Isodose Contours Research Articles

Independent calculation of monitor units for VMAT and SPORT.

Dose and monitor units (MUs) represent two important facets of a radiation therapy treatment. In current practice, verification of a treatment plan is commonly done in dose domain, in which a phantom measurement or forward dose calculation is performed to examine the dosimetric accuracy and the MU settings of a given treatment plan. While it is desirable to verify directly the MU settings, a computational framework for obtaining the MU values from a known dose distribution has yet to be developed. This work presents a strategy to calculate independently the MUs from a given dose distribution of volumetric modulated arc therapy (VMAT) and station parameter optimized radiation therapy (SPORT). The dose at a point can be expressed as a sum of contributions from all the station points (or control points). This relationship forms the basis of the proposed MU verification technique. To proceed, the authors first obtain the matrix elements which characterize the dosimetric contribution of the involved station points by computing the doses at a series of voxels, typically on the prescription surface of the VMAT/SPORT treatment plan, with unit MU setting for all the station points. An in-house Monte Carlo (MC) software is used for the dose matrix calculation. The MUs of the station points are then derived by minimizing the least-squares difference between doses computed by the treatment planning system (TPS) and that of the MC for the selected set of voxels on the prescription surface. The technique is applied to 16 clinical cases with a variety of energies, disease sites, and TPS dose calculation algorithms. For all plans except the lung cases with large tissue density inhomogeneity, the independently computed MUs agree with that of TPS to within 2.7% for all the station points. In the dose domain, no significant difference between the MC and Eclipse Anisotropic Analytical Algorithm (AAA) dose distribution is found in terms of isodose contours, dose profiles, gamma index, and dose volume histogram (DVH) for these cases. For the lung cases, the MC-calculated MUs differ significantly from that of the treatment plan computed using AAA. However, the discrepancies are reduced to within 3% when the TPS dose calculation algorithm is switched to a transport equation-based technique (Acuros™). Comparison in the dose domain between the MC and Eclipse AAA/Acuros calculation yields conclusion consistent with the MU calculation. A computational framework relating the MU and dose domains has been established. The framework does not only enable them to verify the MU values of the involved station points of a VMAT plan directly in the MU domain but also provide a much needed mechanism to adaptively modify the MU values of the station points in accordance to a specific change in the dose domain.

Establishing implantation uncertainties for focal brachytherapy with I-125 seeds for the treatment of localized prostate cancer

Background. The efficacy of focal continuous low dose-rate brachytherapy (CLDR-BT) for prostate cancer requires that appropriate margins are applied to ensure robust target coverage. In this study we propose a method to establish such margins by emulating a focal treatment in patients treated with CLDR-BT to the entire gland.Material and methods. In 15 patients with localized prostate cancer, prostate volumes and dominant intra-prostatic lesions were delineated on pre-treatment magnetic resonance imaging (MRI). Delineations and MRI were registered to trans-rectal ultrasound images in the operating theater. The patients received CLDR-BT treatment to the total prostate volume. The implantation consisted of two parts: an experimental focal plan covering the dominant intra-prostatic lesion (F-GTV), followed by a plan containing additional seeds to achieve entire prostate coverage. Isodose surfaces were reconstructed using follow-up computed tomography (CT). The focal dose was emulated by reconstructing seeds from the focal plan only. The distance to agreement between planned and delivered isodose surfaces and F-GTV coverage was determined to calculate the margin required for robust treatment.Results. If patients had been treated only focally, the target volume would have been reduced from an average of 40.9 cm3 for the entire prostate to 5.8 cm3 for the focal plan. The D90 for the F-GTV in the focal plan was 195 ± 60 Gy, the V100 was 94% [range 71–100%]. The maximum distance (cd95) between the planned and delivered isodose contours was 0.48 cm.Conclusions. This study provides an estimate of 0.5 cm for the margin required for robust coverage of a focal target volume prior to actually implementing a focal treatment protocol.

SU-E-T-363: Experimentally Validated Pencil Beam Scanning Source Model in TOPAS

Purpose: Monte Carlo method can provide the most accurate dose calculation for pencil beam scanning (PBS) proton therapy if the proton sources and particle interaction mechanisms are correct. Methods: TOPAS 1.8, a simulation tool based on Geant4.9.6, was utilized to simulate proton spot profiles. Proton sources, placed at the phantom surface, were modelled with three two-dimensional Gaussian functions to fit measured in-air spot profiles up to 100 mm radius. Simulations were compared with profiles measured using EBT3 film in Solidwater phantoms at various depths from the surface to the end of range for 100, 115, 150, 180, 210 and 225 MeV proton beams with phantom surface locations at 270 mm upstream and at isocentre. Results: Simulation can reproduce one-dimensional integral dose versus radius within 1 mm/1% and two-dimensional profiles within 1 mm out to 0.1% of the central spot dose for all the studied depths of all energies. For two-dimensional 0.01% isodose, simulation can reproduce all 210 MeV proton beam measurements, but cannot predict the diamond-shaped isodose distributions of the 115 MeV beam. When the proton spots are scanned 100 mm off axis, no profile difference can be found between the central axis and the off axis locations for the 210 MeV beam. For the 115 MeV beam, differences of up to 2 mm are observed at the ends of the diamond-shaped 0.01% isodose contour, implying there are lower energy components in the energy spectrum which are less than 4% of the primary energy. Subsequent simulation demonstrates that adopting such an energy spectrum within the source model can improve the distance agreement by 5–6 mm for isodoses below 0.1% near the end of range of the 115 MeV proton beam. Conclusion: Excellent agreement between simulation and measurement validates our PBS source model and the particle interaction mechanism embedded in TOPAS.

TH‐E‐BRF‐04: Characterizing the Response of Texture‐Based CT Image Features for Quantification of Radiation‐Induced Normal Lung Damage

Purpose:Radiation induced lung damage (RILD) is an important dose‐limiting toxicity for patients treated with radiation therapy. Scoring systems for RILD are subjective and limit our ability to find robust predictors of toxicity. We investigate the dose and time‐related response for texture‐based lung CT image features that serve as potential quantitative measures of RILD.Methods:Pre‐ and post‐RT diagnostic imaging studies were collected for retrospective analysis of 21 patients treated with photon or proton radiotherapy for NSCLC. Total lung and selected isodose contours (0–5, 5–15, 15–25Gy, etc.) were deformably registered from the treatment planning scan to the pre‐RT and available follow‐up CT studies for each patient. A CT image analysis framework was utilized to extract 3698 unique texture‐based features (including co‐occurrence and run length matrices) for each region of interest defined by the isodose contours and the total lung volume. Linear mixed models were fit to determine the relationship between feature change (relative to pre‐RT), planned dose and time post‐RT.Results:Seventy‐three follow‐up CT scans from 21 patients (median: 3 scans/patient) were analyzed to describe CT image feature change. At the p=0.05 level, dose affected feature change in 2706 (73.1%) of the available features. Similarly, time affected feature change in 408 (11.0%) of the available features. Both dose and time were significant predictors of feature change in a total of 231 (6.2%) of the extracted image features.Conclusion:Characterizing the dose and time‐related response of a large number of texture‐based CT image features is the first step toward identifying objective measures of lung toxicity necessary for assessment and prediction of RILD. There is evidence that numerous features are sensitive to both the radiation dose and time after RT. Beyond characterizing feature response, further investigation is warranted to determine the utility of these features as surrogates of clinically significant lung injury.

Fast dose kernel interpolation using Fourier transform with application to permanent prostate brachytherapy dosimetry.

Boyer and Mok proposed a fast calculation method employing the Fourier transform (FT), for which calculation time is independent of the number of seeds but seed placement is restricted to calculation grid points. Here an interpolation method is described enabling unrestricted seed placement while preserving the computational efficiency of the original method. The Iodine-125 seed dose kernel was sampled and selected values were modified to optimize interpolation accuracy for clinically relevant doses. For each seed, the kernel was shifted to the nearest grid point via convolution with a unit impulse, implemented in the Fourier domain. The remaining fractional shift was performed using a piecewise third-order Lagrange filter. Implementation of the interpolation method greatly improved FT-based dose calculation accuracy. The dose distribution was accurate to within 2% beyond 3 mm from each seed. Isodose contours were indistinguishable from explicit TG-43 calculation. Dose-volume metric errors were negligible. Computation time for the FT interpolation method was essentially the same as Boyer's method. A FT interpolation method for permanent prostate brachytherapy TG-43 dose calculation was developed which expands upon Boyer's original method and enables unrestricted seed placement. The proposed method substantially improves the clinically relevant dose accuracy with negligible additional computation cost, preserving the efficiency of the original method.

Monte Carlo dosimetry for 103Pd, 125I, and 131Cs ocular brachytherapy with various plaque models using an eye phantom

Purpose:To investigate dosimetry for ocular brachytherapy for a range of eye plaque models containing103Pd, 125I, or 131Cs seeds with model‐based dose calculations.Methods:Five representative plaque models are developed based on a literature review and are compared to the standardized COMS plaque, including plaques consisting of a stainless steel backing and acrylic insert, and gold alloy backings with: short collimating lips and acrylic insert, no lips and silicone polymer insert, no lips and a thin acrylic layer, and individual collimating slots for each seed within the backing and no insert. Monte Carlo simulations are performed using the EGSnrc user‐code BrachyDose for single and multiple seed configurations for the plaques in water and within an eye model (including nonwater media). Simulations under TG‐43 assumptions are also performed, i.e., with the same seed configurations in water, neglecting interseed and plaque effects. Maximum and average doses to ocular structures as well as isodose contours are compared for simulations of each radionuclide within the plaque models.Results:The presence of the plaque affects the dose distribution substantially along the plaque axis for both single seed and multiseed simulations of each plaque design in water. Of all the plaque models, the COMS plaque generally has the largest effect on the dose distribution in water along the plaque axis. Differences between doses for single and multiple seed configurations vary between plaque models and radionuclides. Collimation is most substantial for the plaque with individual collimating slots. For plaques in the full eye model, average dose in the tumor region differs from those for the TG‐43 simulations by up to 10% for125I and 131Cs, and up to 17% for 103Pd, and in the lens region by up to 29% for 125I, 34% for 103Pd, and 28% for 131Cs. For the same prescription dose to the tumor apex, the lowest doses to critical ocular structures are generally delivered with plaques containing 103Pd seeds.Conclusions:The combined effects of ocular and plaque media on dose are significant and vary with plaque model and radionuclide, suggesting the importance of model‐based dose calculations employing accurate ocular and plaque media and geometries for eye plaque brachytherapy.

Model‐based dose calculations for COMS eye plaque brachytherapy using an anatomically realistic eye phantom

To investigate the effects of the composition and geometry of ocular media and tissues surrounding the eye on dose distributions for COMS eye plaque brachytherapy with(125)I, (103)Pd, or (131)Cs seeds, and to investigate doses to ocular structures. An anatomically and compositionally realistic voxelized eye model with a medial tumor is developed based on a literature review. Mass energy absorption and attenuation coefficients for ocular media are calculated. Radiation transport and dose deposition are simulated using the EGSnrc Monte Carlo user-code BrachyDose for a fully loaded COMS eye plaque within a water phantom and our full eye model for the three radionuclides. A TG-43 simulation with the same seed configuration in a water phantom neglecting the plaque and interseed effects is also performed. The impact on dose distributions of varying tumor position, as well as tumor and surrounding tissue media is investigated. Each simulation and radionuclide is compared using isodose contours, dose volume histograms for the lens and tumor, maximum, minimum, and average doses to structures of interest, and doses to voxels of interest within the eye. Mass energy absorption and attenuation coefficients of the ocular media differ from those of water by as much as 12% within the 20-30 keV photon energy range. For all radionuclides studied, average doses to the tumor and lens regions in the full eye model differ from those for the plaque in water by 8%-10% and 13%-14%, respectively; the average doses to the tumor and lens regions differ between the full eye model and the TG-43 simulation by 2%-17% and 29%-34%, respectively. Replacing the surrounding tissues in the eye model with water increases the maximum and average doses to the lens by 2% and 3%, respectively. Substituting the tumor medium in the eye model for water, soft tissue, or an alternate melanoma composition affects tumor dose compared to the default eye model simulation by up to 16%. In the full eye model simulations, the average dose to the lens is larger by 7%-9% than the dose to the center of the lens, and the maximum dose to the optic nerve is 17%-22% higher than the dose to the optic disk for all radionuclides. In general, when normalized to the same prescription dose at the tumor apex, doses delivered to all structures of interest in the full eye model are lowest for(103)Pd and highest for (131)Cs, except for the tumor where the average dose is highest for (103)Pd and lowest for (131)Cs. The eye is not radiologically water-equivalent, as doses from simulations of the plaque in the full eye model differ considerably from doses for the plaque in a water phantom and from simulated TG-43 calculated doses. This demonstrates the importance of model-based dose calculations for eye plaque brachytherapy, for which accurate elemental compositions of ocular media are necessary.

Simulational study of a dosimetric comparison between a Gamma Knife treatment plan and an intensity-modulated radiotherapy plan for skull base tumors

Fractionated stereotactic radiotherapy (SRT) is performed with a linear accelerator-based system such as Novalis. Recently, Gamma Knife Perfexion (PFX) featured the Extend system with relocatable fixation devices available for SRT. In this study, the dosimetric results of these two modalities were compared from the viewpoint of conformity, heterogeneity and gradient in target covering. A total of 14 patients with skull base tumors were treated with Novalis intensity-modulated (IM)-SRT. Treatment was planned on an iPlan workstation. Five- to seven-beam IM-SRT was performed in 14–18 fractions with a fraction dose of 2.5 or 3 Gy. With these patients' data, additional treatment planning was simulated using a GammaPlan workstation for PFX-SRT. Reference CT images with planning structure contour sets on iPlan, including the planning target volume (PTV, 1.1–102.2 ml) and organs at risk, were exported to GammaPlan in DICOM-RT format. Dosimetric results for Novalis IM-SRT and PFX-SRT were evaluated in the same prescription doses. The isocenter number of PFX was between 12 and 50 at the isodose contour of 50–60%. The PTV coverage was 95–99% for Novalis and 94–98% for PFX. The conformity index (CI) was 1.11–1.61 and 1.04–1.15, the homogeneity index (HI) was 1.1–3.62 and 2.3–3.25, and the gradient index (GI) was 3.72–7.97 and 2.54–3.39 for Novalis and PFX, respectively. PTV coverage by Novalis and PFX was almost equivalent. PFX was superior in CI and GI, and Novalis was better in HI. Better conformality would be achieved by PFX, when the homogeneity inside tumors is less important.

Reduced-order constrained optimization (ROCO): Clinical application to head-and-neck IMRT

The authors present the application of the reduced order constrained optimization (ROCO) method, previously successfully applied to the prostate and lung sites, to the head-and-neck (H&N) site, demonstrating that it can quickly and automatically generate clinically competitive IMRT plans. We provide guidelines for applying ROCO to larynx, oropharynx, and nasopharynx cases, and report the results of a live experiment that demonstrates how an expert planner can save several hours of trial-and-error interaction using the proposed approach. The ROCO method used for H&N IMRT planning consists of three major steps. First, the intensity space of treatment plans is sampled by solving a series of unconstrained optimization problems with a parameter range based on previously treated patient data. Second, the dominant modes in the intensity space are estimated by dimensionality reduction using principal component analysis (PCA). Third, a constrained optimization problem over this basis is quickly solved to find an IMRT plan that meets organ-at-risk (OAR) and target coverage constraints. The quality of the plan is assessed using evaluation tools within Memorial Sloan-Kettering Cancer Center (MSKCC)'s treatment planning system (TPS). The authors generated ten H&N IMRT plans for previously treated patients using the ROCO method and processed them for deliverability by a dynamic multileaf collimator (DMLC). The authors quantitatively compared the ROCO plans to the previously achieved clinical plans using the TPS tools used at MSKCC, including DVH and isodose contour analysis, and concluded that the ROCO plans would be clinically acceptable. In our current implementation, ROCO H&N plans can be generated using about 1.6 h of offline computation followed by 5-15 min of semiautomatic planning time. Additionally, the authors conducted a live session for a plan designated by MSKCC performed together with an expert H&N planner. A technical assistant set up the first two steps, which were performed without further human interaction, and then collaborated in a virtual meeting with the expert planner to perform the third (constrained optimization) step. The expert planner performed in-depth analysis of the resulting ROCO plan and deemed it to be clinically acceptable and in some aspects superior to the clinical plan. This entire process took 135 min including two constrained optimization runs, in comparison to the estimated 4 h that would have been required using traditional clinical planning tools. The H&N site is very challenging for IMRT planning, due to several levels of prescription and a large, variable number (6-20) of OARs that depend on the location of the tumor. ROCO for H&N shows promise in generating clinically acceptable plans both more quickly and with substantially less human interaction.

Water and tissue equivalence of a new PRESAGE® formulation for 3D proton beam dosimetry: A Monte Carlo study

To evaluate the water and tissue equivalence of a new PRESAGE(®) 3D dosimeter for proton therapy. The GEANT4 software toolkit was used to calculate and compare total dose delivered by a proton beam with mean energy 62 MeV in a PRESAGE(®) dosimeter, water, and soft tissue. The dose delivered by primary protons and secondary particles was calculated. Depth-dose profiles and isodose contours of deposited energy were compared for the materials of interest. The proton beam range was found to be ≈27 mm for PRESAGE(®), 29.9 mm for soft tissue, and 30.5 mm for water. This can be attributed to the lower collisional stopping power of water compared to soft tissue and PRESAGE(®). The difference between total dose delivered in PRESAGE(®) and total dose delivered in water or tissue is less than 2% across the entire water∕tissue equivalent range of the proton beam. The largest difference between total dose in PRESAGE(®) and total dose in water is 1.4%, while for soft tissue it is 1.8%. In both cases, this occurs at the distal end of the beam. Nevertheless, the authors find that PRESAGE(®) dosimeter is overall more tissue-equivalent than water-equivalent before the Bragg peak. After the Bragg peak, the differences in the depth doses are found to be due to differences in primary proton energy deposition; PRESAGE(®) and soft tissue stop protons more rapidly than water. The dose delivered by secondary electrons in the PRESAGE(®) differs by less than 1% from that in soft tissue and water. The contribution of secondary particles to the total dose is less than 4% for electrons and ≈1% for protons in all the materials of interest. These results demonstrate that the new PRESAGE(®) formula may be considered both a tissue- and water-equivalent 3D dosimeter for a 62 MeV proton beam. The results further suggest that tissue-equivalent thickness may provide better dosimetric and geometric accuracy than water-equivalent thickness for 3D dosimetry of this proton beam.

A novel methodology for 3D deformable dosimetry

Interfraction and intrafraction variation in anatomic structures is a significant challenge in contemporary radiotherapy. The objective of this work is to develop a novel tool for deformable structure dosimetry, using a tissue-equivalent deformable gel dosimeter that can reproducibly simulate targets subject to deformation. This will enable direct measurement of integrated doses delivered in different deformation states, and the verification of dose deforming algorithms. A modified version of the nPAG polymer gel has been used as a deformable 3D dosimeter and phantom to investigate doses delivered to deforming tissue-equivalent geometry. The deformable gel (DEFGEL) dosimeter/phantom is comprised of polymer gel in a latex membrane, moulded (in this case) into a cylindrical geometry, and deformed with an acrylic compressor. Fifteen aluminium fiducial markers (FM) were implanted into DEFGEL phantoms and the reproducibility of deformation was determined via multiple computed tomography (CT) scans in deformed and nondeformed states before and after multiple (up to 150) deformations. Dose was delivered to the DEFGEL phantom in three arrangements: (i) without deformation, (ii) with deformation, and (iii) cumulative exposures with and without deformation, i.e., dose integration. Irradiations included both square field and a stereotactic multiple dynamic arc treatment adapted from a patient plan. Doses delivered to the DEFGEL phantom were read out using cone beam optical CT. Reproducibility was verified by observation of interscan shifts of FM locations (as determined via CT), measured from an absolute reference point and in terms of inter-FM distance. The majority (76%) of points exhibited zero shift, with others shifting by one pixel size consistent with setup error as confirmed with a control sample. Comparison of dose profiles and 2D isodose distributions from the three arrangements illustrated complex spatial redistribution of dose in all three dimensions occurring as a result of the change in shape of the target between irradiations, even for a relatively simple deformation. Discrepancies of up to 30% of the maximum dose were evident from dose difference maps for three orthogonal planes taken through the isocenter of a stereotactic field. This paper describes the first use of a tissue-equivalent, 3D dose-integrating deformable phantom that yields integrated or redistributed dosimetric information. The proposed methodology readily yields three-dimensional (3D) dosimetric data from radiation delivery to the DEFGEL phantom in deformed and undeformed states. The impacts of deformation on dose distributions were readily seen in the isodose contours and line profiles from the three arrangements. It is demonstrated that the system is potentially capable of reproducibly emulating the physical deformation of an organ, and therefore can be used to evaluate absorbed doses to deformable targets and organs at risk in three dimensions and to validate deformation algorithms applied to dose distributions.

CyberKnife with Tumor Tracking: An Effective Treatment for High-Risk Surgical Patients with Stage I Non-Small Cell Lung Cancer

Published data suggests that wedge resection for stage I non-small cell lung cancer (NSCLC) is associated with improved overall survival compared to stereotactic body radiation therapy. We report CyberKnife outcomes for high-risk surgical patients with biopsy-proven stage I NSCLC. PET/CT imaging was completed for staging. Three-to-five gold fiducial markers were implanted in or near tumors to serve as targeting references. Gross tumor volumes (GTVs) were contoured using lung windows; the margins were expanded by 5 mm to establish the planning treatment volume (PTV). Treatment plans were designed using a mean of 156 pencil beams. Doses delivered to the PTV ranged from 42 to 60 Gy in three fractions. The 30 Gy isodose contour extended at least 1 cm from the GTV to eradicate microscopic disease. Treatments were delivered using the CyberKnife system with tumor tracking. Examination and PET/CT imaging occurred at 3 month follow-up intervals. Forty patients (median age 76) with a median maximum tumor diameter of 2.6 cm (range, 1.4–5.0 cm) and a mean post-bronchodilator percent predicted forced expiratory volume in 1 s (FEV1) of 57% (range, 21–111%) were treated. A median dose of 48 Gy was delivered to the PTV over 3–13 days (median, 7 days). The 30 Gy isodose contour extended a mean 1.9 cm from the GTV. At a median 44 months (range, 12–72 months) follow-up, the 3 year Kaplan–Meier locoregional control and overall survival estimates compare favorably with contemporary wedge resection outcomes at 91 and 75%, respectively. CyberKnife is an effective treatment approach for stage I NSCLC that is similar to wedge resection, eradicating tumors with 1–2 cm margins in order to preserve lung function. Prospective randomized trials comparing CyberKnife with wedge resection are necessary to confirm equivalence.

The effect of rib and lung heterogeneities on the computed dose to lung in Ir-192 High-Dose-Rate breast brachytherapy: Monte Carlo versus a treatment planning system

This study investigates to what extent the dose received by lungs from a commercially available treatment planning system, Ir-192 high-dose-rate (HDR), in breast brachytherapy, is accurate, with the emphasis on tissue heterogeneities, and taking into account the presence of ribs, in dose delivery to the lung. A computed tomography (CT) scan of a breast was acquired and transferred to the 3-D treatment planning system and was also used to construct a patient-equivalent phantom. An implant involving 13 plastic catheters and 383 programmed source dwell positions were simulated, using the Monte Carlo N-Particle eXtended (MCNPX) code. The Monte Carlo calculations were compared with the corresponding commercial treatment planning system (TPS) in the form of percentage isodose and cumulative dose-volume histogram (DVH) in the breast, lungs, and ribs. The comparison of the Monte Carlo results and the TPS calculations showed that a percentage of isodose greater than 75% in the breast, which was located rather close to the implant or away from the breast curvature surface and lung boundary, were in good agreement. TPS calculations overestimated the dose to the lung for lower isodose contours that were lying near the breast surface and the boundary of breast and lung and were relatively away from the implant. Taking into account the ribs and entering the actual data for breasts, ribs, and lungs, revealed an average overestimation of the dose by a factor of 8% in the lung for TPS calculations. Therefore, the accuracy of the TPS results may be limited to regions near the implants where the treatment is planned, and is a more conservative approach for regions at boundaries with curvatures or tissues with a different material than that in the breast.

TU‐E‐BRB‐01: Is AcurosXBTM Equivalent in Accuracy to Monte Carlo for Complex IMRT and VMAT Dose Calculations?

Purpose: To assess the accuracy of a new algorithm, a deterministic solution of the Boltzmann Transport Equations (AcurosXB,TM, Varian Medical Systems), in intensity‐modulated (IMRT) and volumetric‐modulated (VMAT) planning against a benchmark Monte Carlo system and a standard clinical algorithm (AAA, Varian Medical Systems) Method and Materials: Four sites were chosen: prostate, lung, oropharynx and nasopharynx, each exhibiting characteristic challenges for planning. CT‐based plans using 6MV IMRT and VMAT delivery were developed for each site and calculated as dose‐to‐water using Acuros v. 11 and AAA v. 10. Plans were exported to and calculated on a 36‐CPU implementation of the EGSnrc‐BEAMnrc MonteCarlo code with better than 1% statistical uncertainty. Dose and DVH differences were evaluated in the Varian‐Eclipse environment for the target PTVs and relevant critical OARs. Results: Dosimetric differences between Acuros and MonteCarlo were in general much smaller than between AAA and MonteCarlo. For Acuros, the greatest differences occurred in low density lung (densities < 0.1 g/cc) or in air cavities, and were less than 5% of the target dose, extending over regions no greater than 5mm. Corresponding AAA differences were as high as 7% and extended for up to 10mm into higher‐density tissues. As a result, the mean lung PTV dose difference was less than 0.5% for Acuros, but 1.7% for AAA. For the other three sites, the Acuros dose differences were within 3% over more than 97% of a representative volume defined by the 60% isodose contours. AAA dose differences were within 3% over 94% of the volume. Conclusions: Within normal tissue‐like materials (densities up to 1.6g/cc) and in complex clinical planning scenarios involving dynamic beam modulation, Acuros and MonteCarlo generally agreed within the statistical precision of the MonteCarlo dose calculations. Small discrepancies beyond this level could be attributed to differences in treatment head and MLC modelling between Eclipse and BEAMnrc.Partial funding provided through a research agreement with Varian Medical Systems

TH-E-BRC-03: Monte Carlo Dosimetry for 125I Brachytherapy of Stage I Non-Small Cell Lung Carcinoma

Purpose: To retrospectively calculate three dimensional dose distributions in the lung and surrounding tissues using Monte Carlo(MC) simulations to evaluate the effect of including media heterogeneities for patients treated with I‐125 brachytherapy for resected non‐small cell lungcancer. Methods: The EGSnrc user‐code BrachyDose is used to perform simulations including media heterogeneities for patients treated with intraoperative I‐125 mesh brachytherapy for stage I non‐small cell lungcancer at Mayo Clinic. Heterogeneous simulation phantoms are generated using patient CT data: voxel medium (air, lung, soft tissue, bone) and density assignment is based on the densities in the CTimages. Seeds are fully modeled. Three‐ dimensional dose distributions in the lung are calculated and are compared with the TG‐43 formalism via DVHs and dose metrics for normal tissue volumes and volumes consisting of various margins (5–20 mm) surrounding the surgical suture (PTV10 for 10 mm margin). Results: Differences in dose metrics for TG‐43 and heterogeneous MC calculations vary dramatically between patients; differences of up to 17% in V100, 26% in D90, and 20% in the mean dose are observed within PTV10. MCdoses to voxels within PTV10 can differ by up to +/−20 Gy (20% of prescription dose) from TG‐43 doses. Correspondingly, large differences are observed in isodose contours for the two calculation methods, particularly in lungtissues; differences are smaller for soft tissue regions. Dose differences are highly sensitive to source positioning within the lung and on lung composition (i.e. media heterogeneities). Conclusions: Doses computed using full MC simulations differ considerably from TG‐43 calculated doses for I‐125 mesh brachytherapy. Accurate treatment planning and evaluation requires the inclusion of non‐water media based on imaging data in dose calculations. BrachyDose calculation times are sufficiently fast for clinical use.

SU‐E‐T‐373: Treatment Plan Validation of the XPS (Interstitial) Applicator for Use with the Xoft Axxent 50kVp X‐Ray Source

Purpose: To determine the dosimetric properties of a new interstitial applicator designed for the Xoft 50 kVp X‐ray Electronic Brachytherapy (eBx) system. Methods: A 0.64 cm diameter polymer sheath applicator interfaces with the Xoft 50kVp x‐ray source to provide radiation treatment to interstitial and other small target areas using eBx. Measurements of the dose profiles in a plane parallel to the applicator axis were determined. The absolute dose delivered to a prescription point at 90° off the central axis 1 cm from the source center, was determined using GafChromic EBT‐2 radiochromic film. The dose distribution measured in this parallel plane is compared to the distribution determined by the treatment planning software (Varian Brachyvision V8). Three x‐ray sources were paired with three interstitial applicators for the tests. Results: Using a single position treatment plan, the absolute dose at the defined prescription point was determined from calibrated film images. The measured dose agreed with the dose predicted by the treatment plan to within ±5%, with the actual range from −4.6% to +3.1% for the three samples. The measured isodose contours, including the 1 cm prescription point contour, agreed with the contours predicted by the treatment plan to within 5% (1 sigma) and the applicator does not perturb the water‐based dose distribution. The absolute dose determined from each of these contours agreed with the predicted dose to within +1/−6%. Conclusions: The XPS applicator can be used to treat small cavities using eBx with the Xoft X‐ray source. The applicator delivers dose profiles in good agreement with those predicted from the treatment planning software. It can be used for treatment in lightly shielded rooms due to the low energies employed. There are also intriguing possibilities for this applicator to be used intra‐operatively.Research supported by Xoft, Inc. — an iCAD Company

Comparison and Evaluation of the Effects of Rib and Lung Inhomogeneities on Lung Dose in Breast Brachytherapy using a Treatment Planning System and the MCNPX Code

Introduction: This study investigates to what extent the computed dose received by lung tissue in a commercially available treatment planning system (TPS) for 192Ir high-dose-rate breast brachytherapy is accurate in view of tissue inhomogeneities and presence of ribs. Materials and Methods: A CT scan of the breast was used to construct a patient-equivalent phantom in the clinical treatment planning system. An implant involving 13 plastic catheters and 383 programmed source dwell positions were simulated using the MCNPX code. Results: The results were compared with the corresponding commercial TPS in the form of isodoses and cumulative dose–volume histogram in breast, lung and ribs. The comparison of Monte Carlo results and TPS calculation showed that the isodoses greater than 62% in the breast that were located rather close to the implant or away from the breast curvature surface and lung boundary were in good agreement. TPS calculations, however, overestimated dose in the lung for lower isodose contours and points that were lying near the breast-air boundary and relatively away from the implant. Discussion and Conclusions: Taking into account the ribs and entering the actual data for breast, rib and lung, revealed an average overestimation of dose in lung in the TPS calculation.

Design and dosimetry characteristics of a commercial applicator system for intra‐operative electron beam therapy utilizing ELEKTA Precise accelerator

The design concept and dosimetric characteristics of a new applicator system for intraoperative radiation therapy (IORT) are presented in this work. A new hard‐docking commercial system includes polymethylmethacrylate (PMMA) applicators with different diameters and applicator end angles and a set of secondary lead collimators. A telescopic device allows changing of source‐to‐surface distance (SSD). All measurements were performed for 6, 9, 12 and 18 MeV electron energies. Output factors and percentage depth doses (PDD) were measured in a water phantom using a plane‐parallel ion chamber. Isodose contours and radiation leakage were measured using a solid water phantom and radiographic films. The dependence of PDD on SSD was checked for the applicators with the smallest and the biggest diameters. SSD dependence of the output factors was measured. Hardcopies of PDD and isodose contours were prepared to help the team during the procedure on deciding applicator size and energy to be chosen. Applicator output factors are a function of energy, applicator size and applicator type. Dependence of SSD correction factors on applicator size and applicator type was found to be weak. The same SSD correction will be applied for all applicators in use for each energy. The radiation leakage through the applicators is clinically acceptable. The applicator system enables effective collimation of electron beams for IORT. The data presented are sufficient for applicator, energy and monitor unit selection for IORT treatment of a patient.PACS number: 87.00.00, 29.20.‐c

SU‐GG‐J‐104: Inclusion of Effects of Image Registration Errors in Accumulated Dose Distributions

Purpose: To consider the effects of image‐registration error in dose accumulation across different geometry instances and to display the resultant errors in a meaningful and useful way. Method and Materials: 4DCT phases acquired of a lung cancer patient were non‐rigidly registered by a B‐spline interpolation method (www.plastimatch.org) to the first phase chosen as a reference geometry. Dose was calculated on each phase and warped back to the reference phase by linear interpolation. The error in the deformation values at each voxel were assumed to be independent of that of neighboring voxels or of adjacent phases. This allows an error instance to be simulated by randomly perturbing the deformation used in dose accumulation by a value sampled from a Gaussian distribution for each voxel. The dose was accumulated for each instance and thirty instances were simulated to sample the total error distribution. Three methods of assigning the standard deviation of the error distribution to each voxel were considered: (i) fixed value of 3mm; (ii) intensity‐matching method: the local value is scaled by a distance to agreement metric calculated by comparing the registered phase image with the reference image; and (iii) vector‐discrepancy method: the local value is scaled by evaluating the invertability of the deformation maps. The effects of the simulated error were visualized by a spread of the accumulated isodose contours. Results: The vector‐discrepancy method resulted in the representation of reduced errors in the posterior region of the target where the extra image detail provided by the ribs and vertebrae increased the accuracy of the registration. Differences between the intensity‐matching method and fixed‐error method were less noticeable. Conclusion: A method of including image registration error in dose accumulation has been developed. Further work is required to compare results to a gold standard on a number of image datasets.

CyberKnife radiosurgery for inoperable stage IA non-small cell lung cancer: 18F-fluorodeoxyglucose positron emission tomography/computed tomography serial tumor response assessment

ObjectiveTo report serial 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/computed tomography (CT) tumor response following CyberKnife radiosurgery for stage IA non-small cell lung cancer (NSCLC).MethodsPatients with biopsy-proven inoperable stage IA NSCLC were enrolled into this IRB-approved study. Targeting was based on 3-5 gold fiducial markers implanted in or near tumors. Gross tumor volumes (GTVs) were contoured using lung windows; margins were expanded by 5 mm to establish the planning treatment volumes (PTVs). Doses ranged from 42-60 Gy in 3 equal fractions. 18F-FDG PET/CT was performed prior to and at 3-6-month, 9-15 months and 18-24 months following treatment. The tumor maximum standardized uptake value (SUVmax) was recorded for each time point.ResultsTwenty patients with an average maximum tumor diameter of 2.2 cm were treated over a 3-year period. A mean dose of 51 Gy was delivered to the PTV in 3 to 11 days (mean, 7 days). The 30-Gy isodose contour extended an average of 2 cm from the GTV. At a median follow-up of 43 months, the 2-year Kaplan-Meier overall survival estimate was 90% and the local control estimate was 95%. Mean tumor SUVmax before treatment was 6.2 (range, 2.0 to 10.7). During early follow-up the mean tumor SUVmax remained at 2.3 (range, 1.0 to 5.7), despite transient elevations in individual tumor SUVmax levels attributed to peritumoral radiation-induced pneumonitis visible on CT imaging. At 18-24 months the mean tumor SUVmax for controlled tumors was 2.0, witha narrow range of values (range, 1.5 to 2.8). A single local failure was confirmed at 24 months in a patient with an elevated tumor SUVmax of 8.4.ConclusionLocal control and survival following CyberKnife radiosurgery for stage IA NSCLC is exceptional. Early transient increases in tumor SUVmax are likely related to radiation-induced pneumonitis. Tumor SUVmaxvalues return to background levels at 18-24 months, enhancing 18F-FDG PET/CT detection of local failure. The value of 18F-FDG PET/CT imaging for surveillance following lung SBRT deserves further study.