Accuracy Of Treatment Planning System Research Articles

Skin dose measurement and estimating the dosimetric effect of applicator misplacement in gynecological brachytherapy: A patient and phantom study.

To evaluate skin dose differences between TPS (treatment planning system) calculations and TLD (thermo-luminescent dosimeters) measurements along with the dosimetric effect of applicator misplacement for patients diagnosed with gynecological (GYN) cancers undergoing brachytherapy. The skin doses were measured using TLDs attached in different locations on patients' skin in pelvic regions (anterior, left, and right) for 20 patients, as well as on a phantom. In addition, the applicator surface dose was calculated with TLDs attached to the applicator. The measured doses were compared with TPS calculations to find TPS accuracy. For the phantom, different applicator shifts were applied to find the effect of applicator misplacement on the surface dose. The mean absolute dose differences between the TPS and TLDs results for anterior, left, and right points were 3.14±1.03, 6.25±1.88, and 6.20±1.97 %, respectively. The mean difference on the applicator surface was obtained 1.92±0.46 %. Applicator misplacements of 0.5, 2, and 4 cm (average of three locations) resulted in 9, 36, and 61%, dose errors respectively. The surface/skin differences between the calculations and measurements are higher in the left and right regions, which relate to the higher uncertainty of TPS dose calculation in these regions. Furthermore, applicator misplacements can result in high skin dose variations, therefore it can be an appropriate quality assurance method for future research.

Test cases for commissioning an add-on micro multi-leaf collimator Apex for stereotactic radiosurgery treatments

Two comprehensive test cases are presented for dosimetric commissioning of a radiosurgery system, in a hospital where dedicated phantoms are not available. The system consisted of an Elekta Precise linear accelerator, an Apex micro multi-leaf collimator, and a Monaco treatment planning system (TPS). The purpose of Test I was to assess the dose accuracy with coplanar arc beams. Test II was an end-to-end type test, a rigid Leksell stereotactic frame was fixed to a watermelon phantom and Ergo++ TPS was used for stereotactic coordinates definition. The purpose of Test II was to assess the dose accuracy with non-coplanar arc beams and the influence of geometrical accuracy in the whole process. Ionization chambers were used for dose measurements. Results of Test I showed that discrepancies below 1% are achievable, while results of Test II allowed detection of geometric shifts < 1 mm with dose discrepancies lower than 1%. To the best of our knowledge, there are not published works reporting test cases for commissioning a stereotactic radiosurgery system like the one tested in this work. The designed test cases showed adequacy for assessment of TPS accuracy in complex treatment configurations, like those used in stereotactic radiosurgery.

Assessing the accuracy of treatment planning system based radiotherapy structure volumes

AbstractAim:The purpose of the present study was to assess the accuracy of radiotherapy (RT) structure volume generated by the Monaco treatment planning system (TPS) for three different computed tomography (CT) slice thicknesses. Further, this study addressed the important issue of ‘different volumes of the same RT structure shown at different places’ in the Monaco TPS. Also, the practical impact of this difference in structure volumes has been studied for brain or head and neck patients.Materials and Methods:Objects of known volumes were scanned with different CT slice thicknesses and contoured as an RT structure in Monaco TPS and two different volumes provided by the TPS for each RT structure were noted and compared with the real volumes of these objects. In addition, correlation was also assessed between TPS provided volumes and real volumes of these objects. The study was further extended to obtain correlation of volumes in cases of organs that exist in pairs (e.g., eye) in the human body.Results:Monaco TPS overestimates structure volumes except for objects with sharp corners. Although, volumes shown at different places of the same structure have nearly a linear correlation, volumes under structure table are more accurate than those shown under dose–volume histogram (DVH) statistics (total volume) table. Difference in magnitude between these two volumes has no correlation if this difference is analysed for paired organs.Findings:This study confirmed that Monaco TPS provides ‘different value at different places’ of the volume of a given contoured structure. It is recommended that this issue should be reviewed and resolved by the supplier.

Three dimensional film dosimetry of photon beam in small field sizes and beyond the heterogeneous regions using a GAFchromic films array

IntroductionInaccurate dose prediction by treatment planning system (TPS) beyond the heterogeneous regions has been a great challenge in radiotherapy, particularly when small field sizes are used. Better understanding of this inaccuracy that depends on dosimetry methods is very crucial. Recently, the film stack dosimeter has been purposed for 3D comprehensive dosimetry. This study proposes an approach to investigate the accuracy of TPS for predicting dose distribution in small field sizes in the presence of nasopharynx heterogeneities by 3D stack film. Material & methodsThe dose distribution and dose profile of rectangular heterogeneous nasopharynx (RHN) phantom with two air cavities and bone equivalent were measured with a 3D stack film dosimeter containing 9 GAFchromic-EBT2 films positioned beyond the heterogeneity regions. The film was validated using MCNPX-Monte Carlo (MC) simulation. The evaluation of TPS inaccuracy in small field sizes (3 × 3, 4 × 4, 5 × 5 cm2) was determined by comparing reconstructed 3D dose distribution of stack film dosimeter with TPS based on full scatter convolution (FSC) using in-house Matlab code. ResultsThe film measurements had a good agreement with the MC calculation for small fields in RHN phantom. The results showed a large discrepancy between stack film measurements and TPS calculations in the volumes enclosed high isodoses, so that the biggest difference occurred in 3 × 3 cm2 field size (relative mean difference = 1.72 ± 0.24, 1.5 ± 0.16 and P-value = 0.002, 0.001 for V95 and V90 respectively) and it decreased in 5 × 5 cm2 field size (relative mean difference = 0.46 ± 0.05, 0.35 ± 0.04 and P-value = 0.001, 0.002 for V95 and V90 respectively). ConclusionThe results suggested that 3D stack film dosimetry can be successfully used as a reliable dosimeter for QA procedure of heterogeneous region in small field sizes. Comparison of the measured and calculated dose volumes demonstrated that the TPS based FSC algorithm has a notable inaccuracy in estimation of dose distribution beyond the nasopharynx heterogeneities. Therefore: it seems heterogeneity corrections should be considered for nasopharynx treatment.

Assessment of leakage dose in vivo in patients undergoing radiotherapy for breast cancer.

Background and purposeAccurate quantification of the relatively small radiation doses delivered to untargeted regions during breast irradiation in patients with breast cancer is of increasing clinical interest for the purpose of estimating long-term radiation-related risks. Out-of-field dose calculations from commercial planning systems however may be inaccurate which can impact estimates for long-term risks associated with treatment. This work compares calculated and measured dose out-of-field and explores the application of a correction for leakage radiation.Materials and methodsDose calculations of a Boltzmann transport equation solver, pencil beam-type, and superposition-type algorithms from a commercial treatment planning system (TPS) were compared with in vivo thermoluminescent dosimetry (TLD) measurements conducted out-of-field on the contralateral chest at points corresponding to the thyroid, axilla and contralateral breast of eleven patients undergoing tangential beam radiotherapy for breast cancer.ResultsOverall, the TPS was found to under-estimate doses at points distal to the radiation field edge with a modern linear Boltzmann transport equation solver providing the best estimates. Application of an additive correction for leakage (0.04% of central axis dose) improved correlation between the measured and calculated doses at points greater than 15 cm from the field edge.ConclusionsApplication of a correction for leakage doses within peripheral regions is feasible and could improve accuracy of TPS in estimating out-of-field doses in breast radiotherapy.

Evaluation of dose calculations accuracy of a commercial treatment planning system for the head and neck region in radiotherapy.

The objective was to quantify dose calculation accuracy of TiGRT TPS for head and neck region in radiotherapy. In radiotherapy of head and neck cancers, treatment planning is difficult, due to the complex shape of target volumes and also to spare critical and normal structures. These organs are often very near to the target volumes and have low tolerance to radiation. In this regard, dose calculation accuracy of treatment planning system (TPS) must be high enough. Thermoluminescent dosimeter-100 (TLD-100) chips were used within RANDO phantom for dose measurement. TiGRT TPS was also applied for dose calculation. Finally, difference between measured doses (Dmeas) and calculated doses (Dcalc) was obtained to quantify the dose calculation accuracy of the TPS at head and neck region. For in-field regions, in some points, the TiGRT TPS overestimated the dose compared to the measurements and for other points underestimated the dose. For outside field regions, the TiGRT TPS underestimated the dose compared to the measurements. For most points, the difference values between Dcalc and Dmeas for the in-field and outside field regions were less than 5% and 40%, respectively. Due to the sensitive structures to radiation in the head and neck region, the dose calculation accuracy of TPSs should be sufficient. According to the results of this study, it is concluded that the accuracy of dose calculation of TiGRT TPS is enough for in-field and out of field regions.

Assessment of Dose Calculation Accuracy of TiGRT Treatment Planning System for Physical Wedged fields in Radiotherapy

Introduction Wedge modifiers are commonly applied in external beam radiotherapy to change the dose distribution corresponding to the body contour and to obtain a uniform dose distribution within the target volume. Since the radiation dose delivered to the target must be within ±5% of the prescribed dose, accurate dose calculation by a treatment planning system (TPS) is important. The objective of the present study was to quantify the dose calculation accuracy of TiGRT TPS for physical wedged fields in radiotherapy. Materials and Methods A Semiflex™ ionization chamber was used for dose measurements in a water phantom; TiGRT TPS was also applied for dose calculations. The central axis (i.e., high dose-small dose gradient), build-up (i.e., high dose-large dose gradient), off-axis (i.e., high dose-small dose gradient), and out-of-field (i.e., low dose-small dose gradient) regions were evaluated in this study. Finally, the confidence limit values were obtained to quantify the dose calculation accuracy of TPS in these regions. Results The confidence limit values for the central axis, build-up, off-axis, and out-of-field regions were 1.01, 8.62, 1.79, and 55.24, respectively. Furthermore, the results showed that TiGRT TPS underestimated the dose of build-up and out-of-field regions for most points. Conclusion According to the results of the present study, it can be concluded that the dose calculation accuracy of TiGRT TPS for physical wedged fields in the central axis, build-up, and off-axis regions is adequate, while it is insufficient for out-of-field regions.

Dosimetric evaluation of the dose calculation accuracy of different algorithms for two different treatment techniques during whole breast irradiation

Purpose In this study, dosimetric comparison of field in field (FIF) and intensity modulated radiation therapy (IMRT) techniques used for treatment of whole breast radiotherapy (WBRT) were made. The dosimetric accuracy of treatment planning system (TPS) for Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB) algorithms in predicting PTV and OAR doses was also investigated. Methods Two different treatment planning techniques of left-sided breast cancer were generated for rando phantom. FIF and IMRT plans were compared for doses in PTV and OAR volumes including ipsilateral lung, heart, left ascending coronary artery, contralateral lung and the contralateral breast. PTV and OARs doses and homogeneity and conformality indexes were compared between two techniques. The accuracy of TPS dose calculation algorithms was tested by comparing PTV and OAR doses measured by thermoluminescent dosimetry with the dose calculated by the TPS using AAA and AXB for both techniques. Results IMRT plans had better conformality and homogeneity indexes than FIF technique and it spared OARs better than FIF. While both algorithms overestimated PTV doses they underestimated all OAR doses. For IMRT plan, PTV doses, overestimation up to 2.5% was seen with AAA algorithm but it decreased to 1.8% when AXB algorithm was used. Based on the results of the anthropomorphic measurements for OAR doses, underestimation greater than 7% is possible by the AAA. The results from the AXB are much better than the AAA algorithm. However, underestimations of 4.8% were found in some of the points even for AXB. For FIF plan, similar trend was seen for PTV and OARs doses in both algorithm. Conclusions When using the Eclipse TPS for breast cancer, AXB the should be used instead of the AAA algorithm, bearing in mind that the AXB may still underestimate all OAR doses.

Dosimetric evaluation of Monte Carlo based treatment planning system in antorphomorpic phantom

Introduction Implementation of advanced radiotherapy techniques makes high demands on the treatment planning system (TPS). Purpose Dosimetric evaluation of calculation algorithm built in Monte Carlo (MC) based TPS. Materials and methods Dose distributions of different complexity were calculated using Elekta Monaco TPS. Both calculation methods (Dose to Water-D2W and Dose to Medium-D2M) for 6MV photon beam were verified using 2D detector and ionization chambers of different volumes in inhomogenous phantom. Results Comparison of measured and calculated data in point of ‘tissue’ and low density medium show good agreement regardless the calculation method. In high density medium deviation was much higher. Also, deviations of measured and values calculated as D2W and D2M, respectively, were larger and of opposite signs. To overcome this we propose to define minimum volume of water cylinder to be placed within high density medium. This volume should allow that D2W and D2M calculated doses differ less than 0.5% and consequently good agreement with measurements. Furthermore, 2D methodology used for patient specific dosimetry was used to evaluate TPS accuracy, but antrophomorphic thorax phantom was used instead of the homogeneous one. Anlayze shows that gamma criterion 3% or 3mm was not fulfilled when inhogeneities are inside treated volume. Conclusion MC TPS evaluation using point dose measurements show large uncertainties within high density medium. It can be overcome by using proposed approach. Also, despite the manufacturer’s claim of 2D verification superiority over point measurements, our investigation shows otherwise when measurements in inhomogeneous phantom are considered.

SU-F-T-363: Monte Carlo Dose Verification for Volumetric Modulated Arc Therapy in Multiple Brain Metastases

Purpose: To verify the calculation accuracy of treatment planning system (TPS)-VMAT plans in multiple brain metastases, by EGSnrc Monte Carlo (MC)-based VMAT calculations. Methods: First, six VMAT plans for four patients with multiple brain metastases were created by analytical anisotropic algorithm in an Eclipse Ver. 8.6.17 (AAA8). The accuracy of TPS-VMAT dose distributions for three algorithms of AAA8 and AAA10 and Acuros XB (AXB10) in the Eclipse Ver. 10.0.24 was verified by MCbased VMAT calculations. Similarly, a set of TPS-VMAT dose distributions were also verified by MC under the identical calculation parameters as patients in a water-equivalent phantom. Dose distributions for PTVs and OARs were quantitatively verified from dose volume histograms (DVHs) and passing rates by three dimensional (3-D) gamma analysis. Results: The dose differences between each algorithm and MC for six VMAT plans were 12.2% at a maximum and 3.5 ± 2.4% on average for D9 5 in PTVs, and 1.8% or less for mean doses in OARs in patients. Similarly, the dose differences in a water-equivalent phantom were improved up to 5.1% and 0.6 ± 1.9% for PTVs and 1.8% or less for OARs. The passing rates with 3%/3 mm criteria in PTVs were up to 57.3% in patients and 80.0% in a waterequivalent phantom. Dose differences of DVHs and the reduction of passing rates for AAA in patients may be attributed to the calculation accuracy for inhomogeneous tissues like bone around target. Similarly, for AXB10 they may result from differences in CT numbers and mass densities of materials assigned to CT images in MC and AXB10, and cut-off energy (AXB10: 0.5 MeV, MC: 0.189 MeV). Conclusion: DVHs and passing rates by 3-D gamma analysis for every structure based on MC-based VMAT dose distributions are useful for dose verification of VMAT plans with multiple brain metastases.

SU‐F‐SPS‐04: Dosimetric Evaluation of the Dose Calculation Accuracy of Different Algorithms for Two Different Treatment Techniques During Whole Breast Irradiation

Purpose:In this study, dosimetric comparison of field in field (FIF) and intensity modulated radiation therapy (IMRT) techniques used for treatment of whole breast radiotherapy (WBRT) were made. The dosimetric accuracy of treatment planning system (TPS) for Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB) algorithms in predicting PTV and OAR doses was also investigated.Methods:Two different treatment planning techniques of left‐sided breast cancer were generated for rando phantom. FIF and IMRT plans were compared for doses in PTV and OAR volumes including ipsilateral lung, heart, left ascending coronary artery, contralateral lung and the contralateral breast. PTV and OARs doses and homogeneity and conformality indexes were compared between two techniques. The accuracy of TPS dose calculation algorithms was tested by comparing PTV and OAR doses measured by thermoluminescent dosimetry with the dose calculated by the TPS using AAA and AXB for both techniques.Results:IMRT plans had better conformality and homogeneity indexes than FIF technique and it spared OARs better than FIF. While both algorithms overestimated PTV doses they underestimated all OAR doses. For IMRT plan, PTV doses, overestimation up to 2.5 % was seen with AAA algorithm but it decreased to 1.8 % when AXB algorithm was used. Based on the results of the anthropomorphic measurements for OAR doses, underestimation greater than 7 % is possible by the AAA. The results from the AXB are much better than the AAA algorithm. However, underestimations of 4.8 % were found in some of the points even for AXB. For FIF plan, similar trend was seen for PTV and OARs doses in both algorithm.Conclusion:When using the Eclipse TPS for breast cancer, AXB the should be used instead of the AAA algorithm, bearing in mind that the AXB may still underestimate all OAR doses.

Dosimetric accuracy of a treatment planning system for actively scanned proton beams and small target volumes: Monte Carlo and experimental validation

This study was performed to evaluate the accuracy of a commercial treatment planning system (TPS), in optimising proton pencil beam dose distributions for small targets of different sizes (5–30 mm side) located at increasing depths in water. The TPS analytical algorithm was benchmarked against experimental data and the FLUKA Monte Carlo (MC) code, previously validated for the selected beam-line. We tested the Siemens syngo® TPS plan optimisation module for water cubes fixing the configurable parameters at clinical standards, with homogeneous target coverage to a 2 Gy (RBE) dose prescription as unique goal. Plans were delivered and the dose at each volume centre was measured in water with a calibrated PTW Advanced Markus® chamber. An EBT3® film was also positioned at the phantom entrance window for the acquisition of 2D dose maps. Discrepancies between TPS calculated and MC simulated values were mainly due to the different lateral spread modeling and resulted in being related to the field-to-spot size ratio. The accuracy of the TPS was proved to be clinically acceptable in all cases but very small and shallow volumes. In this contest, the use of MC to validate TPS results proved to be a reliable procedure for pre-treatment plan verification.

Monte Carlo calculation of shielded colpostat effect on rectum received dose in high dose rate brachytherapy with Cobalt-60 sources

Background: In the current study the effect of shielded colpostat on rectum received dose was calculated in cervical brachytherapy using Monte Carlo (MC) method. Materials and Methods: Two 60 Co sources of GZP6 brachytherapy unit used for intracavitary treatments were simulated using MCNPX Monte Carlo code. Also the two types of colpostats including shielded and unshielded were simulated inside a water phantom. The radial dose func'on, depth doses and dose distribu'on around sources were calculated with spa'al dose resolu'on of 2×2×2 mm. The effect of tungsten shields on rectum dose was studied. Moreover, the accuracy of treatment planning system (TPS) was verified comparing to MC results. Results : Our results showed that shielded colpostats reduce the rectum dose up to 7% according to the MC calcula'ons. Also, it was found less than 3% dif ference between TPS and MC results in percentage depth dose and radial doses for unshielded case. Conclusion: It can be concluded that the applica'on of shielded colpostats provide a rela'vely mild protec'on (7% reduc'on in dose) for the rectum in intracavitary brachytherapy due to the higher energy of 60 Co

SU-E-T-586: Field Size Dependence of Output Factor for Uniform Scanning Proton Beams: A Comparison of TPS Calculation, Measurement and Monte Carlo Simulation

Purpose: Output dependence on field size for uniform scanning beams, and the accuracy of treatment planning system (TPS) calculation are not well studied. The purpose of this work is to investigate the dependence of output on field size for uniform scanning beams and compare it among TPS calculation, measurements and Monte Carlo simulations. Methods: Field size dependence was studied using various field sizes between 2.5 cm diameter to 10 cm diameter. The field size factor was studied for a number of proton range and modulation combinations based on output at the center of spread out Bragg peak normalized to a 10 cm diameter field. Three methods were used and compared in this study: 1) TPS calculation, 2) ionization chamber measurement, and 3) Monte Carlos simulation. The XiO TPS (Electa, St. Louis) was used to calculate the output factor using a pencil beam algorithm; a pinpoint ionization chamber was used for measurements; and the Fluka code was used for Monte Carlo simulations. Results: The field size factor varied with proton beam parameters, such as range, modulation, and calibration depth, and could decrease over 10% from a 10 cm to 3 cm diameter field for a large range proton beam. The XiO TPS predicted the field size factor relatively well at large field size, but could differ from measurements by 5% or more for small field and large range beams. Monte Carlo simulations predicted the field size factor within 1.5% of measurements. Conclusion: Output factor can vary largely with field size, and needs to be accounted for accurate proton beam delivery. This is especially important for small field beams such as in stereotactic proton therapy, where the field size dependence is large and TPS calculation is inaccurate. Measurements or Monte Carlo simulations are recommended for output determination for such cases.

A Brief Note on the Limitation of Treatment Planning in Proton Therapy

Currently, proton therapy is being used for the cancer treatment at 11 centers in the United States of America [1], and there is an increasing interest in using proton therapy at many other institutions, which have proton centers either at the planning stage or under construction. [1] The primary reasons of using protons over photons for the cancer treatment is because of proton's physical properties such as finite beam range, near-zero exit dose, and sharp lateral penumbra. [2,3] Treatment planning in proton therapy is done with an aim of depositing the majority of the dose in the tumor, which is typically covered by the spread-out Bragg peak (SOBP) and avoiding the proton beam ranging into the critical structure. In uniform scanning proton therapy, the beam delivery system deposits an uniform dose for a near rectangular scanning area after the degraded proton beam is scanned laterally. [4] One of the commercially available treatment planning systems (TPS) for the uniform scanning proton therapy is XiO (CMS Inc., St. Louis, MO). The commissioning data in the XiO TPS are generally based on the measurements performed in water medium. Recently, it was reported that the XiO TPS could overestimate the lateral penumbra in uniform scanning proton therapy planning, and this study was based on the measurements done in the homogeneous (water) medium. [5] In real clinical situations, however, proton beams may pass through the inhomogeneous media consisting of tissues with different electron densities. Although the impact of heterogeneities on the measured lateral penumbra may be minimal [6], it is necessary to investigate the accuracy of TPS in predicting lateral penumbra in the presence of low-and high-density heterogeneities for various beam conditions. Additionally, it is imperative to investigate the limitation of TPS in predicting proton range for different clinical situations. The dependency of range uncertainties on the tumor location also needs further investigation since same range uncertainty value may not be applicable for all the proton beams. At present, commercially available TPS in proton therapy do not incorporate dose contribution due to secondary neutrons. [7] The results from different studies on neutron dose in proton therapy may not agree with each other due to dependency of neutron production on the treatment conditions and beam shaping components. [7] Hence, it is recommended to carry out an independent neutron study at the proton center before treating cancer patients using proton therapy technique.

A comprehensive dosimetric comparison between 131Cs and 125I brachytherapy sources for COMS eye plaque implant

Purpose To verify the dosimetric characteristics of 131Cs source in the Collaborative Ocular Melanoma Study (COMS) eye plaque brachytherapy, to compare 131Cs with 125I in a sample implant, and to examine the accuracy of treatment planning system in dose calculation. Methods and Materials Monte Carlo (MC) technique was used to generate three-dimensional dose distributions of a 16-mm COMS eye plaque loaded with 131Cs and 125I brachytherapy sources separately. A spherical eyeball, 24.6 mm in diameter, and an ellipsoidal tumor, 6 mm in height and 12 mm in diameter, were used to evaluate the doses delivered. The simulations were carried out both with and without the gold and gold alloy plaque. A water-equivalent seed carrier was used instead of the silastic insert designed for the traditional COMS eye plaque. The 13 sources involved were also individually simulated to evaluate the intersource effect. In addition, a treatment planning system was used to calculate the doses at the central axis for comparison with MC data. Results The gold plaque had significantly reduced the dose in the tumor volume; at the prescription point of this study, that is, 6 mm from the edge of inner sclera, the gold plaque reduced the dose by about 7% for both types of 131Cs and 125I sources, but the gold alloy plaque reduced the dose only by 4% for both types of sources. The intersource effect reduced the dose by 2% for both types of sources. At the same prescription dose, the treatment with the gold plaque applicator tended to create more hot regions for either type of sources than were seen with the homogeneous water phantom. The doses of TPS agree with the MC. Conclusion The 131Cs source is comparable to the 125I source in the eye plaque brachytherapy. The TPS can provide accurate dose calculations for eye plaque implants with either type of sources.

Application of Monte Carlo calculations for validation of a treatment planning system in high dose rate brachytherapy

Aim The accuracy of treatment planning systems is of vital importance in treatment outcomes in brachytherapy. In the current study the accuracy of dose calculations of a high dose rate (HDR) brachytherapy treatment planning system (TPS) was validated using the Monte Carlo method. Materials and methods Three 60Co sources of the GZP6 afterloading brachytherapy system were modelled using MCNP4C Monte Carlo (MC) code. The dose distribution around all the sources was calculated by MC and a dedicated treatment planning system. The results of both methods were compared. Results There was good agreement (<2%) between TPS and MC calculated dose distributions except at a point near the sources (<1 cm) and beyond the tip of the sources. Conclusions Our study confirmed the accuracy of TPS calculated dose distributions for clinical use in HDR brachytherapy.

SU‐FF‐T‐139: Dosimetric Effect of Jaws for Small MLC Fields in 6 and 15 MV Photon Beam

Purpose: This study measures how the relative position of the jaws above the MLC could affect the dose characteristics of a small field. The study focused on the variations in the percentage depth dose (PDD), beam profile and output of the small field when the jaw‐ends were moved away from the leaf‐ends, that is, when the leaf leakage/transmission and jaw scattering were changed. Method and Materials: A big scanning water tank system (RFA 300, Scanditronix Medical AB) generally used in the commissioning was used to measure the beam characteristics. A photon diode (Scanditronix Medical AB, PDF‐3G) was used to measure both the PDD and beam profile for both 6 and 15 MV photon beam. To measure the output, a micro‐ionization chamber (Scanditronix Medical AB, RK8304) was used. Results: It is found that moving the jaw to different positions away from the leaf‐ends will increase the output and penumbra width for the small fields. Such increase is particularly more significant when the field size is small (0.5 × 0.5 cm2), and when the jaw‐end is at about 1 to 2 cm from the leaf‐end, when the degree of increase changes quickly. Conclusion: Measurement is important in Intensity Modulated Radiotherapy because the jaw cannot cover all the leaf‐ends in a segment of irregular field completely. This results in additional dose contributed by (1) the end surface of the jaw, (2) the leaf‐end and (3) the inter‐ and intra‐leaf leakage/transmissions during the dosimetric measurement. In addition, most of the conventional treatment planning systems ignore these effects in the dose calculation. It is suggested that similar measurements will be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system's accuracy with regard to the leaf leakage/transmission and the jaw effects.

IMRT with the sliding window: Comparison of the static and dynamic methods. Dosimetric and spectral analysis

Background and purpose Aim of the study is the critical appraisal of the two delivery techniques for intensity-modulated treatments commonly known as dynamic and static step and shoot, in the framework of a sliding window multileaf sequencing method. Methods The study was performed using the solution commercialised by Varian with the Eclipse treatment planning system (TPS) and the Clinac accelerator. For a set of intensity modulated fluences, the calculated dose maps and the dose delivered to films were compared for the static and dynamic modes to verify the capability of the TPS to accurately model both the delivery modes. For these investigations, the γ concept of Low et al. [Low D, Harms W, Mutic S, Purdy J. A technique for the quantitative evaluation of dose distributions. Med Phys 1998;25: 656–60] was applied demonstrating, in general, optimal modeling for both static and dynamic tests. Optimal and actual fluences, were analysed to ascertain the degree of the TPS accuracy in converting ‘ideal’ maps into realistic leaf motions. Results Among the methods used, the Webb's Modulation Index [Webb S. Use of a quantitative index of beam modulation to characterize dose conformality: illustration by a comparison of full beamlet IMRT, few-segment IMRT and conformal unmodulated radiotherapy. Phys Med Biol. 2003;48: 2051–2062] proved to be useful. Also, dose volume histogram analysis was applied on deliverable plans using more clinical tools. Conclusion The dynamic delivery method seems to offer a more reliable agreement with the optimal calculations and, clinically, a slightly superior performance in terms of target coverage.

Dosimetric effect of collimating jaws for small multileaf collimated fields

The dosimetric effects from the jaw positioned close to the small field (0.5 x 0.5, 1 x 1, and 2 x 2 cm2) side-edge generated by a single-focused multileaf collimator (MLC) were measured and studied. The measurement is important in intensity modulated radiotherapy (IMRT) because generally the jaw cannot perfectly cover all the leaf-ends in a segment of irregular field. This leads to additional dose contributed by (1) the end surface of the jaw, (2) the leaf-end, and (3) the inter- and intraleaf leakage/transmissions during the dosimetric measurement. Moreover, most of the conventional treatment planning systems ignore these effects in the dose calculation. In this study, measurements were made using a Varian 21 EX linear accelerator with 6 MV photon beam through a MLC containing 120 leaves. Percentage depth dose, beam profile, and output for small fields were measured by varying the jaw at different positions away from the leaf-ends in the field side-edge. Moving the jaw away from the leaf-ends increases the output and penumbra width for the small fields. Such increase is particularly significant when the field size is small (0.5 x 0.5 cm2) and the degree of increase changes quickly when the jaw-end is at about 1-2 cm from the leaf-end. It is suggested that measurements should be carried out in the IMRT commissioning to provide information to physicists in reviewing the treatment planning system's accuracy regarding leaf leakage/transmission and jaw effects.