Use of a liquid ionization chamber for stereotactic radiotherapy dosimetry

Wagner, A.; Crop, F.; Lacornerie, T.; Vandevelde, F; Reynaert, N.

doi:10.1088/0031-9155/58/8/2445

Cited by 24 publications

(34 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…the fact that the sensitive volume is not small compared to the dimensions of the beam). This is possible through the use of Monte Carlo simulations 5 . With those aspects taken into account, global correction factors can be applied to the LIC readings obtained in clinical routine measurements (output factors, percentage depth doses, dose profiles) in order to fully eliminate the perturbations.…”

Section: Discussionmentioning

confidence: 99%

See 1 more Smart Citation

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Wagner

Crop

Lacornerie

et al. 2014

JoVE

View full text Add to dashboard Cite

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via stereotactic radiotherapy where positioning accuracy allows delivering very high doses per fraction in a small volume of the patient. Dosimetric measurements on medical accelerators are conventionally realized using air-filled ionization chambers. However, in small beams these are subject to nonnegligible perturbation effects. This study focuses on liquid ionization chambers, which offer advantages in terms of spatial resolution and low fluence perturbation. Ion recombination effects are investigated for the microLion detector (PTW) used with the Cyberknife system (Accuray). The method consists of performing a series of water tank measurements at different source-surface distances, and applying corrections to the liquid detector readings based on simultaneous gaseous detector measurements. This approach facilitates isolating the recombination effects arising from the high density of the liquid sensitive medium and obtaining correction factors to apply to the detector readings. The main difficulty resides in achieving a sufficient level of accuracy in the setup to be able to detect small changes in the chamber response.

show abstract

Section: Discussionmentioning

confidence: 99%

“…Moreover the sensitive medium has a density close to that of water, reducing the fluence perturbations associated with an air cavity. These aspects make the LIC an interesting candidate for small beam dosimetry [3][4][5] .…”

Section: Introductionmentioning

confidence: 99%

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Wagner

Crop

Lacornerie

et al. 2014

JoVE

View full text Add to dashboard Cite

show abstract

“…3 The use of natural diamond for radiation detectors has been investigated for such fields, owing to its high sensitivity, low leakage, resistance to radiation damage and near tissue equivalence; however, dose-rate dependence and variability of individual detector volume of these detectors makes depth-dose determination difficult. 4 In this work, a new commercially available diamond detector, PTW-60019 microDiamond™ (PTW, Freiberg, Germany), has been evaluated for dosimetry measurements on a clinical CyberKnife VSI system.…”

mentioning

confidence: 99%

Evaluation of a synthetic single-crystal diamond detector for relative dosimetry measurements on a CyberKnife™

Chalkley¹,

Heyes²

2014

BJR

View full text Add to dashboard Cite

Objective: To evaluate a new commercial PTW-60019 microDiamond™ (PTW, Freiburg, Germany) synthetic singlecrystal diamond detector for relative dosimetry measurements on a clinical CyberKnife™ VSI (Accuray Inc., Sunnyvale, CA) system. Methods: Relative output factors (ROFs) were measured for collimator diameters from 5 to 60 mm, and compared with diode [PTW-60017, PTW-60018 and IBA Dosimetry (Schwarzenbruck, Germany) SFD] and ionization chamber (PTW-31014 PinPoint® and PTW-31010 Semiflex) measurements. Beam profiles were measured at a range of depths, and collimator sizes, with the detector stem oriented both parallel and perpendicular to the central axis (CAX). Percentage depth-dose (PDD) curves were obtained for the 60-mm collimator and compared with natural Diamond Detector (PTW-60003) and ionization chamber curves to evaluate energy dependence. Results: Penumbral broadening was noted on profile measurements made with the microDiamond oriented with the stem parallel to the CAX, in comparison with diodes. Oriented perpendicular to the CAX, the profile penumbra was sharper, but stem effects could not be ruled out. The PDD measurements were within 0.5% of ionization chamber measurements, indicating insignificant dose-rate dependence. The ROF for the microDiamond fell between diode and ionization chamber results. Published Monte Carlo-derived CyberKnifespecific factors were applied to the PTW-60017, PTW-60018 and PTW-31014 ROFs, and the microDiamond factors agreed within 2.0% of the mean of these. Conclusion: Over a range of small field relative dosimetry measurements, the microDiamond detector shows excellent spatial resolution, dose-rate independence and water equivalence. Advances in knowledge: The microDiamond is a suitable tool for commissioning stereotactic systems.The CyberKnife™ (Accuray Inc., Sunnyvale, CA) system allows submillimetre positional accuracy for stereotactic radiosurgery (SRS) and stereotactic ablative body radiotherapy treatments. It is operated without a flattening filter, with small field sizes (5-to 60-mm-diameter circular beams) and at a dose rate of the order of 10 Gy min 21, as defined at the depth of maximum dose (15 mm deep) for a 60-mm collimator at an 800-mm source-axis distance. This places an increased demand on the accuracy of dosimetric measurements used for commissioning and quality assurance. The absence of a flattening filter in radiotherapy machines is becoming more widespread, which in turn is leading to an increase in clinical dose rates. It is therefore important to consider dose-rate effects of detectors used for commissioning both stereotactic and conventional fields.Although standard detectors are recognized to be unsuitable for small field applications, owing to dose averaging and perturbation effects, SRS-specific detectors have also been shown to have limitations. Silicon diode detectors exhibit an over-response to low-energy scatter. The relative contribution of this scatter increases with field size, and, for regular field sizes, shielding has been used to compen...

show abstract

“…In the current work, dosimetrically equivalent tissue subsets are generated using Monte Carlo calculations in homogeneous phantoms and from preconceived results of Vanderstraeten et al (5) regarding the approximate number of intervals and the boundaries between them. Simulations were performed with DOSXYZnrc (11) using a phase space of a CyberKnife (Accuray Inc., Sunnyvale, CA) unit, previously modeled (12) . The result is an almost continuously changing tissue composition in consecutive subsets and, thus, a continuously changing hydrogen content, with a continuously changing density (from the mass density calibration curve).…”

Section: Methodsmentioning

confidence: 99%

“…The CyberKnife accelerator has been modeled using BEAMnrc (and the egs++ geometry package to model the iris collimator), generating phase space files at the exit plane of the secondary collimator (12) . The system allows selecting a treatment plan established on the MultiPlan (Accuray, Inc.) treatment planning system (TPS), and to compare the dose distribution and dose‐volume histograms (DVH) between MultiPlan and the Monte Carlo dose engine.…”

Section: Methodsmentioning

confidence: 99%

Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy

Demol

Viard²,

Reynaert

2015

J Applied Clin Med Phys

View full text Add to dashboard Cite

The purpose of this study was to demonstrate that Monte Carlo treatment planning systems require tissue characterization (density and composition) as a function of CT number. A discrete set of tissue classes with a specific composition is introduced. In the current work we demonstrate that, for megavoltage photon radiotherapy, only the hydrogen content of the different tissues is of interest. This conclusion might have an impact on MRI‐based dose calculations and on MVCT calibration using tissue substitutes. A stoichiometric calibration was performed, grouping tissues with similar atomic composition into 15 dosimetrically equivalent subsets. To demonstrate the importance of hydrogen, a new scheme was derived, with correct hydrogen content, complemented by oxygen (all elements differing from hydrogen are replaced by oxygen). Mass attenuation coefficients and mass stopping powers for this scheme were calculated and compared to the original scheme. Twenty‐five CyberKnife treatment plans were recalculated by an in‐house developed Monte Carlo system using tissue density and hydrogen content derived from the CT images. The results were compared to Monte Carlo simulations using the original stoichiometric calibration. Between 300 keV and 3 MeV, the relative difference of mass attenuation coefficients is under 1% within all subsets. Between 10 keV and 20 MeV, the relative difference of mass stopping powers goes up to 5% in hard bone and remains below 2% for all other tissue subsets. Dose‐volume histograms (DVHs) of the treatment plans present no visual difference between the two schemes. Relative differences of dose indexes D98,D95,D50,D05,D02, and Dmean were analyzed and a distribution centered around zero and of standard deviation below 2% (3σ) was established. On the other hand, once the hydrogen content is slightly modified, important dose differences are obtained. Monte Carlo dose planning in the field of megavoltage photon radiotherapy is fully achievable using only hydrogen content of tissues, a conclusion that might impact MRI dose calculation, but can also help selecting the optimal tissue substitutes when calibrating MVCT devices.PACS numbers: 87.55.D‐, 87.55.dk, 87.55.K‐, 87.57.Q‐

show abstract

Use of a liquid ionization chamber for stereotactic radiotherapy dosimetry

Cited by 24 publications

References 23 publications

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Evaluation of a synthetic single-crystal diamond detector for relative dosimetry measurements on a CyberKnife™

Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy

Contact Info

Product

Resources

About