J Napoli

Dosimetry for 131Cs and 125I seeds in solid water phantom using radiochromic EBT film.

PURPOSE To measure the 2D dose distributions with submillimeter resolution for (131)Cs (model CS-1 Rev2) and (125)I (model 6711) seeds in a Solid Water phantom using radiochromic EBT film for radial distances from 0.06cm to 5cm. To determine the TG-43 dosimetry parameters in water by applying Solid Water to liquid water correction factors generated from Monte Carlo simulations. METHODS Each film piece was positioned horizontally above and in close contact with a (131)Cs or (125)I seed oriented horizontally in a machined groove at the center of a Solid Water phantom, one film at a time. A total of 74 and 50 films were exposed to the (131)Cs and (125)I seeds, respectively. Different film sizes were utilized to gather data in different distance ranges. The exposure time varied according to the seed air-kerma strength and film size in order to deliver doses in the range covered by the film calibration curve. Small films were exposed for shorter times to assess the near field, while larger films were exposed for longer times in order to assess the far field. For calibration, films were exposed to either 40kV (M40) or 50kV (M50) x-rays in air at 100.0cm SSD with doses ranging from 0.2Gy to 40Gy. All experimental, calibration and background films were scanned at a 0.02cmpixel resolution using a CCD camera-based microdensitometer with a green light source. Data acquisition and scanner uniformity correction were achieved with Microd3 software. Data analysis was performed using ImageJ, FV, IDL and Excel software packages. 2D dose distributions were based on the calibration curve established for 50kV x-rays. The Solid Water to liquid water medium correction was calculated using the MCNP5 Monte Carlo code. Subsequently, the TG-43 dosimetry parameters in liquid water medium were determined. RESULTS Values for the dose-rate constants using EBT film were 1.069±0.036 and 0.923±0.031cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The corresponding values determined using the Monte Carlo method were 1.053±0.014 and 0.924±0.016cGyU(-1)h(-1) for (131)Cs and (125)I seed, respectively. The radial dose functions obtained with EBT film measurements and Monte Carlo simulations were plotted for radial distances up to 5cm, and agreed within the uncertainty of the two methods. The 2D anisotropy functions obtained with both methods also agreed within their uncertainties. CONCLUSION EBT film dosimetry in a Solid Water phantom is a viable method for measuring (131)Cs (model CS-1 Rev2) and (125)I (model 6711) brachytherapy seed dose distributions with submillimeter resolution. With the Solid Water to liquid water correction factors generated from Monte Carlo simulations, the measured TG-43 dosimetry parameters in liquid water for these two seed models were found to be in good agreement with those in the literature.

SU‐FF‐T‐147: Determination of TG43 Parameters for Cs‐131 Model CS‐1R2 Seed Using Radiochromic EBT Film Dosimetry

Purpose: To measure the 2D dose distributions for Cs‐131 seed model CS‐1R2 for distances from 0.06cm to 4cm using radiochromic EBT film dosimetry. TG43 dosimetric parameters were generated. Method and Materials: Each radiochromic film (GAFCHROMIC® EBT lot ♯35076) was in contact with a model CS‐1R2 seed (IsoRay Medical) at the center of a solid water phantom, 30×30×20cm. A multiple film technique was employed. More than 50 films were separately exposed to 10 seeds, with the product of air kerma strength and exposure time between 14 and 800 Uhr. The seed strength ranged from 10 to 4U (NIST traceable) during the experimental runs. For calibration, 15 films (of the same lot) were exposed to 50kV x‐ray (M50) in air at 100cm SSD with doses ranging from 0.2 to 20Gy. Since the EBT film response is almost identical for M50 and M40 x‐rays, the energy response can be considered to be flat in this region and thus also the same for the Cs‐131 energy. All experimental, calibration and background films were scanned (pixel resolution 0.2mm) using a CCD camera‐based densitometer, with red and green light sources. Conversion from net optical density readings to doses were achieved based on the calibration curve established for each light source used in scanning. The 2D dose values in cylindrical coordinates were converted to polar coordinates, and the TG43 parameters were generated. Results: The dose rate constant, radial dose function and 2D anisotropy function were determined, and compared with those reported by other authors. General agreement was found. Conclusion: It is feasible to determine TG43 parameters for Cs‐131 model CS‐1R2 seed in solid water phantom using radiochromic EBT film. This method is superior to TLDdosimetry (1) in providing data with high spatial resolution for distances down to 0.06 cm and (2) within reasonably achievable time frame.

SU-E-T-149: Brachytherapy Patient Specific Quality Assurance for a HDR Vaginal Cylinder Case

Purpose: Commonly Ir-192 HDR treatment planning system commissioning is only based on a single absolute measurement of source activity supplemented by tabulated parameters for multiple factors without independent verification that the planned distribution corresponds to the actual delivered dose. The purpose on this work is to present a methodology using Gafchromic film with a statistically valid calibration curve that can be used to validate clinical HDR vaginal cylinder cases by comparing the calculated plan dose distribution in a plane with the corresponding measured planar dose. Methods: A vaginal cylinder plan was created with Oncentra treatment planning system. The 3D dose matrix was exported to a Varian Eclipse work station for convenient extraction of a 2D coronal dose plane corresponding to the film position. The plan was delivered with a sheet of Gafchromic EBT3 film positioned 1mm from the catheter using an Ir-192 Nucletron HDR source. The film was then digitized with an Epson 10000 XL color scanner. Film analysis is performed with MatLab imaging toolbox. A density to dose calibration curve was created using TG43 formalism for a single dwell position exposure at over 100 points for statistical accuracy. The plan and measured film dose planes were registered using a known dwell position relative to four film marks. The plan delivered 500 cGy to points 2 cm from the sources. Results: The distance to agreement of the 500 cGy isodose between the plan and film measurement laterally was 0.5 mm but can be as much as 1.5 mm superior and inferior. The difference between the computed plan dose and film measurement was calculated per pixel. The greatest errors up to 50 cGy are near the apex. Conclusion: The methodology presented will be useful to implement more comprehensive quality assurance to verify patient-specific dose distributions

SU‐GG‐T‐569: A Real‐Time Graphical User Interface‐Driven Radiotherapy Treatment Planning Simulator

Purpose: To present a stand‐alone RadiotherapyTreatment Planning simulator (RTPsim) that physicists can use for interactive classroom instruction and provide to students for independent study. Method and Materials: A PC with MATLAB programming language and the imaging toolbox is required. The RTPsim components are (1) a set of grayscale images for dose computation with regions of interest for dose area histogram analysis; (2) a double Gaussian pencil beam algorithm to create treatment fields; (3) dose distribution calculation routines for individual open or wedged beams and for a composite of weighted beams; and (4) a Graphical User Interface (GUI) to run the system with such user friendly features as command buttons to run algorithms, text or cursor variable input, list boxes for patient selection, and radio buttons for beam modifiers such as wedges. Results: The percent depth dose (%DD) for a typical 10×10 cm 6MV beam created by pencil beams closely approximates the actual clinical data. By computing and storing a correction factor table, the %DD values match exactly. The incorporation of divergence and a low dose region beyond the geometrical field edge result in excellent agreement in calculated versus clinical data for open beam profiles. Wedged field profiles show reasonable agreement with our clinical system. Several test cases from texts were reproduced accurately for interactive teaching purposes. As an example, an RTPsim wedged tangent field breast plan comparable to a clinical plan was produced. Variations in wedge, weighting, and normalization point can be demonstrated in real time. Use of a MATLAB GUI is suitable for planning demonstration; physicists and dosimetrists found the GUI convenient and easy to use. Conclusion: The user‐friendly RadiotherapyTreatment Planning simulator can simulate a clinical treatment planningcomputer with sufficient accuracy and speed for interactive educational purposes and independent learning.

SU‐FF‐T‐51: Robotically‐Assisted Minimally Invasive Brachytherapy: Dosimetric Aspects

Purpose: To investigate the dosimetric aspects of robotically‐assisted minimally invasive brachytherapy.Materials and Methods: Using the Intuitive Surgical DaVinci S System, the following were inserted into a porcine abdominopelvic cavity through a 12 mm auxiliary port and sutured directly to the pelvic sidewall: three 6‐French catheters; the Xoft Axxent delivery system; a prototype minimally invasive applicator (MIA). Plans were generated in Nucletrons treatment planning software for an Ir‐192 source and the Xoft 50 kV source. The ability of the prototype MIA to reproducibly position catheters was tested using filmdosimetry. Other applicators were designed to allow an en face application of the source, and various catheter arrangements were investigated. For the en face application, the sensitivity of the delivered dose to random deviations from normal incidence of the catheters (⩽ 5 degrees) and regular catheter spacing were tested. Results: All three trials for introducing the catheters were performed with ease by the surgeon. After the apparatus was sutured into place, it maintained a fixed geometry. The dosimetric measurements using GafChromic EBT film showed a uniform distribution of dose. The treatment plan for the en‐face arrangement using the Xoft source with 16 catheters at random angles showed a deviation from the planned dose of ⩽ 0.39%. Conclusions: The feasibility of performing robotically‐assisted minimally invasive brachytherapy was demonstrated for techniques utilizing both parallel and en face geometries. A customized applicator was designed to deliver a uniform dose to the target. The reproducibility of the device was confirmed with filmdosimetry. For the geometries tested, the en face application was found to be insensitive to random deviations of the catheters of up to 5 degrees from normal incidence. These techniques may be used for minimally invasive intra‐operative or post‐operative brachytherapy procedures using real‐time or atlas‐based planning.

SU‐GG‐T‐07: I‐125 Seed Dosimetry in the Near Field Using Gafchromic® EBT Film

Purpose: To perform radiochromic EBT film dosimetry in the near field of model MED3631A/M I‐125 seed and to determine the TG‐43 dosimetric parameters. Method and Materials: Gafchromic® model EBT films (lot♯35076) were horizontally positioned, one at a time, above and in contact with a NAS model MED3631A/M I‐125 seed horizontally placed at the center of a solid water phantom (RMI457, 30×30×20cm3). A multiple‐film technique was employed. To cover the distance range of 0.06–5cm, 50 EBT films of different sizes (4×4–12×12cm2) were irradiated by 10 individual seeds (NIST traceable air kerma strengths ∼10U, based on current NAS calibration) using different exposure times (1–295hr). A separate set of 19 calibration films was exposed to 50kV x‐rays at 100 cm SSD for doses of 0.2–40Gy at the University of Wisconsin. All experimental, calibration and background films were scanned using a CCD100 densitometer using a green light source with fine spatial resolution (0.2mm). Based on the established calibration curve, dose conversion for the experimental films and generation of the TG‐43 dosimetric parameters (for L=4.2mm) were achieved using IDL v6.0. Results: The dose rate constant was 1.004 cGy/Uh. The radial dose function was obtained for the radial distances from 0.06 to 5cm. The 2D anisotropy functions for distances from 0.1 to 5cm were determined. General agreement with published values and those recommended by TG‐43U1 report for r⩾0.5cm was found. The near field data for 0.25cm is close to Rivards Monte Carlo results for the “ideal capsule orientation”, but different from the TG43U1 consensus values based on Rivards weighted “diagonal” and “vertical” capsule orientations. Conclusion: Using EBT film dosimetry with the multiple‐film technique, TG‐43 dosimetric parameters were determined for model MED3631A/M I‐125 seed in the near field and out to 5cm, allowing for accurate treatment planning calculations. Research supported partially by North American Scientific Inc.

SU‐GG‐T‐39: Optimized Planning of a High Dose Rate Prostate Brachytherapy Treatment Using Constrained Nonlinear Programming

Purpose: To describe an efficient prostate HDR optimization strategy using constrained nonlinear programming. Method and Materials: Calculation points, contours and source positions were exported from a clinical prostate HDR implant case to a PC running MatLab. A matrix Dij is computed for the dose rate to each calculation point i from each dwell position j. The dosed = Dij*t for dwell time t was calculated using the TG‐43 formalism. Both the prostate and urethra are planned for a uniform dosed0 with equal weights. The MatLab optimization toolbox is used to compute the minimum of an arbitrary nonlinear cost function ƒ( t ) = Σ abs ( d 0 − d )∧ n subject to linear inequality constraint Dij*t ⩾ d0 with t values between any lower and upper bound. We investigated the effects of performing an unbounded optimization and setting negative dwell times to zero; linear (n=1) versus quadratic (n=2) cost function with positive dwell times; and lowering the upper bound. These methods were compared to manual optimization performed by an expert user. Results: Utilizing a quadratic cost function, nearly perfect target coverage is obtained with acceptable urethral dose when negative dwell times are allowed. Setting negative dwell times to zero seriously compromises dose homogeneity. The effect of applying a linear cost function as well as that of lowering the upper dwell time bound is to significantly increase the hot spots. A quadratic cost function with positive dwell times and upper dwell time limit of nine times the mean provided a clinically optimal treatment plan. Compared to manual plans, optimized plans had superior target coverage, lower inhomogeneity and were calculated in minutes instead of hours. Conclusion: Constrained nonlinear programming is an effective tool to optimize prostate HDR treatment planning. A relatively high upper dwell time limit with a quadratic cost function yielded the best clinical results.

SU‐FF‐T‐86: Rapid (<15 Minutes) Automated Tangential Breast IMRT Treatment Planning With Cutplane Evaluation

Purpose: To develop an automated, dose-based IMRT planning technique for tangential breast irradiation and to introduce a novel technique for efficient plan evaluation in three-dimensions. Method and Materials: Patients were CT-simulated using an isocentric SAD technique with the medial and lateral tangents designed to encompass the breast tissue. The CT data and the beam information were transferred to the treatment planning system (TPS). The dose was then calculated for standard open equally weighted split beams. The planar dose rate matrix for a beams-eye-view plane was calculated and exported to a PC. An in-house program was developed to rescale this matrix and automatically create a series of dose-based MLC segments. Creation of the MLC segments took less than a minute. The MLC segments were imported back into the TPS where utilities were used to determine the optimal segment weights. An efficient plan evaluation tool was developed that presented the dose distributions in only three or less cutplanes parallel to the plane formed by the posterior beam border. The mathematical formalism for determining the cutplane orientation will be demonstrated. The planning and evaluation tools were tested on three representative case studies and compared to wedged tangent plans. To ensure unbiased comparison all plans were normalized to identical ICRU Reference Points. Results: The entire planning process, from CT import to plan completion, took approximately 15 minutes. For all cases studied the IMRT plans demonstrated superior coverage and dose homogeneity as determined by both isodose coverage and the ICRU Conformity Index. Cutplanes through the lung, mid breast, and apex were found to be the most clinically useful. Conclusion: The automated procedure is able to consistently produce optimized IMRT breast treatment plans within a few minutes. Isodose distributions in three cutplanes parallel to the posterior border are an effective tool for rapid plan evaluation.