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Title: SU-F-T-554: Dark Current Effect On CyberKnife Beam Dosimetry

Abstract

Purpose: All RF linear accelerators produce dark current to varying degrees when an accelerating voltage and RF input is applied in the absence of electron gun injection. This study is to evaluate how dark current from the linear accelerator of CyberKnife affect the dose in the reference dosimetry. Methods: The G4 CyberKnife system with 6MV photon beam was used in this study. Using the ion chamber and the diode detector, the dose was measured in water with varying time delay between acquiring charges and staring beam-on after applying high-voltage into the linear accelerator. The dose was measured after the time delay with over the range of 0 to 120 seconds in the accelerating high-voltage mode without beam-on, applying 0, 10, 50, 100, and 200 MUs. For the measurements, the collimator of 60 mm was used and the detectors were placed at the depths of 10 cm with the source-to-surface distance of 80 cm. Results: The dark current was constant over time regardless of MU. The dose due to the dark current increased over time linearly with the R-squared value of 0.9983 up to 4.4 cGy for the time 120 seconds. In the dose rate setting of 720 MU/min, the relativemore » dose when applying the accelerating voltage without beam-on was increased over time up to 0.6% but it was less than the leakage radiation resulted from the accelerated head. As the reference dosimetry condition, when 100 MU was delivered after 10 seconds time delay, the relative dose increased by 0.7% but 6.7% for the low MU (10 MU). Conclusion: In the dosimetry using CyberKnife system, the constant dark current affected to the dose. Although the time delay in the accelerating high-voltage mode without beam-on is within 10 seconds, the dose less than 100 cGy can be overestimated more than 1%.« less

Authors:
;  [1]
  1. Soonchunhyang University Hospital, Seoul (Korea, Republic of)
Publication Date:
OSTI Identifier:
22649129
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; DOSE RATES; DOSIMETRY; ELECTRIC POTENTIAL; IONIZATION CHAMBERS; LINEAR ACCELERATORS; PHOTON BEAMS; TIME DELAY

Citation Formats

Kim, H, and Chang, A. SU-F-T-554: Dark Current Effect On CyberKnife Beam Dosimetry. United States: N. p., 2016. Web. doi:10.1118/1.4956739.
Kim, H, & Chang, A. SU-F-T-554: Dark Current Effect On CyberKnife Beam Dosimetry. United States. doi:10.1118/1.4956739.
Kim, H, and Chang, A. 2016. "SU-F-T-554: Dark Current Effect On CyberKnife Beam Dosimetry". United States. doi:10.1118/1.4956739.
@article{osti_22649129,
title = {SU-F-T-554: Dark Current Effect On CyberKnife Beam Dosimetry},
author = {Kim, H and Chang, A},
abstractNote = {Purpose: All RF linear accelerators produce dark current to varying degrees when an accelerating voltage and RF input is applied in the absence of electron gun injection. This study is to evaluate how dark current from the linear accelerator of CyberKnife affect the dose in the reference dosimetry. Methods: The G4 CyberKnife system with 6MV photon beam was used in this study. Using the ion chamber and the diode detector, the dose was measured in water with varying time delay between acquiring charges and staring beam-on after applying high-voltage into the linear accelerator. The dose was measured after the time delay with over the range of 0 to 120 seconds in the accelerating high-voltage mode without beam-on, applying 0, 10, 50, 100, and 200 MUs. For the measurements, the collimator of 60 mm was used and the detectors were placed at the depths of 10 cm with the source-to-surface distance of 80 cm. Results: The dark current was constant over time regardless of MU. The dose due to the dark current increased over time linearly with the R-squared value of 0.9983 up to 4.4 cGy for the time 120 seconds. In the dose rate setting of 720 MU/min, the relative dose when applying the accelerating voltage without beam-on was increased over time up to 0.6% but it was less than the leakage radiation resulted from the accelerated head. As the reference dosimetry condition, when 100 MU was delivered after 10 seconds time delay, the relative dose increased by 0.7% but 6.7% for the low MU (10 MU). Conclusion: In the dosimetry using CyberKnife system, the constant dark current affected to the dose. Although the time delay in the accelerating high-voltage mode without beam-on is within 10 seconds, the dose less than 100 cGy can be overestimated more than 1%.},
doi = {10.1118/1.4956739},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • This article is intended to improve the certainty of the absorbed dose determination for reference dosimetry in CyberKnife beams. The CyberKnife beams do not satisfy some conditions of the standard reference dosimetry protocols because of its unique treatment head structure and beam collimating system. Under the present state of affairs, the reference dosimetry has not been performed under uniform conditions and the beam quality correction factor k{sub Q} for an ordinary 6 MV linear accelerator has been temporally substituted for the k{sub Q} of the CyberKnife in many sites. Therefore, the reference conditions and k{sub Q} as a function ofmore » the beam quality index in a new way are required. The dose flatness and the error of dosimeter reading caused by radiation fields and detector size were analyzed to determine the reference conditions. Owing to the absence of beam flattening filter, the dose flatness of the CyberKnife beam was inferior to that of an ordinary 6 MV linear accelerator. And if the absorbed dose is measured with an ionization chamber which has cavity length of 2.4, 1.0 and 0.7 cm in reference dosimetry, the dose at the beam axis for a field of 6.0 cm collimator was underestimated 1.5%, 0.4%, and 0.2% on a calculation. Therefore, the maximum field shaped with a 6.0 cm collimator and ionization chamber which has a cavity length of 1.0 cm or shorter were recommended as the conditions of reference dosimetry. Furthermore, to determine the k{sub Q} for the CyberKnife, the realistic energy spectrum of photons and electrons in water was simulated with the BEAMnrc. The absence of beam flattening filter also caused softer photon energy spectrum than that of an ordinary 6 MV linear accelerator. Consequently, the k{sub Q} for ionization chambers of a suitable size were determined and tabulated as a function of measurable beam quality indexes in the CyberKnife beam.« less
  • External beam therapy (EBT) GAFCHROMIC registered film is evaluated for dosimetry and characterization of the CyberKnife registered radiation beams. Percentage depth doses, lateral beam profiles, and output factors are measured in solid water using EBT GAFCHROMIC registered film (International Specialty Products, Wayne, NJ) for the 6 MV radiation beams of diameter 5 to 60 mm produced by the CyberKnife registered (Accuray, Sunnyvale, CA). The data are compared to those measured with the PTW 60008 diode and the Wellhofer CC01 ion chamber in water. For the small radiation field sizes used in stereotactic radiosurgery, lateral electronic disequilibrium and steep dose gradientsmore » exist in a large portion of these fields, requiring the use of high-resolution measurement techniques. For small beams, the detector size approaches the dimensions of the beam and adversely affects measurement accuracy in regions where the gradient varies across the detector. When film is the detector, the scanning system is usually the resolution-limiting component. Radiographic films based upon silver halide (AgH) emulsions are widely used for relative dosimetry of external radiation treatment beams in the megavoltage energy range, because of their good spatial resolution and capability to provide integrated dosimetry over two dimensions. Film dosimetry, however, has drawbacks due to its steep energy dependence at low photon energies as well as film processor and densitometer artifacts. EBT radiochromic film, introduced in 2004 specifically for IMRT dosimetry, may be a detector of choice for the characterization of small radiosurgical beams, because of its near-tissue equivalence, radiation beam energy independence, high spatial resolution, and self developing properties. For radiation beam sizes greater than 10 mm, the film measurements were identical to those of the diode and ion chamber. For the smaller beam diameters of 7.5 and 5 mm, however, there were differences in the data measured with the different detectors, which are attributed to their different spatial resolution and non-water-equivalence.« less
  • Background: We tested our ability to approximate the dose (38 Gy), fractionation (four fractions), and distribution of high-dose-rate (HDR) brachytherapy for prostate cancer with CyberKnife (CK) stereotactic body radiotherapy (SBRT) plans. We also report early clinical observations of CK SBRT treatment. Methods and Materials: Ten patients were treated with CK. For each CK SBRT plan, an HDR plan was designed using common contour sets and simulated HDR catheters. Planning target volume coverage, intraprostatic dose escalation, and urethra, rectum, and bladder exposure were compared. Results: Planning target volume coverage by the prescription dose was similar for CK SBRT and HDR plans,more » whereas percent of volume of interest receiving 125% of prescribed radiation dose (V125) and V150 values were higher for HDR, reflecting higher doses near HDR source dwell positions. Urethra dose comparisons were lower for CK SBRT in 9 of 10 cases, suggesting that CK SBRT may more effectively limit urethra dose. Bladder maximum point doses were higher with HDR, but bladder dose falloff beyond the maximum dose region was more rapid with HDR. Maximum rectal wall doses were similar, but CK SBRT created sharper rectal dose falloff beyond the maximum dose region. Second CK SBRT plans, constructed by equating urethra radiation dose received by point of maximum exposure of volume of interest to the HDR plan, significantly increased V125 and V150. Clinically, 4-month post-CK SBRT median prostate-specific antigen levels decreased 86% from baseline. Acute toxicity was primarily urologic and returned to baseline by 2 months. Acute rectal morbidity was minimal and transient. Conclusions: It is possible to construct CK SBRT plans that closely recapitulate HDR dosimetry and deliver the plans noninvasively.« less
  • Purpose: In highly-conformal radiotherapy, due to the complexity of both beam configurations and dose distributions, traditional in vivo dosimetry is unpractical or even impossible. The ideal dosimeter would be implanted inside the planning treatment volume so that it can directly measure the total delivered dose during each fraction with no additional uncertainty due to calculation models. The aim of this work is to verify if implantable metal oxide semiconductors field effect transistors (MOSFETs) can achieve a sufficient degree of dosimetric accuracy when used inside extracranial targets undergoing radiotherapy treatments using the Cyberknife system. Methods: Based on the preliminary findings ofmore » this study, new prototypes for high dose fractionations were developed to reduce the time dependence for long treatment delivery times. These dosimeters were recently cleared and are marketed as DVS-HFT. Multiple measurements were performed using both Virtual Water and water phantoms to characterize implantable MOSFETs under the Cyberknife beams, and included the reference-dosimetry consistency, the dependence of the response on the collimator size, on the daily delivered dose, and the time irradiation modality. Finally a Cyberknife prostate treatment simulation using a body phantom was conducted, and both MOSFET and ionization readings were compared to Monte Carlo calculations. The feasibility analysis was conducted based on the ratios of the absorbed dose divided by the dose reading, named as ''further calibration factor'' (FCF). Results: The average FCFs resulted to be 0.98 for the collimator dependence test, and about 1.00 for the reference-dosimetry test, the dose-dependence test, and the time-dependence test. The average FCF of the prostate treatment simulation test was 0.99. Conclusions: The obtained results are well within DVS specifications, that is, the factory calibration is still valid for such kind of treatments using the Cyberknife system, with no need of further calibration factors to be applied. The final accuracy of implantable MOSFETs when used for such kind of treatments was estimated to be within {+-}4%. Additional investigations using dose/fraction higher than 12 Gy, different beam configurations, and tracking systems could extend the present findings to other kind of treatments. MOSFET technology was proven to have high versatility in fast adaptation of existing detectors to new applications. It is plausible to expect a general feasibility of implantable MOSFET technology for in vivo dosimetry of the extracranial-targets treatments using the Cyberknife, provided each particular application will be validated by suitable both physical and clinical studies.« less