Steep Dose Gradient Regions Research Articles

Characterization of Fricke-gelatin dosimeters for intraoperative Radiation Therapy dosimetry

In this work, the usability of Fricke gels in Intraoperative Radiation Therapy (IORT) dosimetry was investigated. Irradiated gel systems are characterized by means of UV/Vis spectrophotometric analysis in order to define the main dosimetric indexes of interest and to evaluate their post-irradiation stability. A measurement of Output Factors for different field sizes is also attempted to study the application of this system for routine quality control. Ad-hoc manufactured thin-layers of gel are employed to perform acquisitions of planar transverse dose profiles and to quantify the effect of Fe(III) diffusion. The Fricke gel system presents an adequate dosimetric performance, with excellent response linearity up to 30 Gy. Comparison of dosimetric response over a wide range of irradiation dose rates allows to conclude that the gel response is not influenced significantly by this parameter. Transverse dose profiles acquired with thin-layers present an agreement within 3% with reference values, even in very steep dose gradient regions. Finally, computed values for the diffusion coefficient of ferric ions are in good agreement with those reported in literature for dosimetric gels of similar composition. Overall performance of the gel system appears promising for further applications in IORT dosimetry.

Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method.

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.

Dosimetric characterisation of the optically-stimulated luminescence dosimeter in cobalt-60 high dose rate brachytherapy system.

This study investigates the characteristics and application of the optically-stimulated luminescence dosimeter (OSLD) in cobalt-60 high dose rate (HDR) brachytherapy, and compares the results with the dosage produced by the treatment planning system (TPS). The OSLD characteristics comprised linearity, reproducibility, angular dependence, depth dependence, signal depletion, bleaching rate and cumulative dose measurement. A phantom verification exercise was also conducted using the Farmer ionisation chamber and in vivo diodes. The OSLD signal indicated a supralinear response (R2 = 0.9998). It exhibited a depth-independent trend after a steep dose gradient region. The signal depletion per readout was negligible (0.02%), with expected deviation for angular dependence due to off-axis sensitive volume, ranging from 1 to 16%. The residual signal of the OSLDs after 1day bleached was within 1.5%. The accumulated and bleached OSLD signals had a standard deviation of ± 0.78 and ± 0.18Gy, respectively. The TPS was found to underestimate the measured doses with deviations of 5% in OSLD, 17% in the Farmer ionisation chamber, and 7 and 8% for bladder and rectal diode probes. Discrepancies can be due to the positional uncertainty in the high-dose gradient. This demonstrates a slight displacement of the organ at risk near the steep dose gradient region will result in a large dose uncertainty. This justifies the importance of in vivo measurements in cobalt-60 HDR brachytherapy.

Analysis of the response of PVA-GTA Fricke-gel dosimeters with clinical magnetic resonance imaging

Abstract Fricke gel dosimeters produced with a matrix of Poly-vinyl alcohol (PVA) cross-linked with glutaraldehyde (GTA) were analyzed with magnetic resonance imaging (MRI). Previous studies based on spectrophotometry showed valuable dosimetric features of these gels in terms of X-ray sensitivity and diffusion of the ferric ions produced after irradiation. In this study, MRI was performed on the gels at 1.5 T with a clinical scanner in order to optimize the acquisition parameters and obtain high contrast between irradiated and non-irradiated samples. The PVA gels were found to offer good linearity in the range of 0–10 Gy and a stable signal for several hours after irradiation. The sensitivity was about 40% higher compared to gels produced with agarose as gelling agent. The effect of xylenol orange (XO) on the MRI signal was also investigated: gel dosimeters made without XO show higher sensitivity to x-rays than those made with XO. The dosimetric accuracy of the 3D gels was investigated by comparing their MRI response to percentage depth dose and transversal dose profile measurements made with an ionization chamber in a water phantom. The comparison of PVA-GTA gels with and without XO showed that the chelating agent reduces the MRI sensitivity of the gels. Depth-dose and transversal dose profiles acquired by PVA-GTA gels without XO are more accurate and consistent with the ionization chamber data. However, diffusion effects hinder accurate measurements in the steep dose gradient regions and they should be further reduced by modifying the gel matrix and/or by minimizing the delay between irradiation and imaging.

SU‐F‐T‐449: Dosimetric Comparison of Acuros XB, Adaptive Convolve in Intensity Modulated Radiotherapy for Head and Neck Cancer

Purpose:There have been several publications focusing on dose calculation in lung for a new dose calculation algorithm of Acuros XB (AXB). AXB could contribute to dose calculation for high‐density media for bone and dental prosthesis rather than in lung. We compared the dosimetric performance of AXB, Adaptive Convolve (AC) in head and neck IMRT plans.Methods:In a phantom study, the difference in depth profile between AXB and AC was evaluated using Kodak EDR2 film sandwiched with tough water phantoms. 6 MV x‐ray using the TrueBeam was irradiated. In a patient study, 20 head and neck IMRT plans had been clinically approved in Pinnacle3 and were transferred to Eclipse. Dose distribution was recalculated using AXB in Eclipse while maintaining AC‐calculated monitor units and MLC sequence planned in Pinnacle. Subsequently, both the dose‐volumetric data obtained using the two different calculation algorithms were compared.Results:The results in the phantom evaluation for the shallow area ahead of the build‐up region shows over‐dose for AXB and under‐dose for AC, respectively. In the patient plans, AXB shows more hot spots especially around the high‐density media than AC in terms of PTV (Max difference: 4.0%) and OAR (Max. difference: 1.9%). Compared to AC, there were larger dose deviations in steep dose gradient region and higher skin‐dose.Conclusion:In head and neck IMRT plans, AXB and AC show different dosimetric performance for the regions inside the target volume around high‐density media, steep dose gradient regions and skin‐surface. There are limitations in skin‐dose and complex anatomic condition using even inhomogeneous anthropomorphic phantom Thus, there is the potential for an increase of hot‐spot in AXB, and an underestimation of dose in substance boundaries and skin regions in AC.

Dosimetric quality, accuracy, and deliverability of modulated radiotherapy treatments for spinal metastases

Cancer often metastasizes to the vertebra, and such metastases can be treated successfully using simple, static posterior or opposed-pair radiation fields. However, in some cases, including when re-irradiation is required, spinal cord avoidance becomes necessary and more complex treatment plans must be used. This study evaluated 16 sample intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) treatment plans designed to treat 6 typical vertebral and paraspinal volumes using a standard prescription, with the aim of investigating the advantages and limitations of these treatment techniques and providing recommendations for their optimal use in vertebral treatments. Treatment plan quality and beam complexity metrics were evaluated using the Treatment And Dose Assessor (TADA) code. A portal-imaging–based quality assurance (QA) system was used to evaluate treatment delivery accuracy, and radiochromic film measurements were used to provide high-resolution verification of treatment plan dose accuracy, especially in the steep dose gradient regions between each vertebral target and spinal cord. All treatment modalities delivered approximately the same doses and the same levels of dose heterogeneity to each planning target volume (PTV), although the minimum PTV doses in the vertebral plans were substantially lower than the prescription, because of the requirement that the plans meet a strict constraint on the dose to the spinal cord and cord planning risk volume (PRV). All plans met required dose constraints on all organs at risk, and all measured PTV-cord dose gradients were steeper than planned. Beam complexity analysis suggested that the IMRT treatment plans were more deliverable (less complex, leading to greater QA success) than the VMAT treatment plans, although the IMRT plans also took more time to deliver. The accuracy and deliverability of VMAT treatment plans were found to be substantially increased by limiting the number of monitor units (MU) per beam at the optimization stage, and thereby limiting beam modulation complexity. The VMAT arcs that were optimized with MU limitation had higher QA pass rates as well as higher modulation complexity scores (less complexity), lower modulation indices (less modulation), lower MU per beam, larger beam segments, and fewer small apertures than the VMAT arcs that were optimized without MU limitation. It is recommended that VMAT treatments for vertebral volumes, where the PTV abuts or surrounds the spinal cord, should be optimized with MU limitation. IMRT treatments may be preferable to the VMAT treatments, for dosimetry and deliverability reasons, but may be inappropriate for some patients because of their increased treatment delivery time.

SU‐E‐T‐727: The Application of Regularization‐Based Multi‐Channel Radiochromic Film Dosimetry for Patient‐Specific QA of Stereotactic Body Radiation Therapy (SBRT) Treatment

Purpose:To examine the feasibility of using radiochromic film with advanced calibration techniques for absolute and relative dose measurement in patient‐specific SBRT QA.Methods:Ten SBRT cases were randomly selected. The treatment plans were mapped to a cylindrical Perspex phantom for QA measurement. A radiochromic film of 8”x10” was cut into a large piece of 20×21cm2 and 2 small pieces of 4×4cm2 each. The large piece was inserted in the mid‐plane of the phantom which was irradiated with the treatment beam. One of the small pieces was irradiated with known dose. All irradiated films were allowed to stabilize for 24 hours before scanning. The protocol of single‐scan with recalibration was followed and the 2 small films were used to adjust the calibration curve. The treatment film was calibrated with the dose gradient regularization method by an in‐house developed MATLab program. The calculated dose planes were then exported to RIT113V4.4. Both treatment films and planned dose planes were normalized to the corresponding prescribed fractional dose for analysis.Results:The analysis was performed by normalizing both the treatment film and the planned dose planes to the prescribed fractional dose. Isodose line comparison was performed to visually check the alignment of the isodose level of steep dose gradient regions. Good agreements were observed in all 10 cases. Gamma index analysis with tolerance of 3% and 2mm was also carried out for quantitative analysis. All 10 cases were reported with a gamma index passing percentage of at least 98.9%, which were higher than the institute's passing criteria of over 95%.Conclusion:By employing recalibration and regularization‐based calibration techniques, radiochromic film can measure both the absolute and relative dose of SBRT treatment plans. This study suggested that radiochromic film, with proper application of advanced calibration methods, can be a promising tool for patient‐specific SBRT QA.

On the Nonequilibrium of Radiation-Induced Bystander Effects in Tumor Surface and Its Implications in Radiation Therapy

This work aims to theoretically show the development of a nonequilibrium of radiation-induced bystander effect (RIBE) under steep dose gradient regions that typically occur in the field edges of a beam. We applied the kinetics model proposed by (McMahon et al. 2013) for in vivo conditions coupled with a hypothesis called “Layer-limited bystander signaling (LLBS)” to demonstrate 1) an enhancement in TCP (i.e. Enhanced TCP or ETCP) due to bystander signals, 2) the development of nonequilibrium of RIBE under steep dose gradient regions and 3) the reduction in ETCP in the surface of Clinical Target Volume (CTV) due to the non-equilibrium of RIBE. We incorporated the elements of RIBE directly in the existing Poisson LQ model available in Pinnacle3 TPS (Version 9.10.0) to compute the percentage reduction of ETCP in the tumor surface due to nonequilibrium of RIBE. The percentage improvement in TCP obtained in tumor surface by accounting for RIBE is about 46% lower than that obtained in the interior of the tumor. This suggests that relatively more number of cancerous cells might survive in the vicinity of tumor surface. The result obtained from the study is indicative of an additional uncertainty component associated with radiation treatment. Hence, this paper suggests that the radiation treatments employing steep dose gradients could be biophysically different in many ways.

An investigation of a PRESAGE® in vivo dosimeter for brachytherapy

Determining accurate in vivo dosimetry in brachytherapy treatment with high dose gradients is challenging. Here we introduce, investigate, and characterize a novel in vivo dosimeter and readout technique with the potential to address this problem. A cylindrical (4 mm × 20 mm) tissue equivalent radiochromic dosimeter PRESAGE® in vivo (PRESAGE®-IV) is investigated. Two readout methods of the radiation induced change in optical density (OD) were investigated: (i) volume-averaged readout by spectrophotometer, and (ii) a line profile readout by 2D projection imaging utilizing a high-resolution (50 micron) telecentric optical system. Method (i) is considered the gold standard when applied to PRESAGE® in optical cuvettes. The feasibility of both methods was evaluated by comparison to standard measurements on PRESAGE® in optical cuvettes via spectrophotometer. An end-to-end feasibility study was performed by a side-by-side comparison with TLDs in an 192Ir HDR delivery. 7 and 8 Gy was delivered to PRESAGE®-IV and TLDs attached to the surface of a vaginal cylinder. Known geometry enabled direct comparison of measured dose with a commissioned treatment planning system. A high-resolution readout study under a steep dose gradient region showed 98.9% (5%/1 mm) agreement between PRESAGE®-IV and Gafchromic® EBT2 Film. Spectrometer measurements exhibited a linear dose response between 0–15 Gy with sensitivity of 0.0133 ± 0.0007 ΔOD/(Gy ⋅ cm) at the 95% confidence interval. Method (ii) yielded a linear response with sensitivity of 0.0132 ± 0.0006 (ΔOD/Gy), within 2% of method (i). Method (i) has poor spatial resolution due to volume averaging. Method (ii) has higher resolution (∼1 mm) without loss of sensitivity or increased noise. Both readout methods are shown to be feasible. The end-to-end comparison revealed a 2.5% agreement between PRESAGE®-IV and treatment plan in regions of uniform high dose. PRESAGE®-IV shows promise for in vivo dose verification, although improved sensitivity would be desirable. Advantages include high-resolution, convenience and fast, low-cost readout.

Dose verification of intensity modulated radiotherapy in head and neck tumors

Purpose: To evaluate the agreement between measured and calculated doses for head and neck tumors using different gamma criteria and to establish quality assurance protocol for the delivery of IMRT in The National Cancer Institute in Cairo. Methods : The dose is calculated for 30 patients using CMS Treatment Planning System. The ionization chamber (0.6 cm 3 Farmer type) is used for point dose measurements. The 2D-array (PTW 729) and GafChromic films (EBT2) are used for 2D graphical dose distribution. Four different gamma criteria of dose difference (DD) and distance to agreement (DTA) (3%/3 mm, 3%/5 mm, 4%/4 mm and 5%/5 mm DD / DTA) are selected. These criteria are evaluated while suppressing the dose of 10%, 20% or 30% from dose distribution. Results: Point dose evaluations using the ion chamber ranged from -2.6% to 3.7% (mean and standard deviation of 0.46 ± 1.7) . Significant differences are observed between the films and 2D-array for all criteria except the 3%/5 mm criteria (96.89 ± 2.2% vs. 94.81 ± 4.2% (p < 0.01)). Conclusion : Differences may exceed about 3% when the ionization chamber is present in steep dose gradient regions. The present results suggest the gamma criteria of 3%/5 mm as the most suitable criteria for IMRT quality assurance. This gamma criterion of 3%/5 mm favorably exceeds 95% in case of maximum dose while suppressing the dose of 20%.The use of 2D-array can reduce the IMRT QA workload. ------------------------------ Cite this article as: Elawady RA, Attalla EM, Elshemey WM, Shouman T, Alsayed AA. Dose verification of intensity modulated radiotherapy in head and neck tumors. Int J Cancer Ther Oncol 2014; 2 (3):02037. DOI : 10.14319/ijcto.0203.7

Does the γ dose distribution comparison technique default to the distance to agreement test in clinical dose distributions?

To determine the validity of the assumption that the γ dose distribution comparison tool defaults to the distance to agreement test under conditions of clinically relevant steep dose gradients and γ test criteria. The assumption was tested by computing the angle θ between the dose axis and γ vector for clinical treatment plans. θ was a function of the evaluated dose distribution dose gradient and the ratio (α) of the dose difference to distance to agreement (DTA) criteria. Dose distributions from prostate, head and neck, and lung clinical treatment plans were examined: 50 treatment plans were selected for each of the prostate and head neck sites and 27 treatment plans were selected for lung. Dose-gradient histograms were prepared for each of the treatment plans using α = 1%/mm (e.g., 3%, 3 mm dose difference and DTA criteria, respectively). To determine how frequently different values of α were used in publications, papers that referenced the original γ paper were examined to identify the dose difference and DTA criteria used in those publications. In order to compare θ calculated using α = 1%/mm to θ for other values of α, the relationship between θ and α was determined. For most of the targets and critical structures, the maximum value of θ approached 90°, so the assumption that the γ tool defaulted to the DTA test in steep dose gradients was correct. Most of the published papers using the γ tool employed the 3%, 3 mm dose difference and DTA criteria, respectively. Most of the other evaluations used criteria such that α ≥ 1%/mm, so the conclusions relating to the examined dose distributions applied. There were a few papers employing very small values of α (including one where α = 0.17%/mm), breaking the assumption that the γ dose comparison tool defaulted to the DTA tool in steep dose gradients. Most published cases utilized values of α ≥ 1%/mm, and for those the implicit assumption that the γ dose comparison tool defaults to the DTA test in steep dose gradient regions was true. There were a few cases for which α was small enough to potentially invalidate this assumption. Care should be taken by investigators when selecting the γ test criteria to assure that the γ test will appropriately default to the DTA test in the steepest dose gradients being evaluated.

SU‐E‐T‐20: Validating That the Gamma Dose Distribution Comparison Defaults to the Distance‐To‐Agreement (DTA) Test in Steep Dose Gradients for Clinical Dose Distributions

Purpose: To test the validity of the central assumption that the gamma dose comparison test defaults to a distance‐to‐agreement (DTA) test in steep dose gradient regions of clinical dose distributions for typically employed dose difference and DTA criteria. Methods: The angle θ between the dose axis and the gamma vector was computed usingθ = tan−1(Λ), where Λ was the local dose gradient of the evaluated dose distribution. θ depended only on the ratio α between the two test criteria and its computation required only one dose distribution. While the most common test criteria were 3%/3mm (α = 1%/mm), we also evaluated 2%/3mm (α = 0.67%/mm) and 3%/2mm (α = 1.5%/mm). Clinical treatment plans were used to calculate θ using the three values of α. 50 cases of both prostate and head and neck and 27 cases for lung cancer were examined. The planning target volumes and selected critical structures were examined. The maximum values of θ were determined for these structures to determine if these maximum θ angles were close to 90 degrees for the selected a values. Results: Most examined targets and critical structures exhibited large values of θ. The exception was for some parotid glands where the doses were very low and relatively homogeneous. The fact that we found that most structures had at least some voxels with sufficiently large dose gradients to yield θ values near 90 degrees validated our hypothesis that for steep gradients, the gamma test defaulted to distance‐to‐agreement test for clinical dose gradients. Conclusion: The implicit assumption that the gamma dose comparison tool defaults to the DTA test in steep dose gradient regions was validated for prostate, had‐and‐neck, and lung cancer sites. While dose distributions from other sites have not been examined, it is likely that they too will yield similar results.

SU‐E‐T‐372: Optimal Selection of Plane and Alignment Based on Quantitative Evaluation Using Three‐Dimensional (3D) Dose Data for IMRT and VMAT Dosimetry

Purpose: Gamma, DTA and dose difference (DD) evaluations could take into account only the two axes on the evaluation plane for IMRT and VMAT plan verification dosimetry. However, some steep dose gradient region on another axis with the errors of measurement setup and the laser alignment can decrease the agreement between the calculation and measurement on the plane. In this study, we developed an optimal selection method of finding optimal plane of the evaluation and adequate alignment of the measurement data using three‐dimensional dose data. Methods: After the isocenter alignment of the measurement data manually, the optimal selection engine reconstructs dose planes at certain locations on the axis perpendicular to the evaluation plane from the 3D data of dose calculation. then, the calculation planes are compared with the measurement plane using gamma evaluation in order to choose the candidates of the best plane of the evaluation. Using the candidates with the combinations with rotations and shifts, DD evaluation is conducted to find out the best plane and the best combination of shift and rotation. To assess the effectiveness of the method, a head and neck IMRT plan measurement was performed and the path ratios of gamma evaluation (3%/3mm criteria) were measured with optimal selection and non‐optimal selection method, respectively. Results: Average pass ratio of gamma evaluation with the optimal selection method was14.2%‐better than with the non‐optimal selection method. Additionally, the method found 0.5mm‐ and 1.5mm‐deviations in the two‐axis on the evaluation plane, respectively. 1.0mm‐deviation was also measured on the axis perpendicular to the plane. Conclusions: IMRT and VMAT verification measurement with the method could provide us not only the dosimetric check of the plan with better result but also the mechanical test of isocenter laser alignment.

SU-E-T-80: Development of IMRT Postal Audit Phantom Using Radiophotoluminescence Glass Dosimeter

Purpose: The purpose of this study is to develop IMRT postal audit phantom using radiophotoluminescence glass dosimeter, and to report initial experiences of the phantom. Method and Materials:We developed the IMRT postal audit phantom. The phantom can be inserted with both RPL glass dosimeter, GD‐302M (Asahi Techno Glass) for point dose verification and Radiochromic film, EBT2 (ISP) for relative planar dose verification. In the dedicated phantom, RPL glass dosimeter can be placed in 4 positions corresponding to target and 1 position to OAR. The EBT2 film can be inserted on the central transverse plane in the phantom. The phantom was made of water substitute material, Tough Water WE‐211 (Kyoto kagaku). for the initial experiences, 7 fields segmental MLCIMRT was planned with XiO (Elekta) using 6 & 10 MV from Clinac iX‐S. In addition, to evaluate the measurement accuracy of RPL glass dosimeter, point doses with ionization chamber (PTW 30013 Farmer type and PTW 31016 Pinpoint 3D) were measured under the same conditions. Results: Differences between the point dose with RPL glass dosimeter and that with Pinpoint 3D chamber agreed within 2% at all positions (high and low dose regions). Point dose differences between with Farmer chamber and with Pinpoint 3D chamber agreed within 1% at target (high dose region). At OAR (low dose and steep dose gradient region), however, the dose difference was exceeded by 5%. This is due to volume averaging effect of Farmer chamber.Conclusion: Feasibility study of IMRT postal audit phantom was performed. The measurement dose with RPL glass dosimeter was good agreement with Pinpoint 3D ionization chamber. Therefore, the dedicated phantom using RPL glass dosimeter shows considerable potential for the IMRT postal audit. To establish IMRT postal audit in Japan, we are starting dummy run now.

PIXEL-BASED CORRECTION METHOD FOR GAFCHROMIC®EBT FILM DOSIMETRY

In this paper, a new approach using a pixel-based correction method was developed to fix the non-uniform responses of flat-bed type scanners used for radiochromic film dosimetry. In order to validate the method’s performance, two cases were tested: the first consisted of simple dose distributions delivered by a single port; the second was a complicated dose distribution composed of multiple beams. In the case of the simple individual dose condition, ten different doses, from 8.3 cGy to 307.1 cGy, were measured, horizontal profiles were analyzed using the pixel-based correcton method and compared with results measured by an ionization chamber and results corrected using the existing correction method. A complicated inverse pyramid dose distribution was made by piling up four different field shapes, which were measured with GAFCHROMIC ® EBT film and compared with the Monte Carlo calculation; as well as the dose distribution corrected using a conventional method. The results showed that a pixel-based correction method reduced dose difference from the reference measurement down to 1% in the flat dose distribution region or 2 mm in a steep dose gradient region compared to the reference data, which were ionization chamber measurement data for simple cases and the MC computed data for the complicated case, with an exception for very low doses of less than about 10 cGy in the simple case. Therefore, the pixel-based scanner correction method is expected to enhance the accuracy of GAFCHROMIC ® EBT film dosimetry, which is a widely used tool for two-dimensional dosimetry.

Gamma Dose Distribution Evaluation Tool

Quantitative comparisons of dose distributions are an integral component of a medical physicist's responsibility of assuring high quality radiation therapy dose delivery. While overlays and other displays of multiple dose distributions are useful for such evaluations, quantitative evaluations require a mathematical comparison. The dose-difference is the most straightforward method for comparing two dose distributions, but it can show large differences in steep dose gradient regions, even for relatively small misalignments. A tool, termed y,was developed to take both dose and spatial difference into account. It does this automatically, evaluating distributions for dose difference and spatial discrepancies in regions of shallow and steep dose gradients, respectively. The tool has been used extensively in commercial dose measurement and evaluation software. This chapter describes the tool, some alternative techniques, and limitations of the tool.

Investigating the feasibility of 3D dosimetry in the RPC IMRT H&N phantom

An urgent requirement for 3D dosimetry has been recognized because of high failure rate (∼25%) in RPC credentialing, which relies on point and 2D dose measurements. Comprehensive 3D dosimetry is likely to resolve more errors and improve IMRT quality assurance. This work presents an investigation of the feasibility of PRESAGE/optical-CT 3D dosimetry in the Radiologic Physics Center (RPC) IMRT H&N phantom. The RPC H&N phantom (with standard and PRESAGE dosimetry inserts alternately) was irradiated with the same IMRT plan. The TLD and EBT film measurement data from standard insert irradiation was provided by RPC. The 3D dose measurement data from PRESAGE insert irradiation was readout using the OCTOPUS™ 5X optical-CT scanner at Duke. TLD, EBT and PRESAGE dose measurements were inter-compared with Eclipse calculations to evaluate consistency of planning and delivery. Results showed that the TLD point dose measurements agreed with Eclipse calculations to within 5% dose-difference. Relative dose comparison between Eclipse dose, EBT dose and PRESAGE dose was conducted using profiles and gamma comparisons (4% dose-difference and 4 mm distance-to-agreement). Profiles showed good agreement between measurement and calculation except along steep dose gradient regions where Eclipse modelling might be inaccurate. Gamma comparisons showed that the measurement and calculation showed good agreement (>96%) if edge artefacts in measurements are ignored. In conclusion, the PRESAGE/optical-CT dosimetry system was found to be feasible as an independent dosimetry tool in the RPC IMRT H&N phantom.

137Cs source dose distribution using the Fricke Xylenol Gel dosimetry

Dosimetric measurements close to radioisotope sources, such as those used in brachytherapy, require high spatial resolution to avoid incorrect results in the steep dose gradient region. In this work the Fricke Xylenol Gel dosimeter was used to obtain the spatial dose distribution. The readings from a 137Cs source were performed using two methods, visible spectrophotometer and CCD camera images. Good agreement with the Sievert summation method was found for the transversal axis dose profile within uncertainties of 4% and 5%, for the spectrophotometer and CCD camera respectively. Our results show that the dosimeter is adequate for brachytherapy dosimetry and, owing to its relatively fast and easy preparation and reading, it is recommended for quality control in brachytherapy applications.

Sci-Fri AM: YIS-08: Prototype fan beam optical computed tomography scanner for three-dimensional dose verification in radiation therapy.

A prototype, rapid, high precision fan beam optical computed tomography (OptCT) scanner for three-dimensional polymer gel dosimetry of complex radiotherapy protocols has been developed. The scanner employs a sixty degree fan beam of 543 nm laser light to interrogate irradiated gel samples up to 19 cm in cross-sectional diameter. Rapid data acquisition with minimal scattered light and minimal refraction and reflection artefacts is achieved with the unique radial design of the front and back apertures of the index matching medium tank, concentric arrangement of a CNC machined collimator and five photodiode detector arrays for light detection. Characteristics of the scanner include: an SNR of optical absorbance in light field projections of up to 100:1; SNR of optical absorbance in transmission projections of up to 92:1 for phantoms with absorbances ranging from 0.1 to 1.0 and a spatial resolution of 0.25 mm. OptCT dose maps extracted from normoxic NIPAM polymer gels irradiated with (a) a rectangular 3×3 cm2 field, (b) a 4-field wedged treatment and (c) a C-shaped IMRT plan are examined and compared to the Eclipse TPS. For these treatments, dose resolution is examined in steep and moderate dose gradient regions using dose difference and gamma function metrics. The effect of different OptCT reconstruction parameters on ring and streaking artefacts is also demonstrated.

SU-FF-T-338: Patterns of Failure for Ion Chamber Based IMRT QA

Purpose: To examine the effect of detector size in the dosimetry of IMRTdelivery. In addition we studied the effect of tongue and groove effect (TGE) and steep dose gradient regions (SDGR) at the point of measurement and their implication in the measured dose.Method and Materials: The advantage of intensity‐modulated radiation therapy(IMRT) over conventional treatment methods is the limited damage to normal tissue, while delivering high doses to target volumes. Because of the complexity associated with the application of IMRT, patient plan quality assurance (QA) is very important. In our department QA measurements can reveal discrepancies larger than 3% of the expected values. Those discrepancies cannot be easily explained by common errors (phantom setup, output variation etc.). Absolute dose measurements of intensity modulated fields were performed in a water tank phantom using ion chambers of three different volumes (0.015cc, 0.056cc and 0.3cc). All measurements were compared to the corresponding calculated doses from the treatment planning system. Film measurements where also taken for every IMRT field in our study in order to examine if the point of measurement was on a SDGR and/or affected by TGE. Results: According to our results a better agreement with the calculated values was shown using the 0.3cc ionization chamber. Discrepancies of more that 3% existed in cases where the point of measurement fell into a SDGR or on areas where TGE was more pronounced, as demonstrated by the film measurements. Conclusions: Preliminary results indicate that using a larger ionization chamber for clinical QA purposes is a possible solution to overcome the positional uncertainty, especially in cases of SDGR or if synchronization is not used for limiting the TGE.