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Title: Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams

Abstract

We have studied the response of factory calibrated implantable MOSFET detectors to absorbed doses from 100 to 400 cGy. The average measured dose is quite close to the true delivered dose, with the standard deviation falling between 1.4 and 3.6%. The measured dose tends to be slightly underestimated for smaller doses, while it tends to be slightly overestimated for larger doses. Thus, although the calibration of the detector is most accurate for doses close to the calibration dose of 200 cGy, it may be used over the range of commonly used doses in fractionated radiotherapy.

Authors:
; ;  [1]
  1. Department of Radiation Physics, Division of Radiation Oncology, University of Texas M.D. Anderson Cancer Center, Houston, Texas, 77030 (United States)
Publication Date:
OSTI Identifier:
20726896
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 32; Journal Issue: 11; Other Information: DOI: 10.1118/1.2065447; (c) 2005 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; CALIBRATION; CALIBRATION STANDARDS; DOSEMETERS; DOSIMETRY; EVALUATION; IN VIVO; MOSFET; PHOTON BEAMS; RADIATION DOSES; RADIOTHERAPY

Citation Formats

Briere, Tina Marie, Beddar, A. Sam, and Gillin, Michael T.. Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams. United States: N. p., 2005. Web. doi:10.1118/1.2065447.
Briere, Tina Marie, Beddar, A. Sam, & Gillin, Michael T.. Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams. United States. doi:10.1118/1.2065447.
Briere, Tina Marie, Beddar, A. Sam, and Gillin, Michael T.. Tue . "Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams". United States. doi:10.1118/1.2065447.
@article{osti_20726896,
title = {Evaluation of precalibrated implantable MOSFET radiation dosimeters for megavoltage photon beams},
author = {Briere, Tina Marie and Beddar, A. Sam and Gillin, Michael T.},
abstractNote = {We have studied the response of factory calibrated implantable MOSFET detectors to absorbed doses from 100 to 400 cGy. The average measured dose is quite close to the true delivered dose, with the standard deviation falling between 1.4 and 3.6%. The measured dose tends to be slightly underestimated for smaller doses, while it tends to be slightly overestimated for larger doses. Thus, although the calibration of the detector is most accurate for doses close to the calibration dose of 200 cGy, it may be used over the range of commonly used doses in fractionated radiotherapy.},
doi = {10.1118/1.2065447},
journal = {Medical Physics},
number = 11,
volume = 32,
place = {United States},
year = {Tue Nov 15 00:00:00 EST 2005},
month = {Tue Nov 15 00:00:00 EST 2005}
}
  • Purpose: To perform a comparison of the daily measured dose at depth in tissue with the predicted dose values from treatment plans for 29 prostate cancer patients involved in a clinical trial. Methods and Materials: Patients from three clinical sites were implanted with one or two dosimeters in or near the prostatic capsule. The implantable device, known as the DVS, is based on a metal-oxide-semiconductor field effect transistor (MOSFET) detector. A portable telemetric readout system couples to the dosimeter antenna (visible on kilovoltage, computed tomography, and ultrasonography) for data transfer. The predicted dose values were determined by the location ofmore » the MOSFET on the treatment planning computed tomography scan. Serial computed tomography images were taken every 2 weeks to evaluate any migration of the device. The clinical protocol did not permit alteration of the treatment parameters using the dosimeter readings. For some patients, one of several image-guided radiotherapy (RT) modalities was used for target localization. Results: The evaluation of dose discrepancy showed that in many patients the standard deviation exceeded the previous values obtained for the dosimeter in a phantom. In some patients, the cumulative dose disagreed with the planned dose by {>=}5%. The data presented suggest that an implantable dosimeter can help identify dose discrepancies (random or systematic) for patients treated with external beam RT and could be used as a daily treatment verification tool for image-guided RT and adaptive RT. Conclusion: The results of our study have shown that knowledge of the dose delivered per fraction can potentially prevent over- or under-dosage to the treatment area and increase the accuracy of RT. The implantable dosimeter could also be used as a localizer for image-guided RT.« less
  • This paper is concerned with the role of the package of MOSFETs used in measurements of gamma dose in mixed gamma-neutron fields, in high energy bremsstrahlung and soft X-ray fields. It is shown that a kovar cap should be avoided for dosimetry applications in the presence of strong thermal neutron fluences. In regions of strong electronic disequilibrium the bare or unencapsulated MOSFET is a unique tool for surface dose measurements and Monte Carlo model validation. For depths where electronic equilibrium exists (i.e. x > depth of D{sub max}) the MOSFET package is not critical. For low energy X-ray fields themore » energy dependence of the dose enhancement factor (DEF) will differ depending upon whether the irradiation is performed in free air geometry or on the surface of a phantom.« less
  • In many radiotherapy clinics an independent verification of the number of monitor units (MU) used to deliver the prescribed dose to the target volume is performed prior to the treatment start. Traditionally this has been done by using methods mainly based on empirical factors which, at least to some extent, try to separate the influence from input parameters such as field size, depth, distance, etc. The growing complexity of modern treatment techniques does however make this approach increasingly difficult, both in terms of practical application and in terms of the reliability of the results. In the present work the performancemore » of a model-based approach, describing the influence from different input parameters through actual modeling of the physical effects, has been investigated in detail. The investigated model is based on two components related to megavoltage photon beams; one describing the exiting energy fluence per delivered MU, and a second component describing the dose deposition through a pencil kernel algorithm solely based on a measured beam quality index. Together with the output calculations, the basis of a method aiming to predict the inherent calculation uncertainties in individual treatment setups has been developed. This has all emerged from the intention of creating a clinical dose/MU verification tool that requires an absolute minimum of commissioned input data. This evaluation was focused on irregular field shapes and performed through comparison with output factors measured at 5, 10, and 20 cm depth in ten multileaf collimated fields on four different linear accelerators with varying multileaf collimator designs. The measurements were performed both in air and in water and the results of the two components of the model were evaluated separately and combined. When compared with the corresponding measurements the resulting deviations in the calculated output factors were in most cases smaller than 1% and in all cases smaller than 1.7%. The distribution describing the calculation errors in the total dose output has a mean value of -0.04% and a standard deviation of 0.47%. In the dose calculations a previously developed correction of the pencil kernel was applied that managed to contract the error distribution considerably. A detailed analysis of the predicted uncertainties versus the observed deviations suggests that the predictions indeed can be used as a basis for creating action levels and tracking dose calculation errors in homogeneous media.« less
  • Purpose: The Exradin A26 is a new design of micro-ionization ion chamber that externally resembles the Exradin A16 model but has significant internal changes to address measurement issues reported in the literature for the A16. This project involved the characterization of two versions of the A26 chamber in high energy x-rays with particular reference to the performance specification laid out in the imminent Addendum to TG-51. Methods: The Exradin A26 was investigated in a range of megavoltage photon beams (6–25 MV). Investigations looked at chamber settling, ion recombination and polarity. Since it has been previously shown that non-ideal performance ismore » most easily identified through ion recombination measurements, the focus was on the determination of Pion. Results: i) Chamber settling - the chamber response stabilizes very quickly (within 3 minutes), even after a large change in the polarizing voltage.ii) The polarity correction was found to be small (within 0.2% of unity)iii) The chamber showed linear behavior for a Jaffe plot (1/reading vs 1/polarizing voltage) for applied voltages ≤ 200 V.iv) The recombination correction showed a linear variation with the doseper- pulse, was not significantly dependent on the polarity of the collecting voltage and was consistent with the chamber dimensions (i.e. agreed with Boag theory). Conclusion: An initial investigation of the Exradin A26 micro chamber suggests that although its performance exceeds the AAPM specification for a reference-class ion chamber for use in megavoltage photon beams it is a significant improvement over the previous A16 design. Further work is required to evaluate long-term stability and determine kQ factors.« less
  • The authors have studied the performance of a new version of the implantable MOSFET detector designed for clinical use. Detectors were irradiated under 6- and 18-MV beams in water at body temperature to the calibration dose of 200 cGy/fraction for 20 daily fractions to determine their response and reproducibility. Additional measurements were performed in a solid phantom under 6-MV irradiation at room temperature to daily doses of 100, 150, 200, 250, 400 and 600 cGy/fraction. Finally, the angular dependence with respect to rotation about the detector's longitudinal axis was studied. At body temperature, the detectors were found to have anmore » average response within {+-}3% of the calibration dose with a standard deviation of 2% or less. At room temperature, doses lower than the calibration dose led to a slight overresponse while doses higher than the calibration dose led to a slight underresponse. Angular dependence was not significant, on average within 1.2% of the mean. When used as specified by the manufacturer, these detectors should provide data useful to verify the delivered dose for external beam radiation therapy within a certain tolerance.« less