Oxygen contamination of a polyacrylamide gel (PAG) dosimeter can have a significant detrimental e... more Oxygen contamination of a polyacrylamide gel (PAG) dosimeter can have a significant detrimental effect upon the performance characteristics of the gel as a dosimeter. Oxygen contamination can occur during preparation, but can also arise from the diffusion of atmospheric oxygen through vessel walls. Even 'trace' amounts of oxygen present in any gaseous space (usually nitrogen filled) between the surface of the PAG and the vessel seal can have a detectable effect. This paper describes a method used to quantify the detrimental effects of a range of oxygen concentrations upon the performance of the PAG. The results of diffusion studies for air and pure oxygen diffusing into the PAG over time are presented and coupled with a novel method of calculating oxygen concentrations in the PAG. Results obtained suggest that the diffusion is Fickian. The coefficient of diffusion D has been calculated to be (8Ā±2)Ć10 ā6 cm 2 s ā1 for oxygen diffusing into PAG stored at 4 ā¢ C, under the assumption that the diffusion mechanism is independent of the concentration of the diffusing species. In addition, a quantitative relation has been established between the transverse relaxation rate R 2 of the PAG and the concentration of oxygen present. The implications of these findings for polymer gel dosimetry are discussed.
New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex... more New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex three-dimensional dose distributions accurately and with good spatial resolution. Polymer gel is an emerging new dosimeter being applied to these challenges. The aim of this review is to present a practical overview of polymer gel dosimetry, including gel manufacture, imaging, calibration and application to radiotherapy verification. The
Although much has been published concerning the various safety risks associated with magnetic res... more Although much has been published concerning the various safety risks associated with magnetic resonance (MR) imaging, little mention is made of the acoustic noise generated during scanning. The existing data are now out of date with regard to currently used imaging protocols and technology. Out of concern for patient safety, measurements were made of the acoustic noise generated during various scan protocols on a high field (1.5 T) Siemens MR imager. These data were compared with published data. The data show that for certain protocols, the exposure to acoustic noise falls outside safety guidelines unless ear protection is used.
New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex... more New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex three-dimensional dose distributions accurately and with good spatial resolution. Polymer gel is an emerging new dosimeter being applied to these challenges. The aim of this review is to present a practical overview of polymer gel dosimetry, including gel manufacture, imaging, calibration and application to radiotherapy veriĀ®cation. The dosimeters consist of a gel matrix within which is suspended a solution of acrylic molecules. These molecules polymerize upon exposure to radiation, with the degree of polymerization being proportional to absorbed dose. The polymer distribution can be measured in two or three dimensions using MRI or optical tomography and, after calibration, the images can be converted into radiation dose distributions. Manufacture of the gel is reported to be reproducible, and measured dose in the range 0Ā±10 Gy is accurate to within 3Ā±5%. In-plane image resolution of 1 mm61 mm, with image slice thicknesses of between 2Ā±5 mm, is typically achievable using clinical 1.5 T MR scanners and standard T 2 weighted imaging sequences. The gels have been used to verify a number of conventional and novel radiotherapy modalities, including brachytherapy, intensity modulated radiotherapy and stereotactic radiosurgery. All the studies have conĀ®rmed the value and versatility of the dosimetry technique.
Radiotherapy is widely used to palliate local symptoms in non-small-cell lung cancer. Using conve... more Radiotherapy is widely used to palliate local symptoms in non-small-cell lung cancer. Using conventional X-ray simulation, it is often difficult to accurately localize the extent of the tumour. We report a randomized, double blind trial comparing target localization with conventional and virtual simulation. Eighty-six patients underwent both conventional and virtual simulation. The conventional simulator films were compared with digitally reconstructed radiographs (DRRs) produced from the computed tomography (CT) data. The treatment fields defined by the clinicians using each modality were compared in terms of field area, position and the implications for target coverage. Comparing fields defined by each study arm, there was a major mis-match in coverage between fields in 66.2% of cases, and a complete match in only 5.2% of cases. In 82.4% of cases, conventional simulator fields were larger (mean 24.5+/-5.1% (95% confidence interval)) than CT-localized fields, potentially contributing to a mean target under-coverage of 16.4+/-3.5% and normal tissue over-coverage of 25.4+/-4.2%. CT localization and virtual simulation allow more accurate definition of the target volume. This could enable a reduction in geographical misses, while also reducing treatment-related toxicity.
BANG-gel dosimetry offers the potential for measuring the dose delivered by a radiotherapy treatm... more BANG-gel dosimetry offers the potential for measuring the dose delivered by a radiotherapy treatment technique, in three dimensions, with high spatial resolution and good accuracy. The ability to measure comprehensively a 3D dose distribution is a major advantage of the gel dosimeter over conventional planar and point-based dosimeter devices, particularly when applied to the verification of complex dose distributions characteristic of intensity-modulated radiotherapy (IMRT). In this paper an in-house manufactured BANG-gel dosimeter was applied to study the dose distributions of two irradiation experiments for which the distributions were known: (i) a dosimetrically simple parallel-opposed irradiation, and (ii) a more complex nine-field 'static tomotherapy' intensity-modulated irradiation delivered with the Nomos MIMiC. The uniform distribution in (i) allowed a study of the magnetic resonance (MR) imaging parameters to achieve an optimal trade-off between noise and image resolution (optimum image resolution for the Siemens 1.5T Vision system was determined to be approximately 0.8 mm2 with a slice thickness of 2 mm). The spatial uniformity of gel sensitivity to radiation was found to depend strongly on the presence of oxygen, which must be eliminated for the gel dosimeter to be of use. The gel dosimeter was found to agree well with predicted dose distributions and accurately measured the steep penumbral fall-off of dose, even after many days, proving its potential for the verification of IMRT distributions. In the nine-field IMRT delivery (ii) the predicted dose was computed by both an in-house 'component-delivery' dose algorithm and the Peacock planning-system dose algorithm. Good agreement was found between the two algorithms despite the latter's omission of the change in penumbral characteristics with aperture-size during delivery, lack of inhomogeneity correction and approximate modelling of leaf leakage. These effects were found to be small for the problem studied. The predicted distribution agreed well with the gel-measured distribution at medium and high doses (50-90% isodose lines) although differences of up to 10% were observed at lower doses (30% isodose line). The gel dosimeter was found to have the potential to verify IMRT distributions but required considerable care to achieve accurate results. Attention was required to achieve uniformity of gel sensitivity (to prevent oxygen contamination), and in the calibration process.
A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is... more A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher accuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately with a 10 x 10 cm2 field end-on in a water bath, such that the characteristic depth-dose curve is recorded in the gel. The calibration is then determined by fitting the depth-dose measured in water, against the measured change in relaxivity with depth in the gel. Increased accuracy is achieved in this simple depth-dose geometry by averaging the relaxivity at each depth. A large number of calibration data points, each with relatively high accuracy, are obtained. Calibration data over the full range of dose (1.6-10 Gy) is obtained by irradiating one test-tube to 10 Gy at dose maximum (Dmax), and the other to 4.5 Gy at Dmax. The new calibration method is compared with a 'standard method' where five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy. The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the new method by a factor of about 4 and 10 respectively, when compared with the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slope of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.
The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluated when used to verify ... more The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluated when used to verify two radiotherapy treatment plans of increasing complexity. The plans investigated were a three-field coplanar arrangement, using the linac jaws for field shaping, and a four-field, conformal, non-coplanar plan using precision-cast lead alloy shielding blocks. Each treatment was performed three times using phantoms and calibration gels manufactured in-house. Two phantoms were specially designed for this work to aid accurate positioning of the gels for irradiation and imaging. All gels were imaged post-irradiation using a Siemens Vision 1.5T MR scanner. T2 relaxation images were calibrated to absorbed dose distributions using a number of smaller calibration vessels to produce distribution maps of relative dose. The relative dose distributions were found to be reproducible, with the standard deviation on the mean areas enclosed by the > or = 50% isodose lines measured in three orthogonal planes being 6.4% and 4.1% for the coplanar and non-coplanar plans respectively. The measured distributions were also consistent with those planned, with isodose lines generally agreeing to within a few millimetres. However, the measured absolute doses were on average 23.5% higher than those planned. Although the polyacrylamide gel dosimetry technique has some limitations, particularly when calibrating distributions to absolute dose, the ability to resolve sharp dose gradients in three dimensions with millimetre precision is invaluable when verifying complex conformal treatment plans, where avoidance of proximal, critical structures is a treatment criterion.
Few quantitative data are available on the kinetics of polymerization reactions in polymer gel (P... more Few quantitative data are available on the kinetics of polymerization reactions in polymer gel (PAG) dosimeters and their long-term stability. Post-irradiation polymerization reactions have been found to continue for several weeks, posing questions regarding dosimeter stability and its achievement. In this paper we report an investigation of polymerization kinetics in PAG dosimeters and the effect of diffusing oxygen into the dosimeter, post irradiation, as a potential method of inhibiting further polymerization and stabilizing the dose distribution. Results show continuous post-irradiation changes in transverse relaxation rate R2 with time over the five week study period and that a steady-state may not be reached for a period of months. An assessment is made of the appropriate time to image the dosimeter which shows that after three to four days the polymerization change is slow compared with imaging time. The implications of the time delay between irradiation and imaging are discussed in terms of the resultant sensitivity of the dosimeter and accuracy of the dose measured. In pairs of dosimeters, one sealed the other open to air, oxygen diffusing into the dosimeter arrests polymerization. However, the diffusion rate is too slow to make this method practicable. The slow diffusion means that while in regions near the air/gel interface polymerization is quickly arrested, in deeper regions it may continue for many hours, causing artefacts in the dose distribution. In the companion paper to this from a collaborating team, a study focusing on modelling oxygen diffusion in dosimeter gel will be presented.
It is well known that the experimental dosimetry of brachytherapy sources presents a challenge. D... more It is well known that the experimental dosimetry of brachytherapy sources presents a challenge. Depending on the particular-dosimeter used, measurements can suffer from poor spatial resolution (ion chambers), lack of 3D information (film) or errors due to the presence of the dosimeter itself distorting the radiation flux. To avoid these problems, we have investigated the dosimetry of a clinical 192Ir source using a polyacrylamide gel (PAG) dosimeter. Experimental measurements of dose versus radial distance from the centre of the source (cross-line plots) were compared with calculations produced with a Nucletron NPS planning system. Good agreement was found between the planning system and gel measurements in planes selected for analysis. Gel dosimeter measurements in a coronal plane through the phantom showed a mean difference between measured absorbed dose and calculated dose of 0.17 Gy with SD = 0.13 Gy. Spatially, the errors at the reference point remain within one image pixel (1.0 mm). The use of polymer gel dosimetry shows promise for brachytherapy applications, offering complete, three-dimensional dose information, good spatial resolution and small measurement errors. Measurements close to the source, however, are difficult, due to some of the limiting properties of the polyacrylamide gel.
Oxygen contamination of a polyacrylamide gel (PAG) dosimeter can have a significant detrimental e... more Oxygen contamination of a polyacrylamide gel (PAG) dosimeter can have a significant detrimental effect upon the performance characteristics of the gel as a dosimeter. Oxygen contamination can occur during preparation, but can also arise from the diffusion of atmospheric oxygen through vessel walls. Even 'trace' amounts of oxygen present in any gaseous space (usually nitrogen filled) between the surface of the PAG and the vessel seal can have a detectable effect. This paper describes a method used to quantify the detrimental effects of a range of oxygen concentrations upon the performance of the PAG. The results of diffusion studies for air and pure oxygen diffusing into the PAG over time are presented and coupled with a novel method of calculating oxygen concentrations in the PAG. Results obtained suggest that the diffusion is Fickian. The coefficient of diffusion D has been calculated to be (8Ā±2)Ć10 ā6 cm 2 s ā1 for oxygen diffusing into PAG stored at 4 ā¢ C, under the assumption that the diffusion mechanism is independent of the concentration of the diffusing species. In addition, a quantitative relation has been established between the transverse relaxation rate R 2 of the PAG and the concentration of oxygen present. The implications of these findings for polymer gel dosimetry are discussed.
New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex... more New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex three-dimensional dose distributions accurately and with good spatial resolution. Polymer gel is an emerging new dosimeter being applied to these challenges. The aim of this review is to present a practical overview of polymer gel dosimetry, including gel manufacture, imaging, calibration and application to radiotherapy verification. The
Although much has been published concerning the various safety risks associated with magnetic res... more Although much has been published concerning the various safety risks associated with magnetic resonance (MR) imaging, little mention is made of the acoustic noise generated during scanning. The existing data are now out of date with regard to currently used imaging protocols and technology. Out of concern for patient safety, measurements were made of the acoustic noise generated during various scan protocols on a high field (1.5 T) Siemens MR imager. These data were compared with published data. The data show that for certain protocols, the exposure to acoustic noise falls outside safety guidelines unless ear protection is used.
New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex... more New, complex radiotherapy delivery techniques require dosimeters that are able to measure complex three-dimensional dose distributions accurately and with good spatial resolution. Polymer gel is an emerging new dosimeter being applied to these challenges. The aim of this review is to present a practical overview of polymer gel dosimetry, including gel manufacture, imaging, calibration and application to radiotherapy veriĀ®cation. The dosimeters consist of a gel matrix within which is suspended a solution of acrylic molecules. These molecules polymerize upon exposure to radiation, with the degree of polymerization being proportional to absorbed dose. The polymer distribution can be measured in two or three dimensions using MRI or optical tomography and, after calibration, the images can be converted into radiation dose distributions. Manufacture of the gel is reported to be reproducible, and measured dose in the range 0Ā±10 Gy is accurate to within 3Ā±5%. In-plane image resolution of 1 mm61 mm, with image slice thicknesses of between 2Ā±5 mm, is typically achievable using clinical 1.5 T MR scanners and standard T 2 weighted imaging sequences. The gels have been used to verify a number of conventional and novel radiotherapy modalities, including brachytherapy, intensity modulated radiotherapy and stereotactic radiosurgery. All the studies have conĀ®rmed the value and versatility of the dosimetry technique.
Radiotherapy is widely used to palliate local symptoms in non-small-cell lung cancer. Using conve... more Radiotherapy is widely used to palliate local symptoms in non-small-cell lung cancer. Using conventional X-ray simulation, it is often difficult to accurately localize the extent of the tumour. We report a randomized, double blind trial comparing target localization with conventional and virtual simulation. Eighty-six patients underwent both conventional and virtual simulation. The conventional simulator films were compared with digitally reconstructed radiographs (DRRs) produced from the computed tomography (CT) data. The treatment fields defined by the clinicians using each modality were compared in terms of field area, position and the implications for target coverage. Comparing fields defined by each study arm, there was a major mis-match in coverage between fields in 66.2% of cases, and a complete match in only 5.2% of cases. In 82.4% of cases, conventional simulator fields were larger (mean 24.5+/-5.1% (95% confidence interval)) than CT-localized fields, potentially contributing to a mean target under-coverage of 16.4+/-3.5% and normal tissue over-coverage of 25.4+/-4.2%. CT localization and virtual simulation allow more accurate definition of the target volume. This could enable a reduction in geographical misses, while also reducing treatment-related toxicity.
BANG-gel dosimetry offers the potential for measuring the dose delivered by a radiotherapy treatm... more BANG-gel dosimetry offers the potential for measuring the dose delivered by a radiotherapy treatment technique, in three dimensions, with high spatial resolution and good accuracy. The ability to measure comprehensively a 3D dose distribution is a major advantage of the gel dosimeter over conventional planar and point-based dosimeter devices, particularly when applied to the verification of complex dose distributions characteristic of intensity-modulated radiotherapy (IMRT). In this paper an in-house manufactured BANG-gel dosimeter was applied to study the dose distributions of two irradiation experiments for which the distributions were known: (i) a dosimetrically simple parallel-opposed irradiation, and (ii) a more complex nine-field 'static tomotherapy' intensity-modulated irradiation delivered with the Nomos MIMiC. The uniform distribution in (i) allowed a study of the magnetic resonance (MR) imaging parameters to achieve an optimal trade-off between noise and image resolution (optimum image resolution for the Siemens 1.5T Vision system was determined to be approximately 0.8 mm2 with a slice thickness of 2 mm). The spatial uniformity of gel sensitivity to radiation was found to depend strongly on the presence of oxygen, which must be eliminated for the gel dosimeter to be of use. The gel dosimeter was found to agree well with predicted dose distributions and accurately measured the steep penumbral fall-off of dose, even after many days, proving its potential for the verification of IMRT distributions. In the nine-field IMRT delivery (ii) the predicted dose was computed by both an in-house 'component-delivery' dose algorithm and the Peacock planning-system dose algorithm. Good agreement was found between the two algorithms despite the latter's omission of the change in penumbral characteristics with aperture-size during delivery, lack of inhomogeneity correction and approximate modelling of leaf leakage. These effects were found to be small for the problem studied. The predicted distribution agreed well with the gel-measured distribution at medium and high doses (50-90% isodose lines) although differences of up to 10% were observed at lower doses (30% isodose line). The gel dosimeter was found to have the potential to verify IMRT distributions but required considerable care to achieve accurate results. Attention was required to achieve uniformity of gel sensitivity (to prevent oxygen contamination), and in the calibration process.
A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is... more A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher accuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately with a 10 x 10 cm2 field end-on in a water bath, such that the characteristic depth-dose curve is recorded in the gel. The calibration is then determined by fitting the depth-dose measured in water, against the measured change in relaxivity with depth in the gel. Increased accuracy is achieved in this simple depth-dose geometry by averaging the relaxivity at each depth. A large number of calibration data points, each with relatively high accuracy, are obtained. Calibration data over the full range of dose (1.6-10 Gy) is obtained by irradiating one test-tube to 10 Gy at dose maximum (Dmax), and the other to 4.5 Gy at Dmax. The new calibration method is compared with a 'standard method' where five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy. The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the new method by a factor of about 4 and 10 respectively, when compared with the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slope of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.
The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluated when used to verify ... more The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluated when used to verify two radiotherapy treatment plans of increasing complexity. The plans investigated were a three-field coplanar arrangement, using the linac jaws for field shaping, and a four-field, conformal, non-coplanar plan using precision-cast lead alloy shielding blocks. Each treatment was performed three times using phantoms and calibration gels manufactured in-house. Two phantoms were specially designed for this work to aid accurate positioning of the gels for irradiation and imaging. All gels were imaged post-irradiation using a Siemens Vision 1.5T MR scanner. T2 relaxation images were calibrated to absorbed dose distributions using a number of smaller calibration vessels to produce distribution maps of relative dose. The relative dose distributions were found to be reproducible, with the standard deviation on the mean areas enclosed by the > or = 50% isodose lines measured in three orthogonal planes being 6.4% and 4.1% for the coplanar and non-coplanar plans respectively. The measured distributions were also consistent with those planned, with isodose lines generally agreeing to within a few millimetres. However, the measured absolute doses were on average 23.5% higher than those planned. Although the polyacrylamide gel dosimetry technique has some limitations, particularly when calibrating distributions to absolute dose, the ability to resolve sharp dose gradients in three dimensions with millimetre precision is invaluable when verifying complex conformal treatment plans, where avoidance of proximal, critical structures is a treatment criterion.
Few quantitative data are available on the kinetics of polymerization reactions in polymer gel (P... more Few quantitative data are available on the kinetics of polymerization reactions in polymer gel (PAG) dosimeters and their long-term stability. Post-irradiation polymerization reactions have been found to continue for several weeks, posing questions regarding dosimeter stability and its achievement. In this paper we report an investigation of polymerization kinetics in PAG dosimeters and the effect of diffusing oxygen into the dosimeter, post irradiation, as a potential method of inhibiting further polymerization and stabilizing the dose distribution. Results show continuous post-irradiation changes in transverse relaxation rate R2 with time over the five week study period and that a steady-state may not be reached for a period of months. An assessment is made of the appropriate time to image the dosimeter which shows that after three to four days the polymerization change is slow compared with imaging time. The implications of the time delay between irradiation and imaging are discussed in terms of the resultant sensitivity of the dosimeter and accuracy of the dose measured. In pairs of dosimeters, one sealed the other open to air, oxygen diffusing into the dosimeter arrests polymerization. However, the diffusion rate is too slow to make this method practicable. The slow diffusion means that while in regions near the air/gel interface polymerization is quickly arrested, in deeper regions it may continue for many hours, causing artefacts in the dose distribution. In the companion paper to this from a collaborating team, a study focusing on modelling oxygen diffusion in dosimeter gel will be presented.
It is well known that the experimental dosimetry of brachytherapy sources presents a challenge. D... more It is well known that the experimental dosimetry of brachytherapy sources presents a challenge. Depending on the particular-dosimeter used, measurements can suffer from poor spatial resolution (ion chambers), lack of 3D information (film) or errors due to the presence of the dosimeter itself distorting the radiation flux. To avoid these problems, we have investigated the dosimetry of a clinical 192Ir source using a polyacrylamide gel (PAG) dosimeter. Experimental measurements of dose versus radial distance from the centre of the source (cross-line plots) were compared with calculations produced with a Nucletron NPS planning system. Good agreement was found between the planning system and gel measurements in planes selected for analysis. Gel dosimeter measurements in a coronal plane through the phantom showed a mean difference between measured absorbed dose and calculated dose of 0.17 Gy with SD = 0.13 Gy. Spatially, the errors at the reference point remain within one image pixel (1.0 mm). The use of polymer gel dosimetry shows promise for brachytherapy applications, offering complete, three-dimensional dose information, good spatial resolution and small measurement errors. Measurements close to the source, however, are difficult, due to some of the limiting properties of the polyacrylamide gel.
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Papers by M. McJury