A Anand

Energy dependence and dose response of Gafchromic EBT2 film over a wide range of photon, electron, and proton beam energies.

PURPOSE Since the Gafchromic film EBT has been recently replaced by the newer model EBT2, its characterization, especially energy dependence, has become critically important. The energy dependence of the dose response of Gafchromic EBT2 film is evaluated for a broad range of energies from different radiation sources used in radiation therapy. METHODS The beams used for this study comprised of kilovoltage x rays (75, 125, and 250 kVp), 137Cs gamma (662 KeV), 60Co gamma (1.17-1.33 MeV), megavoltage x rays (6 and 18 MV), electron beams (6 and 20 MeV), and proton beams (100 and 250 MeV). The films response to each of the above energies was measured over the dose range of 0.4-10 Gy, which corresponds to optical densities ranging from 0.05 to 0.74 for the film reader used. RESULTS The energy dependence of EBT2 was found to be relatively small within measurement uncertainties (1 sigma = +/- 4.5%) for all energies and modalities. CONCLUSION For relative and absolute dosimetry of radiation therapy beams, the weak energy dependence of the EBT2 makes it most suitable for clinical use compared to other films.

Beyond Gaussians: a study of single-spot modeling for scanning proton dose calculation

Active spot scanning proton therapy is becoming increasingly adopted by proton therapy centers worldwide. Unlike passive-scattering proton therapy, active spot scanning proton therapy, especially intensity-modulated proton therapy, requires proper modeling of each scanning spot to ensure accurate computation of the total dose distribution contributed from a large number of spots. During commissioning of the spot scanning gantry at the Proton Therapy Center in Houston, it was observed that the long-range scattering protons in a medium may have been inadequately modeled for high-energy beams by a commercial treatment planning system, which could lead to incorrect prediction of field size effects on dose output. In this study, we developed a pencil beam algorithm for scanning proton dose calculation by focusing on properly modeling individual scanning spots. All modeling parameters required by the pencil beam algorithm can be generated based solely on a few sets of measured data. We demonstrated that low-dose halos in single-spot profiles in the medium could be adequately modeled with the addition of a modified Cauchy-Lorentz distribution function to a double-Gaussian function. The field size effects were accurately computed at all depths and field sizes for all energies, and good dose accuracy was also achieved for patient dose verification. The implementation of the proposed pencil beam algorithm also enabled us to study the importance of different modeling components and parameters at various beam energies. The results of this study may be helpful in improving dose calculation accuracy and simplifying beam commissioning and treatment planning processes for spot scanning proton therapy.

Experimental characterization of the low-dose envelope of spot scanning proton beams

In scanned proton beam radiotherapy, multiple pencil beams are used to deliver the total dose to the target volume. Because the number of such beams can be very large, an accurate dosimetric characterization of every single pencil beam is important to provide adequate input data for the configuration of the treatment planning system. In this work, we present a method to measure the low-dose envelope of single pencil beams, known to play a meaningful role in the dose computation for scanned proton beams. We measured the low-dose proton beam envelope, which extends several centimeters outwards from the center of each single pencil beam, by acquiring lateral dose profile data, down to relative dose levels that were a factor of 10(4) lower than the central axis dose. The overall effect of the low-dose envelope on the total dose delivered by multiple pencil beams was determined by measuring the dose output as a function of field size. We determined that the low-dose envelope can be influential even for fields as large as 20 cm x 20 cm.

Parameterization of multiple Bragg curves for scanning proton beams using simultaneous fitting of multiple curves

Although Bortfelds analytical formula is useful for describing Bragg curves, measured data can deviate from the values predicted by the model. Thus, we sought to determine the parameters of a closed analytical expression of multiple Bragg curves for scanning proton pencil beams using a simultaneous optimization algorithm and to determine the minimum number of energies that need to be measured in treatment planning so that complete Bragg curves required by the treatment planning system (TPS) can be accurately predicted. We modified Bortfelds original analytical expression of Bragg curves to accurately describe the dose deposition resulting from secondary particles. The parameters of the modified analytical expression were expressed as the parabolic cylinder function of the ranges of the proton pencil beams in water. Thirty-nine discrete Bragg curves were measured in our center using a PTW Bragg Peak chamber during acceptance and commission of the scanning beam proton delivery system. The coefficients of parabolic function were fitted by applying a simultaneous optimization algorithm to seven measured curves. The required Bragg curves for 45 energies in the TPS were calculated using our parameterized analytical expression. Finally, the 10 cm width of spread-out Bragg peaks (SOBPs) of beams with maximum energies of 221.8 and 121.2 MeV were then calculated in the TPS and compared with measured data. Compared with Bortfelds original formula, our modified formula improved fitting of the measured depth dose curves at depths around three-quarters of the maximum range and in the beam entrance region. The parabolic function described the relationship between the parameters of the analytic expression of different energies. The predicted Bragg curves based on the parameters fitted using the seven measured curves accurately described the Bragg curves of proton pencil beams of 45 energies configured in our TPS. When we used the calculated Bragg curves as the input to TPS, the standard deviations of the measured and calculated data points along the 10 cm SOBPs created with proton pencil beams with maximum energies of 221.8 and 121.2 MeV were 1.19% and 1.18%, respectively, using curves predicted by the algorithm generated from the seven measured curves. Our method would be a valuable tool to analyze measured Bragg curves without the need for time-consuming measurements and correctly describe multiple Bragg curves using a closed analytical expression.

A procedure to determine the planar integral spot dose values of proton pencil beam spots.

PURPOSE Planar integral spot dose (PISD) of proton pencil beam spots (PPBSs) is a required input parameter for beam modeling in some treatment planning systems used in proton therapy clinics. The measurement of PISD by using commercially available large area ionization chambers, like the PTW Bragg peak chamber (BPC), can have large uncertainties due to the size limitation of these chambers. This paper reports the results of our study of a novel method to determine PISD values from the measured lateral dose profiles and peak dose of the PPBS. METHODS The PISDs of 72.5, 89.6, 146.9, 181.1, and 221.8 MeV energy PPBSs were determined by area integration of their planar dose distributions at different depths in water. The lateral relative dose profiles of the PPBSs at selected depths were measured by using small volume ion chambers and were investigated for their angular anisotropies using Kodak XV films. The peak spot dose along the beams central axis (D(0)) was determined by placing a small volume ion chamber at the center of a broad field created by the superposition of spots at different locations. This method allows eliminating positioning uncertainties and the detector size effect that could occur when measuring it in single PPBS. The PISD was then calculated by integrating the measured lateral relative dose profiles for two different upper limits of integration and then multiplying it with corresponding D(0). The first limit of integration was set to radius of the BPC, namely 4.08 cm, giving PISD(RBPC). The second limit was set to a value of the radial distance where the profile dose falls below 0.1% of the peak giving the PISD(full). The calculated values of PISD(RBPC) obtained from area integration method were compared with the BPC measured values. Long tail dose correction factors (LTDCFs) were determined from the ratio of PISD(full)∕PISD(RBPC) at different depths for PPBSs of different energies. RESULTS The spot profiles were found to have angular anisotropy. This anisotropy in PPBS dose distribution could be accounted in a reasonable approximate manner by taking the average of PISD values obtained using the in-line and cross-line profiles. The PISD(RBPC) values fall within 3.5% of those measured by BPC. Due to inherent dosimetry challenges associated with PPBS dosimetry, which can lead to large experimental uncertainties, such an agreement is considered to be satisfactory for validation purposes. The PISD(full) values show differences ranging from 1 to 11% from BPC measured values, which are mainly due to the size limitation of the BPC to account for the dose in the long tail regions of the spots extending beyond its 4.08 cm radius. The dose in long tail regions occur both for high energy beams such as 221.8 MeV PPBS due to the contributions of nuclear interactions products in the medium, and for low energy PPBS because of their larger spot sizes. The calculated LTDCF values agree within 1% with those determined by the Monte Carlo (MC) simulations. CONCLUSIONS The area integration method to compute the PISD from PPBS lateral dose profiles is found to be useful both to determine the correction factors for the values measured by the BPC and to validate the results from MC simulations.

TH‐C‐BRB‐10: Scanned Proton Pencil Beam's Lateral Profile Library to Describe the Spot Dose Characteristics

Purpose: To create a comprehensive data library of lateral dose profiles to characterize proton pencil beam spots (PPBS) with wide range of energies available for clinical use. Methods: To improve accuracy of dose modeling by the treatment planning system, a comprehensive data library of measured PPBS profiles has been created. At our institution, there are 94 energies available for spot scanned proton beams. For each energy, lateral dose profiles of PPBS in water have been measured at several depths. A pinpoint chamber with an active volume of 0.015 cm3 was used to collect relative dose profiles in water tank using a fixed SSD setup. Both in‐line and cross‐ line relative dose profiles of PPBS have been measured and analyzed. A PTW Bragg peak chamber (BPC) was used to obtain the absolute planar integral spot dose (PISD) of these PPBS at the respective depths of measurements. A structured query language based data repository has been created for query and retrieval of the details of dosimetry data of these PPBS. Results: PPBS characteristics, such as full width half maximum, the PISD (Gy‐cm2/MU) and integral spread of the PISD along the central axis have been tabulated for all available beam energies. The availability of this database allows the study of the spot size distribution as a function of energy and depth. Additionally, the measured data also provides the critical long tail dose correction factors (LTDCF) for the BPC measured PISD as a function of energy and depth. Conclusions: The lateral profiles measured for 94 different energies, has provided useful information about the effect of multiple Coulomb scattering and nuclear halo effect on the propagation of PPBS in water, and spot sizes as a function of energy and depths. The database provides a comprehensive library of lateral profiles of PPBS for clinical and research uses. This project was partially supported by Varian Master Research Agreement, # CS2005‐00012856SP andNIH/NCI Grant 5‐P01CA021239‐30.

SU‐GG‐T‐463: Study of the Magnitude of Detector Size Effect in the Measured Lateral Profiles of Proton Pencil Beam Spots

Purpose: Lateral profiles of therapeuticradiation beams measured by ionization chambers are known to be affected by their sizes. The purpose of this study is to quantify the detector size effect on the measurements of lateral profiles of the proton pencil beam spots (PPBS) of the scanning beam nozzle of the Proton Therapy Center at Houston. Methods and Materials: In‐air and in‐water lateral profiles of 72.5, 163.9 and 221.8 MeV energy PPBS were measured by a PTW 31014 Pinpoint ionization chamber (IC) (radius of 0.1 cm). The measured profiles were fitted to a sum of multiple Gaussians and were deconvolved with a Gaussian detector response kernel, K(x) = A. exp (‐ x2/2σκ2) with σκ taken as the radius of the IC. The deconvolved profiles are also given by the sum of Gaussians with modified σ2 = σm 2 ‐ σκ2, where σm is the sigma of a Gaussian in the fitted measured profiles. The full width at half maxima (FWHM) and full width at 0.9 and 0.01 of the maxima, (FW0.9M, FW0.01M) of the profiles with and without the detector size corrections were compared to quantify the effect of the IC size on the in‐air and inwater lateral profiles of PPBS. Results: The maximum differences between the uncorrected and corrected FWHM, FW0.9M and FW0.01M for the PPBS of all the three energies in this study, were found to be less than 0.3, 0.1, 0.5 mm for in‐air and 0.2, 0.1, 0.3 mm for in‐water lateral profiles respectively. The detector size corrected and the measured lateral profiles were found to be very close to each other. Conclusion: The analytical deconvolution method with a Gaussian detector response kernel indicates that the detector size has a rather small effect on the Gaussian like lateral profiles of PPBS measured with small ionization chambers.

SU‐E‐T‐46: M.D. Anderson Prostate Seed Implant Dose Calculator

Purpose: To develop a prostate seed implant dose calculator for conducting independent verification of Variseed (Varian Medical, Palo Alto, CA) brachytherapytreatment planning system (TPS). Methods: AAPM TG 43 based formalism was used in our method to compute point doses for prostate implant seeds commissioned in Variseed 8.0®. We have investigated four different isotope seeds, I125(Model 6711, 9011), Cs131(Model Cs‐1) and Pd103(Model 200) in this work. Three distinct Methods: point source model using anisotropy factors, line source model using anisotropy factors, and line source model using anisotropy functions were used to develop in‐house dose calculator software. The dose calculator accounts for source strength, geometric dose fall off, attenuation, scatter and time, but does not consider any heterogeneous corrections. Source data parameters obtained from customer technical bulletins were input into both the Variseed TPS and our calculator. Spline and non oscillatory, seed specific fit functions to parameterize dose distribution for these seeds was evaluated for our software. These functions were then imported into the dose calculator to compute point doses as a function of radial distance from various seeds. The functions developed for our modeling were selectively chosen to account for self absorption from the seeds housing. The doses obtained by our secondary dose verification software were then compared against Variseed8.0® computed doses to clinically relevant radial distances. The dose calculator software has been packaged into a GUI module and is currently being tested for its robustness. Results: A good agreement (within 3% for 5 cm radial distance) for the isotopes studied is observed between the TPS and our calculator. Conclusions: We have validated Variseed computed point doses by using our dose calculator. Within all uncertainties as described within TG‐43, our results have been found to be in good agreement between our system and TPS.

TH‐E‐BRB‐04: Dose Response of EBT2 Film Modeled as a Bimolecular Reaction

Purpose: This study investigated if the dose‐response of EBT2 film can be modeled as a bimolecular reaction of the monomers that composes the active layer of the EBT2 film. The LET dependence of EBT2 films was explored using the models developed in this study. Methods: To build a dose‐response curve a set of films was exposed to pristine proton beams of 161.61 MeV, with doses ranging from 0.93 Gy to 14.82 Gy at a depth of 2 cm in water. The same procedure was applied to a film using a lower energy beam, 85.55 MeV. Because the chemical models predict different values for the maximum optical density of the film, another set of films was exposed to a higher dose, about 200 Gy, to determine which chemical model would better predict the film parameters. Fluence‐averaged LET curves were computed by calculating the ratio of the dose and fluence in water. Proton energy spectra were also computed at selected depths for both energies. Results: The unimolecular and the bimolecular models were able to accurately fit the experimental data, with both having R2 = 0.9996. The maximum optical density values found were 0.901 +/− 0.050 by the unimolecular model and 1.276 +/− 0.077 by the bimolecular model. Exposing a set of EBT2 films to 200 Gy yielded a measured optical density of 1.360 +/− 0.070, which indicated larger systematic uncertainties in the unimolecular model compared to the bimolecular model. Conclusions: Although both the unimolecular and the bimolecular models fit the experimental data with similar accuracy, only the bimolecular model could predict the maximum optical density of the EBT2 film with acceptable accuracy. We also observed that the energy spectra at the measurement depths play a role in the LET response of the EBT2 films when described as a bimolecular reaction. This project is supported in part by P01CA021239 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

SU‐E‐T‐69: A Novel Method Using Agfa‐Kodak‐ Computer Radiography System for Routine Quality Assurance Tests on Linear Accelerators

Purpose: Since conventional radiographic films are becoming obsolete, we have developed a novel technique utilizing ComputerRadiography(CR) plates to perform routine Linacquality assurance checks. Methods: A photostimulable CR plate can detect both direct and indirect ionizationradiation, and is even sensitive to photons in the visible range. A technique involving optical bleaching of radiation field on CR plate is being established as the calibration tool for performing routine quality checks such as: light field radiation congruence, symmetric/asymmetric jaw positioning. The CR plate is removed from its cassette with minimal background light and is exposed to a known field size of 6/18 MV photons in air at a fixedsource to surface distance. The jaws are then moved in to form a smaller field size, and light field is then shone on the pre exposed plate. This causes photo bleaching of radiation field proportional to the jaw settings and thereby providing a suitable surrogate to establish the light and radiation coincidence. The plate is then read utilizing a Kodak (ACR 2000i) scanner which reads the photo stimulated luminescence and converts the signal into a digital image pixel map. Analysis using a MATLAB‐based algorithm was done to compute the dose gradient at the junction of the light and radiation field. The technique has been tested against conventional methods of usingfilm and graph paper. Results: All light and radiation field measurements using the CR plate were within 1 mm from measurements. The method has been verified for its reproducibility and estimated uncertainty involving software based analysis has been found to be within +/− 0.2%. Conclusions: A method to efficiently, accurately and reproducibly perform routine QA has been developed. The technique has yielded a reproducible method to quantify the congruence between the light and the radiation field.