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Title: Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry

Abstract

The accuracy of the Leksell GammaPlan registered , the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N{sub 2} gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distributionmore » predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan registered differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z=90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan registered to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.« less

Authors:
; ; ; ; ;  [1];  [2];  [2];  [2]
  1. Institute of Biomedical Engineering, Bogazici University, Istanbul (Turkey)
  2. (Turkey)
Publication Date:
OSTI Identifier:
20951293
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 34; Journal Issue: 5; Other Information: DOI: 10.1118/1.2718732; (c) 2007 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; DOSIMETRY; GELS; ISODOSE CURVES; PHANTOMS; PVC; RADIATION DOSE DISTRIBUTIONS; RADIATION DOSES; RADIOTHERAPY; SINUSES; SURGERY; YTTRIUM 90

Citation Formats

Isbakan, Fatih, Uelgen, Yekta, Bilge, Hatice, Ozen, Zeynep, Agus, Onur, Buyuksarac, Bora, Institute of Radiation Oncology, Istanbul University, Istanbul, Radiation Oncology Department, Marmara University Hospital, Istanbul, and Institute of Biomedical Engineering, Bogazici University, Istanbul. Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry. United States: N. p., 2007. Web. doi:10.1118/1.2718732.
Isbakan, Fatih, Uelgen, Yekta, Bilge, Hatice, Ozen, Zeynep, Agus, Onur, Buyuksarac, Bora, Institute of Radiation Oncology, Istanbul University, Istanbul, Radiation Oncology Department, Marmara University Hospital, Istanbul, & Institute of Biomedical Engineering, Bogazici University, Istanbul. Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry. United States. doi:10.1118/1.2718732.
Isbakan, Fatih, Uelgen, Yekta, Bilge, Hatice, Ozen, Zeynep, Agus, Onur, Buyuksarac, Bora, Institute of Radiation Oncology, Istanbul University, Istanbul, Radiation Oncology Department, Marmara University Hospital, Istanbul, and Institute of Biomedical Engineering, Bogazici University, Istanbul. Tue . "Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry". United States. doi:10.1118/1.2718732.
@article{osti_20951293,
title = {Gamma Knife 3-D dose distribution near the area of tissue inhomogeneities by normoxic gel dosimetry},
author = {Isbakan, Fatih and Uelgen, Yekta and Bilge, Hatice and Ozen, Zeynep and Agus, Onur and Buyuksarac, Bora and Institute of Radiation Oncology, Istanbul University, Istanbul and Radiation Oncology Department, Marmara University Hospital, Istanbul and Institute of Biomedical Engineering, Bogazici University, Istanbul},
abstractNote = {The accuracy of the Leksell GammaPlan registered , the dose planning system of the Gamma Knife Model-B, was evaluated near tissue inhomogeneities, using the gel dosimetry method. The lack of electronic equilibrium around the small-diameter gamma beams can cause dose calculation errors in the neighborhood of an air-tissue interface. An experiment was designed to investigate the effects of inhomogeneity near the paranosal sinuses cavities. The homogeneous phantom was a spherical glass balloon of 16 cm diameter, filled with MAGIC gel; i.e., the normoxic polymer gel. Two hollow PVC balls of 2 cm radius, filled with N{sub 2} gas, represented the air cavities inside the inhomogeneous phantom. For dose calibration purposes, 100 ml gel-containing vials were irradiated at predefined doses, and then scanned in a MR unit. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. Dose distributions are the results of a single shot of irradiation, obtained by collimating all 201 cobalt sources to a known target in the phantom. Both phantoms were irradiated at the same dose level at the same coordinates. Stereotactic frames and fiducial markers were attached to the phantoms prior to MR scanning. The dose distribution predicted by the Gamma Knife planning system was compared with that of the gel dosimetry. As expected, for the homogeneous phantom the isodose diameters measured by the gel dosimetry and the GammaPlan registered differed by 5% at most. However, with the inhomogeneous phantom, the dose maps in the axial, coronal and sagittal planes were spatially different. The diameters of the 50% isodose curves differed 43% in the X axis and 32% in the Y axis for the Z=90 mm axial plane; by 44% in the X axis and 24% in the Z axis for the Y=90 mm coronal plane; and by 32% in the Z axis and 42% in the Y axis for the X=92 mm sagittal plane. The lack of ability of the GammaPlan registered to predict the rapid dose fall off, due to the air cavities behind or near the lesion led to an overestimation of the dose that was actually delivered. Clinically, this can result in underdosing of lesions near tissue inhomogeneities in patients under treatment.},
doi = {10.1118/1.2718732},
journal = {Medical Physics},
number = 5,
volume = 34,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Polymer gel dosimeters offer a practical solution to 3D dose verification for conventional radiotherapy as well as intensity-modulated and stereotactic radiotherapy. In this study, EGSnrc calculated and PAGAT polymer gel dosimeter measured dose volume histograms (DVHs) for single-shot irradiations of the Gamma Knife (GK) unit were used to investigate the effects of the presence of inhomogeneities on 3D dose distribution. The head phantom was a custom-built 16 cm diameter Plexiglas sphere. Inside the phantom, there is a cubic cutout for inserting the gel vials and another cutout for inserting the inhomogeneities. Following irradiation with the GK unit, the polymer gelmore » phantoms were scanned with a 1.5 T MRI scanner. Comparing the results of measurement in homogeneous and heterogeneous phantoms revealed that inserting inhomogeneities inside the homogeneous phantom did not cause considerable disturbances on dose distribution in irradiation with 8 mm collimator within low isodose levels (<50%), which is essential for the dose sparing of sensitive structures. The results of simulation for homogeneous and inhomogeneous phantoms in irradiation with 18 mm collimator of the GK unit showed 23.24% difference in DVH within 90%-100% relative isodose level and also revealed that a significant part of the target (28.56%) received relative doses higher than the maximum dose, which exceeds the acceptance criterion (5%). Based on these results it is concluded that the presence of inhomogeneities inside the phantom can cause considerable errors in dose calculation within high isodose levels with respect to LGP prediction which assumes that the target is a homogeneous material. Moreover, it is demonstrated that the applied MC code is an accurate and stand-alone tool for 3D evaluation of dose distribution in irradiation with the GK unit, which can provide important, 3D plan evaluation criteria used in clinical practice.« less
  • This study reports on the effects of x-ray CT dose in CT imaged normoxic polyacrylamide (nPAG) gel dosimeters. The investigation is partitioned into three sections. First, the CT dose absorbed in nPAG is quantified under a range of typical gel CT imaging protocols. It is found that the maximum absorbed CT dose occurs for volumetric imaging and is in the range of 4.6{+-}0.2 cGy/image. This does scales linearly with image averaging. Second, using Raman spectroscopy, the response of nPAG to CT imaging photon energies (i.e., 120-140 kVp) is established and compared to the well known dose response of nPAG exposedmore » to 6 MV photons. It is found that nPAG exhibits a weaker response (per unit dose) to 140-kVp incident photons as compared to 6 MV incident photons (slopes m{sub 6MV}=-0.0374{+-}0.0006 Gy{sup -1} and m{sub 140kVp}=-0.016{+-}0.001 Gy{sup -1}). Finally, using the above data, an induced change in CT number ({delta}N{sub CT}) is calculated for nPAG imaged using a range of gel imaging protocols. It is found that under typical imaging protocols (120-140 kVp, 200 mAs, {approx}16-32 image averages) a {delta}N{sub CT}<0.2 H is induced in active nPAG dosimeters. This {delta}N{sub CT} is below the current limit of detectability of CT nPAG polymer gel dosimetry. Under expanded imaging protocols (e.g., very high number of image averages) an induced {delta}N{sub CT} of {approx}0.5 H is possible. In these situations the additional polymerization occurring in nPAG due to the imaging process may need to be accounted for.« less
  • Radiation fluence changes caused by the dosimeter itself and poor spatial resolution may lead to lack of 3-dimensional (3D) information depending on the features of the dosimeter and quality assurance of dose distributions for high–dose rate (HDR) iridium-192 ({sup 192}Ir) brachytherapy sources is challenging and experimental dosimetry methods used for brachytherapy sources are limited. In this study, we investigated 3D dose distributions of {sup 192}Ir brachytherapy sources for irradiation with single and multiple dwell positions using a normoxic gel dosimeter and compared them with treatment planning system (TPS) calculations. For dose calibration purposes, 100-mL gel-containing vials were irradiated at predefinedmore » doses and then scanned in an magnetic resonance (MR) imaging unit. Gel phantoms prepared in 2 spherical glasses were irradiated with {sup 192}Ir for the calculated dwell positions, and MR scans of the phantoms were obtained. The images were analyzed with MATLAB software. Dose distributions and profiles derived with 1-mm resolution were compared with TPS calculations. Linearity was observed between the delivered dose and the reciprocal of the T2 relaxation time constant of the gel. The x-, y-, and z-axes were defined as the sagittal, coronal, and axial planes, respectively, the sagittal and axial planes were defined parallel to the long axis of the source while the coronal plane was defined horizontally to the long axis of the source. The differences between measured and calculated profile widths of 3-cm source length and point source for 70%, 50%, and 30% isodose lines were evaluated at 3 dose levels using 18 profiles of comparison. The calculations for 3-cm source length revealed a difference of > 3 mm in 1 coordinate at 50% profile width on the sagittal plane and 3 coordinates at 70% profile width and 2 coordinates at 50% and 30% profile widths on the axial plane. Calculations on the coronal plane for 3-cm source length showed > 3-mm difference in 1 coordinate at 50% and 70% and 2 coordinates at 30% profile widths. The point source measurements and calculations for 50% profile widths revealed a difference > 3 mm in 1 coordinate on the sagittal plane and 2 coordinates on the axial plane. The doses of 3 coordinates on the sagittal plane and 4 coordinates on the axial plane could not be evaluated in 30% profile width because of low doses. There was good agreement between the gel dosimetry and TPS results. Gel dosimetry provides dose distributions in all 3 planes at the same time, which enables us to define the dose distributions in any plane with high resolution. It can be used to obtain 3D dose distributions for HDR {sup 192}Ir brachytherapy sources and 3D dose verification of TPS.« less
  • Purpose: Water equivalent polymer gel dosimeters and magnetic resonance imaging were employed to measure the output factors of the two smallest treatment fields available in a Gamma Knife model C radiosurgery unit, those formed employing the 4 and 8 mm final collimator helmets. Methods: Three samples of the VIP normoxic gel formulation were prepared and irradiated so that a single shot of the field whose output factor is to be measured and a single shot of the reference 18 mm field were delivered in each one. Emphasis is given to the development and benchmarking of a refined data processing methodologymore » of reduced uncertainty that fully exploits the 3D dose distributions registered in the dosimeters. Results: Polymer gel results for the output factor of the 8 mm collimator helmet are found to be in close agreement with the corresponding value recommended by the vendor (0.955{+-}0.007 versus 0.956, respectively). For the 4 mm collimator helmet, however, polymer gel results suggest an output factor 3% lower than the value recommended by the vendor (0.841{+-}0.009 versus 0.870, respectively). Conclusions: A comparison with corresponding measurements published in the literature indicates that output factor results of this work are in agreement with those obtained using dosimetric systems which, besides fine spatial resolution and lack of angular and dose rate dependence of the dosimeter's response, share with polymer gels the favorable characteristic of minimal radiation field perturbation.« less
  • The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performedmore » with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 {mu}Gy m{sup 2} h{sup -1} Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within {+-}2% or 2 mm for 98% of points compared with RCF measurements and to within {+-}3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.« less