo Modeling of Philips RT-250 Orthovoltage Unit for Beam Spectrum Modulation. United States: N. p., 2015. Web. doi:10.1118/1.4924919.
Reynoso, F, & Cho, S. SU-E-T-557: Monte Carlo Modeling of Philips RT-250 Orthovoltage Unit for Beam Spectrum Modulation. United States. doi:10.1118/1.4924919.
Reynoso, F, and Cho, S. Mon . "SU-E-T-557: Monte Carlo Modeling of Philips RT-250 Orthovoltage Unit for Beam Spectrum Modulation". United States. doi:10.1118/1.4924919.
@article{osti_22496272,
title = {SU-E-T-557: Monte Carlo Modeling of Philips RT-250 Orthovoltage Unit for Beam Spectrum Modulation},
author = {Reynoso, F and Cho, S},
abstractNote = {Purpose: To develop and validate a Monte Carlo (MC) model of a Phillips RT-250 orthovoltage unit to test various beam spectrum modulation strategies for in vitro/vivo studies. A model of this type would enable the production of unconventional beams from a typical orthovoltage unit for novel therapeutic applications such as gold nanoparticle-aided radiotherapy. Methods: The MCNP5 code system was used to create a MC model of the head of RT-250 and a 30 × 30 × 30 cm{sup 3} water phantom. For the x-ray machine head, the current model includes the vacuum region, beryllium window, collimators, inherent filters and exterior steel housing. For increased computational efficiency, the primary x-ray spectrum from the target was calculated from a well-validated analytical software package. Calculated percentage-depth-dose (PDD) values and photon spectra were validated against experimental data from film and Compton-scatter spectrum measurements. Results: The model was validated for three common settings of the machine namely, 250 kVp (0.25 mm Cu), 125 kVp (2 mm Al), and 75 kVp (2 mm Al). The MC results for the PDD curves were compared with film measurements and showed good agreement for all depths with a maximum difference of 4 % around dmax and under 2.5 % for all other depths. The primary photon spectra were also measured and compared with the MC results showing reasonable agreement between the two, validating the input spectra and the final spectra as predicted by the current MC model. Conclusion: The current MC model accurately predicted the dosimetric and spectral characteristics of each beam from the RT-250 orthovoltage unit, demonstrating its applicability and reliability for beam spectrum modulation tasks. It accomplished this without the need to model the bremsstrahlung xray production from the target, while significantly improved on computational efficiency by at least two orders of magnitude. Supported by DOD/PCRP grant W81XWH-12-1-0198.},
doi = {10.1118/1.4924919},
journal = {Medical Physics},
number = 6,
volume = 42,
place = {United States},
year = {Mon Jun 15 00:00:00 EDT 2015},
month = {Mon Jun 15 00:00:00 EDT 2015}
}
  • Purpose: To reconstruct phase-space information upstream of patient specific collimators for Monte Carlo simulations using only radiotherapy planning system data. Methods: The proton energies are calculated based on residual ranges, e.g., sum of prescribed ranges in a patient and SSD. The Kapchinskij and Vladimirskij (KV) distribution was applied to sample proton’s x-y positions and momentum direction and the beam shape was assumed to be a circle. Free parameters, e.g., the initial energy spread and the emittance of KV distribution were estimated from the benchmarking with commissioning data in a commercial treatment planning system for an operational proton therapy center. Themore » number of histories, which defines the height of individual pristine Bragg peaks (BP) of Spread-out Bragg peak (SOBP), are weighted based on beam current modulation and a correction factor is applied to take into account the fluence reduction as the residual range decreases due to the rotation of the range modulator wheel. The timedependent behaviors, e.g., the changes of the residual range and histories per a pristine BP, are realized by utilizing TOPAS (Tool for Particle Simulation). Results: Benchmarking simulations for selected SOBPs ranging 7.5 cm to 15.5 cm matched within 2 mm in range and up to 5 mm in SOBP width against measurement data in water phantom. We found this model tends to underestimate entrance dose by about 5 % in comparison to measurement. This was attributed to the situation that the energy distribution used in the model was limited in its granularity at the limit of single energy spectrum for the narrow angle modulator steps used in the proximal pull back region of the SOBPs. Conclusion: Within these limitations the source modeling method proved itself an acceptable alternative of a full treatment head simulation when the machine geometry and materials information are not available.« less
  • Purpose: Mean organ doses from structures located in field and outside of field boundaries during radiotherapy treatment must be considered when looking at secondary effects. Treatment planning in patients with 40 years of follow-up does not include 3-D treatment planning images and did not estimate dose to structures out of the direct field. Therefore, it is of interest to correlate actual clinical events with doses received. Methods: Accurate models of radiotherapy machines combined with whole body computational phantoms using Monte Carlo methods allow for dose reconstructions intended for studies on late radiation effects. The Theratron-780 radiotherapy unit and anatomically realisticmore » hybrid computational phantoms are modeled in the Monte Carlo radiation transport code MCNPX. The major components of the machine including the source capsule, lead in the unit-head, collimators (fixed/adjustable), and trimmer bars are simulated. The MCNPX transport code is used to compare calculated values in a water phantom with published data from BJR suppl. 25 for in-field doses and experimental data from AAPM Task Group No. 36 for out-of-field doses. Next, the validated cobalt-60 teletherapy model is combined with the UF/NCI Family of Reference Hybrid Computational Phantoms as a methodology for estimating organ doses. Results: The model of Theratron-780 has shown to be agree with percentage depth dose data within approximately 1% and for out of field doses the machine is shown to agree within 8.8%. Organ doses are reported for reference hybrid phantoms. Conclusion: Combining the UF/NCI Family of Reference Hybrid Computational Phantoms along with a validated model of the Theratron-780 allows for organ dose estimates of both in-field and out-of-field organs. By changing field size, position, and adding patient-specific blocking more complicated treatment set-ups can be recreated for patients treated historically, particularly those who lack both 2D/3D image sets.« less
  • Purpose To create brass leaves for an orthovoltage MLC and take initial beam profile measurements. Methods The low-energy MLC was designed in previous work. Brass was chosen for its self-lubrication and low cost. Stock brass rectangles (30cm × 1.0cm × 0.5cm) were ordered with pre-cut gear rack along the topmost long edges. Leaf designs were translated into G-code, then cut with a Tormach CNC-1100 mill. Intense bowing was observed in the beam direction, which required straightening via an in-house jig. Straightened leaves were placed into MLC assembly and mounted to a 320 kVp orthovoltage tube. EDR2 film was irradiated inmore » four situations: MLC open so one edge was isocentric, and MLC open more than isocentric, completely closed MLC, and an open field shot with the MLC removed. The first two scans tested penumbra for our rectangular edges due to unfocused design. The final two scans tested transmission and interleaf leakage. All four experiments were set to 120 kVp and 10 mA for two minutes. Results Transmission and interleaf leakage were found to be zero. Interleaf leakage is faintly visible on film, but undetected by our film scanner despite high spatial resolution. Penumbra at isocenter was found to be 0.72mm, which matched the penumbras of true field edges. Penumbra off-isocenter was 1.1mm. Mechanically, leaves are moving smoothly once straightened. Conclusion Beam profiles through our brass MLC are acceptable. Leaves attenuate and move as designed. Looking forward, we intend to animate our MLC to deliver more complicated treatment plans.« less
  • Purpose: Lung-SBRT uses hypo-fractionated dose in small non-IMRT fields with tissue-heterogeneity corrected plans. An independent MU verification is mandatory for safe and effective delivery of the treatment plan. This report compares planned MU obtained from iPlan-XVM-Calgorithm against spreadsheet-based hand-calculation using most commonly used simple TMR-based method. Methods: Treatment plans of 15 patients who underwent for MC-based lung-SBRT to 50Gy in 5 fractions for PTV V100%=95% were studied. ITV was delineated on MIP images based on 4D-CT scans. PTVs(ITV+5mm margins) ranged from 10.1- 106.5cc(average=48.6cc). MC-SBRT plans were generated using a combination of non-coplanar conformal arcs/beams using iPlan XVM-Calgorithm (BrainLAB iPlan ver.4.1.2)more » for Novalis-TX consisting of micro-MLCs and 6MV-SRS (1000MU/min) beam. These plans were re-computed using heterogeneity-corrected Pencil-Beam (PB-hete) algorithm without changing any beam parameters, such as MLCs/MUs. Dose-ratio: PB-hete/MC gave beam-by-beam inhomogeneity-correction-factors (ICFs):Individual Correction. For independent-2nd-check, MC-MUs were verified using TMR-based hand-calculation and obtained an average ICF:Average Correction, whereas TMR-based hand-calculation systematically underestimated MC-MUs by ∼5%. Also, first 10 MC-plans were verified with an ion-chamber measurement using homogenous phantom. Results: For both beams/arcs, mean PB-hete dose was systematically overestimated by 5.5±2.6% and mean hand-calculated MU systematic underestimated by 5.5±2.5% compared to XVMC. With individual correction, mean hand-calculated MUs matched with XVMC by - 0.3±1.4%/0.4±1.4 for beams/arcs, respectively. After average 5% correction, hand-calculated MUs matched with XVMC by 0.5±2.5%/0.6±2.0% for beams/arcs, respectively. Smaller dependence on tumor volume(TV)/field size(FS) was also observed. Ion-chamber measurement was within ±3.0%. Conclusion: PB-hete overestimates dose to lung tumor relative to XVMC. XVMC-algorithm is much more-complex and accurate with tissues-heterogeneities. Measurement at machine is time consuming and need extra resources; also direct measurement of dose for heterogeneous treatment plans is not clinically practiced, yet. This simple correction-based method was very helpful for independent-2nd-check of MC-lung-SBRT plans and routinely used in our clinic. A look-up table can be generated to include TV/FS dependence in ICFs.« less
  • Purpose: The calibration of radiation protection instrumentation including ionization chambers, scintillators, and Geiger Mueller (GM) counters used as survey meters are often done using {sup 137}Cs irradiators. During calibration, irradiators use a combination of attenuators with various thicknesses to modulate the beam to a known air-kerma rate. The variations in energy spectra as a result of these attenuators are not accounted for and may play a role in the energy-dependent response of survey meters. This study uses an experimentally validated irradiator geometry modeled in the MCNP5 transport code to characterize the effects of attenuation on the energy spectrum. Methods: Amore » Hopewell Designs G-10 {sup 137}Cs irradiator with lead attenuators of thicknesses of 0.635, 1.22, 2.22, and 4.32 cm, was used in this study. The irradiator geometry was modeled in MCNP5 and validated by comparing measured and simulated percent depth dose (PDD) and cross-field profiles. Variations in MCNP5 simulated spectra with increasing amounts of attenuation were characterized using the relative intensity of the 662 keV peak and the average energy. Results: Simulated and measured PDDs and profiles agreed within the associated uncertainty. The relative intensity of the 662 keV peak for simulated spectra normalized to the intensity of the unattenuated spectra ranged from 0.16% to 100% based on attenuation thickness. The average energy for simulated spectra for attenuators ranged from 582 keV with no attenuation to 653 keV with 5.54 cm of attenuation. Statistical uncertainty for MCNP5 simulations ranged from 0.11% to 3.69%. Conclusion: This study successfully used MCNP5 to validate a {sup 137}Cs irradiator geometry and characterize variations in energy spectra between different amounts of attenuation. Variations in the average energy of up to 12% were determined through simulations, and future work will aim to determine the effects of these differences on survey meter response and calibration.« less