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Title: Assessing the effect of electron density in photon dose calculations

Abstract

Photon dose calculation algorithms (such as the pencil beam and collapsed cone, CC) model the attenuation of a primary photon beam in media other than water, by using pathlength scaling based on the relative mass density of the media to water. In this study, we assess if differences in the electron density between the water and media, with different atomic composition, can influence the accuracy of conventional photon dose calculations algorithms. A comparison is performed between an electron-density scaling method and the standard mass-density scaling method for (i) tissues present in the human body (such as bone, muscle, etc.), and for (ii) water-equivalent plastics, used in radiotherapy dosimetry and quality assurance. We demonstrate that the important material property that should be taken into account by photon dose algorithms is the electron density, and not the mass density. The mass-density scaling method is shown to overestimate, relative to electron-density predictions, the primary photon fluence for tissues in the human body and water-equivalent plastics, where 6%-7% and 10% differences were observed respectively for bone and air. However, in the case of patients, differences are expected to be smaller due to the large complexity of a treatment plan and of the patient anatomymore » and atomic composition and of the smaller thickness of bone/air that incident photon beams of a treatment plan may have to traverse. Differences have also been observed for conventional dose algorithms, such as CC, where an overestimate of the lung dose occurs, when irradiating lung tumors. The incorrect lung dose can be attributed to the incorrect modeling of the photon beam attenuation through the rib cage (thickness of 2-3 cm in bone upstream of the lung tumor) and through the lung and the oversimplified modeling of electron transport in convolution algorithms. In the present study, the overestimation of the primary photon fluence, using the mass-density scaling method, was shown to be a consequence of the differences in the hydrogen content between the various media studied and water. On the other hand, the electron-density scaling method was shown to predict primary photon fluence in media other than water to within 1%-2% for all the materials studied and for energies up to 5 MeV. For energies above 5 MeV, the accuracy of the electron-density scaling method was shown to depend on the photon energy, where for materials with a high content of calcium (such as bone, cortical bone) or for primary photon energies above 10 MeV, the pair-production process could no longer be neglected. The electron-density scaling method was extended to account for pair-production attenuation of the primary photons. Therefore the scaling of the dose distributions in media other than water became dependent on the photon energy. The extended electron-scaling method was shown to estimate the photon range to within 1% for all materials studied and for energies from 100 keV to 20 MeV, allowing it to be used to scale dose distributions to media other than water and generated by clinical radiotherapy photon beams with accelerator energies from 4 to 20 MV.« less

Authors:
;  [1];  [2]
  1. Francis H. Burr Proton Therapy Center, Massachusetts Hospital, Harvard Medical School, 30 Fruit Street, Boston, Massachusetts 02114 (United States)
  2. (United Kingdom)
Publication Date:
OSTI Identifier:
20775078
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 33; Journal Issue: 2; Other Information: DOI: 10.1118/1.2161407; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; ALGORITHMS; CARCINOMAS; ELECTRON DENSITY; LUNGS; PHOTON BEAMS; PHOTONS; RADIATION DOSES; RADIOTHERAPY; SKELETON

Citation Formats

Seco, J., Evans, P. M., and Joint Department of Physics, Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT. Assessing the effect of electron density in photon dose calculations. United States: N. p., 2006. Web. doi:10.1118/1.2161407.
Seco, J., Evans, P. M., & Joint Department of Physics, Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT. Assessing the effect of electron density in photon dose calculations. United States. doi:10.1118/1.2161407.
Seco, J., Evans, P. M., and Joint Department of Physics, Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT. Wed . "Assessing the effect of electron density in photon dose calculations". United States. doi:10.1118/1.2161407.
@article{osti_20775078,
title = {Assessing the effect of electron density in photon dose calculations},
author = {Seco, J. and Evans, P. M. and Joint Department of Physics, Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT},
abstractNote = {Photon dose calculation algorithms (such as the pencil beam and collapsed cone, CC) model the attenuation of a primary photon beam in media other than water, by using pathlength scaling based on the relative mass density of the media to water. In this study, we assess if differences in the electron density between the water and media, with different atomic composition, can influence the accuracy of conventional photon dose calculations algorithms. A comparison is performed between an electron-density scaling method and the standard mass-density scaling method for (i) tissues present in the human body (such as bone, muscle, etc.), and for (ii) water-equivalent plastics, used in radiotherapy dosimetry and quality assurance. We demonstrate that the important material property that should be taken into account by photon dose algorithms is the electron density, and not the mass density. The mass-density scaling method is shown to overestimate, relative to electron-density predictions, the primary photon fluence for tissues in the human body and water-equivalent plastics, where 6%-7% and 10% differences were observed respectively for bone and air. However, in the case of patients, differences are expected to be smaller due to the large complexity of a treatment plan and of the patient anatomy and atomic composition and of the smaller thickness of bone/air that incident photon beams of a treatment plan may have to traverse. Differences have also been observed for conventional dose algorithms, such as CC, where an overestimate of the lung dose occurs, when irradiating lung tumors. The incorrect lung dose can be attributed to the incorrect modeling of the photon beam attenuation through the rib cage (thickness of 2-3 cm in bone upstream of the lung tumor) and through the lung and the oversimplified modeling of electron transport in convolution algorithms. In the present study, the overestimation of the primary photon fluence, using the mass-density scaling method, was shown to be a consequence of the differences in the hydrogen content between the various media studied and water. On the other hand, the electron-density scaling method was shown to predict primary photon fluence in media other than water to within 1%-2% for all the materials studied and for energies up to 5 MeV. For energies above 5 MeV, the accuracy of the electron-density scaling method was shown to depend on the photon energy, where for materials with a high content of calcium (such as bone, cortical bone) or for primary photon energies above 10 MeV, the pair-production process could no longer be neglected. The electron-density scaling method was extended to account for pair-production attenuation of the primary photons. Therefore the scaling of the dose distributions in media other than water became dependent on the photon energy. The extended electron-scaling method was shown to estimate the photon range to within 1% for all materials studied and for energies from 100 keV to 20 MeV, allowing it to be used to scale dose distributions to media other than water and generated by clinical radiotherapy photon beams with accelerator energies from 4 to 20 MV.},
doi = {10.1118/1.2161407},
journal = {Medical Physics},
number = 2,
volume = 33,
place = {United States},
year = {Wed Feb 15 00:00:00 EST 2006},
month = {Wed Feb 15 00:00:00 EST 2006}
}
  • The authors questioned whether more sophisticated density correction algorithms than those presently used for photon-beam dose calculations are necessary to take full advantage of CT in treatment planning. Predictions obtained with correction equations based on the radiological thickness (effective SSD) and generalized power-law TAR methods, as well as on the more complex equivalent-TAR method, we compared with ionization-chamber measurements in phantoms containing stimulated lung and soft tissues. Various geometric configurations were investigated for /sup 60/Co and 8-MV radiations. While the equivalent-TAR method gave the best general agreement, the power-law method appears to be accurate enough for regular treatment-planning purposes. Themore » effective SSD method has a limited range of validity. At present, complicated methods of density correction do not seem to be justified.« less
  • The authors questioned whether more sophisticated density correction algorithms than those presently used for photon-beam dose calculations are necessary to take full advantage of CT in treatment planning. Predictions obtained with correction equations based on the radiological thickness (effective SSD) and generalized power-law TAR methods, as well as on the more complex equivalent-TAR method, are compared with ionization-chamber measurements in phantoms containing simulated lung and soft tissues. Various geometric configurations were investigated for /sup 60/Co and 8-MV radiations. While the equivalent-TAR method gave the best general agreement, the power-law method appears to be accurate enough for regular treatment-planning purposes. Themore » effective SSD method has a limited range of validity. At present, complicated methods of density correction do not seem to be justified.« less
  • The procedures of radiation therapy consist of a number of steps, one of which is the calculation of the radiation dosage pattern that will result when a particular arrangement of radiation beams or sources is applied to a patient. The purpose of this calculation is two-fold. One is to predict, as part of the dose planning process, what dose distribution can be achieved with a selected beam arrangement. The second is to record what treatment has been given so that post-treatment analysis can be carried out. Both of these are important.
  • Purpose: To develop and evaluate a correction method for lateral electron disequilibrium and tissue inhomogeneities in lung tissues applicable to the BrainSCAN treatment planning system. Methods and Materials: Four noncoplanar 6-MV photon beams with different beam diameters were applied to the right lung of a thorax phantom. The measured/calculated dose value ratio was evaluated as a function of a parameter that describes the degree of the lateral electron disequilibrium based on the primary dose. Results: The dose ratio showed a clearcut linear dependency on the disequilibrium parameter. Applying the proposed correction method, only minor differences between the measured and calculatedmore » doses were found for lesions >1 cm. However, for lesions <1 cm surrounded by lung tissue the difference was {<=}15%. Conclusion: The data have indicated a relevant magnitude of the correction factor only for lung lesions <1 cm.« less
  • Purpose: Validation of iBEAM™ evo couch-top for different relative electron density (RED) combination during photon beam dose calculation in Monaco− TPS. Methods: The iBEAM™ evo couch-top has two layers:outer carbon fiber (CF) and inner foam core (FC). To study the beam intensity attenuation of couch-top, measured doses were compared with doses calculated for different REDs. Measurements were performed in solid water phantom with PTW-0.125cc ion-chamber positioned at center of the phantom with 5.3cm thickness slabs placed above and below the chamber. Similarly, in TPS, iBEAM™ evo couch-top was simulated and doses were calculated for different RED combinations (0.2CF-0.2FC, 0.4CF-0.2FC, 0.6CF-0.2FC,more » 0.8CF-0.2FC, and 1.0CF-0.2FC) by using Monte Carlo dose calculation algorithm in Monaco TPS (V5.1). Doses were measured for every 10 degree gantry angle separation, 10×10cm{sup 2} field size and 6MV photons. Then, attenuation is defined as the ratio of output at posterior gantry angle to output of its opposed anterior gantry angle (e.g.225°/45°). output fluctuation with different gantry angle was within ±0.21%. To confirm above results, dose-planes were measured for five pelvic VMAT plans (360°arc) in PTW two-dimensional array and compared with different calculated dose-planes of above-mentioned couch REDs. Gamma pass rates<1.00) were analyzed for 3%/2mm criteria. Results: Measured and calculated attenuation was in good agreement for the RED combination of 0.2CF-0.2FC and difference was within ±0.515%. However, other density combination showed difference of ±0.9841%, ±1.667%, ±2.9241% and ±2.8832% for 0.4CF-0.2FC, 0.6CF-0.2FC, 0.8CF-0.2FC, and 1.0CF-0.2FC, respectively. Maximum couch-top attenuation was observed at 110°–120° and 240°–250° and decreases linearly as the gantry angle approaches 180°. Moreover, gamma pass rate confirmed the above results and showed maximum pass rate of 96.23% for 0.2CF-0.2FC, whereas others were 95.72%, 95.12%, 94.31% and 93.24%. Conclusion: RED value of 0.2CF-0.2FC was found to be suitable for accurate couch-top modeling for 6MV photon beam Monte Carlo calculations in Monaco TPS.« less