bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐152: Convolution/superposition Algorithm and High‐Z Dental Materials: Dosimetric Study in a Solid Water Slab Phantom
Authors: Spirydovich S , Papiez L , Langer M , Thai V
Published: July 2006

Purpose To address the accuracy of the dose calculated with convolution/superposition algorithm in the presence of high‐Z dental materials. Methods and Materials Three methods were utilized to access the dose: convolution/superposition algorithm, Fluence Map Monte Carlo (FMMC) method, and radiochromic film. We considered a solid water® slab phantom which had an embedded high‐Z material. For dose calculations and measurements we used a 6MV photon beam from a clinically commissioned linear accelerator. Results We observed a close agreement for the dose measured with radiochromic film and the dose calculated with FMMC algorithm. On the other hand, a large discrepancy was discovered for the dose calculated with the convolution/superposition algorithm compared to the dose obtained with measurement or FMMC algorithm. The greatest discrepancy was observed downstream from the high‐Z cerrobend inhomogeneity where the convolution/superposition algorithm calculated a dose which was higher than the dose measured with radiochromic film by 10–20% depending on the size of and the distance from the inhomogeneity. Clinically this finding shows that the delivered dose would be 10– 20% less than the prescribed dose which was calculated with convolution/superposition algorithm. In the region upstream from all the studied high‐Z inhomogeneities the convolution/superposition algorithm was underestimating the delivered dose. The convolution/superposition algorithm was unable to properly estimate the dose enhancement due to the increased backscatter near the inhomogeneity. Conclusions The convolution/superposition algorithm may significantly overestimate the actual dose in the site of the tumor located downstream from the high‐Z dental restorations or prostheses.
SU‐FF‐T‐153: Correlation of ICRU Reference Dose and PTV Dose Descriptors in Intensity Modulated Radiation Therapy
Authors: Yaparpalvi R
Published: July 2006

Objective: The application of IMRT in clinical trials has been hindered by a lack of internationally accepted standards for prescribing dose as in 2D and 3DCRT external beam planning. We aimed to study the relationship of ICRU reference dose to mean, median, and modal PTV doses for IMRT applied to diverse targets. Materials and Methods: DMLC‐IMRT plans for 70 patients treated for prostate (14), gynecological (19), head and neck (19), lung (4), rectal/anal (8), brain (4) and abdominal disease (2) were randomly selected and analyzed. The ICRU reference point was located in each plan following ICRU report 50 and 62 guidelines. The ICRU reference dose, PTV mean, median, and modal doses, and DVHs were calculated with an Eclipse treatment planning system (Varian). PTV range was 50 – 1937 cc. Median PTV was 508 cc. Results: In general, ICRU reference dose was > PTV mean dose (in 82% of the cases studied). The difference between the PTV mean and the ICRU reference doses was 2% or less in 77% of the cases studied (mean difference ‐0.73%, range ‐4.2% to +3.6%). The ICRU reference dose and the PTV median and modal doses were not significantly different (p = 0.3 and 0.15 respectively). Conclusion: The ICRU reference point dose for the IMRT described herein appears to be reflective of PTV median or modal doses but does not represent PTV mean dose. New dose specification standards for IMRT, consistent with methods used to accredit facilities to apply IMRT in clinical trials, may enhance the quality of these trials and of other trials using rapidly emerging radiation planning and delivery techniques.
SU‐FF‐T‐154: Cumulating Static Dose Distributions to Simulate Dynamic Dose Distributions: An Experimental Study
Authors: Ayadi M , Sarrut D , Ginestet C
Published: July 2006

Purpose: To quantify the change of the beam penumbra when irradiating a moving object. To compare dose distributions from irradiated films of an inhomogeneous moving phantom with cumulated static dose distributions (SDD) obtained from the TPS. Method and Materials: A cubic inhomogeneous phantom (“tumor in lung”), consisting of a polyethylene insert embedded in a wood phantom, was used for the study. This phantom was set on a “dynamic” plate which simulated respiration. This plate had a vertical excursion of 2cm with a 4s period. X‐OmatV films were placed at the center of the insert. They were irradiated perpendicularly to the direction of the movement with a 6MV photon beam of 8×8cm2. The movement of the plate was split in 4 intervals of positions. For each position interval, the duration was determined according to the sinusoidal model. SDD were calculated for the different positions of the moving phantom with the superposition convolution algorithm of the Xio TPS. Dynamic dose distributions (DDD) were simulated by summing SDD weighted with the temporal weight. Simulated DDD from films or from the TPS were finally compared with the measured DDD. Penumbrae were measured from dose profiles at the center of the phantom. Results: For a motion amplitude of 2cm, the penumbra increased with a factor of 2.7 while penumbra from calculated dose distributions increased with a factor of 2.4. Conclusion: This preliminary study aims at personalizing margins in lung treatments.
SU‐FF‐T‐155: Derivation of Photon Energy Spectra From Transmission Measurements Using Large Fields
Authors: Nes E , Esquivel C , Waggener R
Published: July 2006

Purpose: To reconstruct a 6 MV photon spectrum using an iterative process based on attenuation measurements performed in large fields. Method and Materials: The main algorithm written in Mathematica® code uses as input data Monte Carlo‐predetermined pencil beam monoenergetic scatter kernels for various water phantom thicknesses, open beam fluences and beam fluences measured in air with phantoms of different thickness. The iteration starts with a flat spectrum used to calculate the polyenergetic kernels for each water thickness. The scattered radiation is calculated by convolving open beam fluences with the corresponding polyenergetic kernels. The primary fluences are determined by subtracting the scatter fluences from the fluences measured with the phantom in place. The reconstructed primary energy spectrum is derived from the transmission values using the simulated annealing technique. The spectrum determined at the end of the loop is compared to the input spectrum of the main algorithm. If the new spectrum does not meet the stopping criterion, it is fed as input for a new iteration. Results: 72 data Monte Carlo monoenergetic scatter kernels are derived for six water thicknesses. The amplitude of the monoenergetic scatter kernels increases with energy and water phantom thickness. For thin phantoms there is a strong dependence of scatter with thickness. For large phantoms the increase is negligible after a certain phantom thickness which depends on beam energy. The average energy of the derived spectrum is 1.9MeV. There is a good agreement (2%) between measured and calculated PDD except at the first portion of the graph (electron contamination from head is not account for). Conclusion: The method is robust, good for portal dosimetry. It can be used to evaluate accurately the photon scatter in portal imaging since is taking into account the energy spectrum dependence of the scatter.
SU‐FF‐T‐156: Determination of Skin Dose for Modulated Electron Radiation Therapy (MERT)
Authors: Stathakis S , Paskalev K , Li J , Ma C
Published: July 2006

Purpose: Estimation of surface dose is very important for patients undergoing radiation therapy. This work is aimed at accurate determination of the dose to the skin at a depth of 0.07 mm, the practical reference depth for skin as recommended by ICRP and ICRU, using ultra thin TLDs, parallel ion chamber and Monte Carlo calculations for patients undergoing energy modulated electron radiation therapy (MERT). Method and Materials: Monte Carlo simulations and measurements were carried out for 5×5 cm2 and 10×10cm2 fields for electron beams of energies ranging from 6 to 21 MeV. The dose at the ICRU reference depth was computed at normal incident angles. For patient undergoing MERT treatments, the SSD to the patients skin is 60cm and the treatment is delivered with the photon MLC. Finally, the dose was measured and calculated for breast MERT plans using the leaf sequence obtained for each case. Results: Good agreement (±3%) was achieved between measurements and calculations. The surface dose at the entrance was increased as the electron beam energy and/or the field size increased. A decrease of the surface dose is observed when the SSD is smaller. The surface dose at 60cm was measured to be 1–4% lower than the one at 100cm SSD. The surface dose was increased under MERT conditions proportional to the modulation in MU of the treatment. Conclusions: The dose at the surface of the patient is mostly dependent on the SSD, the electron beam energy and modality. By correlating the TLD measurements to Monte Carlo calculations, one can predict the dose at the skin surface with great accuracy. Knowing the dose received at the surface of the patient can lead to prediction of skin reactions helping with the design of new treatment techniques and different dose fractionation schemes.
SU‐FF‐T‐157: Determination of the Effective Chamber Position of 2D Ion Chamber Array Using An HDR Afterloading Device
Authors: Biggs P
Published: July 2006

Purpose: Detector arrays are useful devices for quality assurance (QA) since they can record and display 2D data in real time, thus replacing film for many QA procedures. We have carried out tests on one such device*. Methods & Materials: This device consists of 1020 ion chambers, 4.5 mm in diameter, 5 mm high and 7.6 mm center to center. This 32×32 chamber array is located at some depth below the surface of the surface of the container. To determine the effective depth of the chambers below the surface, experiments were carried out using a 4 MV linac and an HDR afterloading device with a source. For the linac, measurements were made at distances between 80 and 135 cm, in 5 cm steps. For the afterloading device, a special jig was made that permitted placement of the catheter holding the source at distances from 1 to 10 cm at 1 cm steps. Plots were made of the inverse root of the charge collected vs. the distance from the top of the detector. Results: The inverse square law was found to be valid in both cases (r2 = 0.9994 [linac] and 0.9998 [HDR]). The linac experiment indicated that the effective distance of the chambers below the surface was −1.9 ± 0.9 cm, while for the HDR experiment it was −0.62 ± 0.03 cm. Thus the error for the HDR measurement is considerably less than for the linac measurement. This is clearly due to the fact that the linac data has to be extrapolated over a large distance, leading to a greater uncertainty. Conclusions: Use of a afterloading source has enabled the effective chamber position of a 2D ion chamber array to be determined to an accuracy better than 0.5 mm. MatriXX *(Scanditronix‐Wellhofer, Bartlett, TN) (Research sponsored by Scanditronix‐Wellhofer GmBH).
SU‐FF‐T‐160: Development and Use of a Dose‐Volume Histogram Analysis Tool
Authors: Bice W , Jurkovic I , Sims M , Prestidge B
Published: July 2006

Purpose: To develop an automated tool providing rapid, consistent analysis of the dose‐volume histograms (DVH) generated by commercial treatment planning systems (TPS). This tool has been used for comparative analysis of competing plans and is currently being used to study and mimic physician decision criteria. Method and Materials: Software was developed to import DVH information stored in RTOG submission format, making it relatively independent of the TPS used to generate the plan. Analysis tools are provided to generate conformity, uniformity and radiobiological quantifiers which describe each treatment plan. These quantifiers are presented separately and as overall plan evaluation values, to include CTI, CN and COIN. Radiobiological quantifiers include NTCP, EUD and EUBED. Results: The software has been used to evaluate competing techniques—(1) conventional, (2) two‐field tangential inverse‐planned IMRT and (3) multiple (3 or more) beam IMRT—of breast irradiation on 20 patients. Plans were adjusted to provide 90% of the prescription dose, 50.4 Gy in 28 fractions, to 90% of the PTV. The superiority of the dose distribution of the IMRT methods was clearly demonstrated as more conformal (CN 0.79 vs. 0.83, p < 0.001) with reduced doses to the lung (mean dose 5.4 Gy vs. 4.6 Gy, p = 0.004) and heart (mean dose 2.3 Gy vs. 1.5 Gy, p = 0.02). The radiobiological advantages of IMRT, although better, were less dramatic (EUD, not significant, NTCP effective volumes significant, but NTCP too small to draw conclusions). Conclusion: : Use of this tool enables easy, consistent interpretation of the DVH and the overall treatment scheme. Choosing between competing plans, developing and adjusting dose limits and weighting used in objective functions, and the ability to describe and mimic the physician decision making process are possible uses of the tool.
SU‐FF‐T‐161: Development of a More Objective and Accurate Film Verification Tool for Stereotactic Radiosurgery
Authors: Shoales J , Mistry V , Boyer A
Published: July 2006

Purpose: To develop a more objective and accurate quality assurance (QA) tool for use in spatial verification of patient treatment isocenter(s) to be used in conjunction with a commercial linear accelerator based stereotactic radiosurgery (SRS) system. Method and Materials: A plastic positioning device was constructed to house an open‐ended cylindrical lead foil ring approximately 0.3 [mm] thick and 1.0 [cm] in diameter. The cylindrical design was used for comparison versus the vendor supplied radio‐opaque ball design. Verification films were acquired using a 1.75 [cm] SRS cone with the same set‐up used for a SRS patient treated at our clinic. Films were acquired at gantry angles of 0°, 90°, and 270° for both the radio‐opaque ball and cylindrical device and repeated three (3) times. The objectivity of the film QA evaluation was determined by calculating the variance in the measurement of the radial offset distance for each gantry angle. In addition, five (5) additional sets of verification film data were acquired to evaluate the detection of a range of deliberate erroneous incremental offsets (0.0, 0.5, 1.0, 1.5, and 2.0 [mm]). The accuracy of the film QA evaluation was determined by measuring the isocenter offsets in the QA films and comparing those results to the known offset using linear regression analysis. Results: Calculated variance values from the QA films acquired at three (3) gantry angles measuring radial offset showed a reduction in user variability from 28% to 21% when the cylindrical device was used. Similarly, linear regression analyses of the incremental offset QA films demonstrated a slightly better correlation to the expected values when using the cylindrical device. Conclusion: The cylindrical SRS positioning device improves the objectivity of the QA procedure for isocenter verification and detects critical set‐up errors more accurately thus improving the ability of the physicist to assess patient set‐up.
SU‐FF‐T‐164: Development of Method to Visualize I‐125 Seed in Postimplant Seed Identification in Prostate Permanent Implant Brachytherapy
Authors: Takahashi Y , Sumida I , Kozuka T , Ito A , Yamashita T
Published: July 2006

Purpose: We investigated the method to visualize I‐125 seeds in CT‐based postimplant seed identification in prostate permanent implant brachytherapy using CT sinogram. Furthermore, we evaluated the impact of manual seed identification using our method on dosimetric parameter compared with automatic seed identification of commercially available planning system. Method and Materials: The reprojected CT sinogram was used to visualize I‐125 seeds. CT images of the metal parts only were separated from the original CT images by setting the threshold for pixel value. Then we performed edge detection. Using these images, sinograms of CT images with and without seeds were obtained by inverse Radon transform, and the sinogram of the metal image was subtracted from that of the original image. Finally, the image was reconstructed using the sinogram by Radon transform. The phantom study was performed to verify whether our method can separate adjacent seeds correctly. For patient study, postimplant analyses of 10 patients were performed. The implanted seeds were identified both by manually using our method and automatically using Variseed auto seed finder system. The differences of the number of seeds, dose delivered to 90 % of prostate volume (D90), and D5 of urethra between two methods were investigated. For statistics, paired t‐test was performed. Results: In phantom study, even two adjacent seeds were clearly separated by our method but not by automatic seed finder. For patient study, the number and orientation of seeds could be clearly determined. In post implant dosimetry, significant differences were observed between our method and automatic seed finder in seed number (p=0.009) and D90 of prostate (p=0.028). Conclusion: Although our method can only be applicable in manual seed identification, it can be useful in postimplant dosimetry in clinical practice.
SU‐FF‐T‐165: Development of Web‐Based Customized Cancer Radiation Treatment Planning Simulation System
Authors: Kum O
Published: July 2006

Customized cancer radiation treatment planning for each patient is very useful for both a patient and a doctor because it makes possible for the most effective treatment with the least possible dose to patient. Radiation planners know that too small a dose to the tumor can result in recurrence of the cancer, while too large a dose to healthy tissue can cause complications or even death. The best solution is to build an accurate planning simulation system to provide better treatment strategies based on each patient's computerized tomography (CT) image. We are developing a web‐based customized cancer radiation therapy simulation system consisting of three important computer codes; the parallel Monte Carlo high‐energy beam code (PMCEPT code) for calculating doses against the target generated from the patient CT image, the parallel mixed integer linear programming code for optimizing the treatment plan, and scientific data visualization code for efficient pre/post evaluation of the results. This independent system with a Beowulf PC cluster of about 100 CPUs is operated by the client‐server system via web because managing a big PC cluster and continuous updating of the complicated MC and optimization codes are not an easy job for a hospital. Another benefit for an independent client‐server system is to provide high quality results to many hospitals, which will eventually lead to decrease the medical cost and provide high‐quality medical service to local area hospitals. At the conference, we are going to introduce the idea and progress of this project. This work was supported, in part, by the SRC/ERC program of MOST/KOSEF (grant number: R11‐1999‐054).
SU‐FF‐T‐168: Does IMRT Planning Increase the Effect of Systematic Patient‐Beam Positioning Uncertainties On the Rectal DVH Compared to Conformal Planning?
Authors: Nguyen TB , Hoole ACF , Thomas SJ , Chantler H , Cowley I , Burnet NG
Published: July 2006

Purpose: The goal of this study is to compare the effect of systematic patient‐beam positioning uncertainties on the rectal DVH between IMRT and conformal planning and to validate the use of the dose constraints V95%, V90%, V75% for plan evaluation. Method and Materials: For a selected prostate case, a 47 segment IMRT plan, with five equally spaced treatment fields, was produced using the XiO treatment planning system (v4.02, CMS, St Louis). Additionally, a further two conformal plans, with three and four treatment fields respectively, were produced using the Addenbrooke's Radiotherapy Planning System (ARPS). In order to simulate the patient‐beam positioning for all plans identically, the XiO plan was imported into ARPS. Patient‐beam positioning errors were simulated in ARPS by shifting the treatment isocenter by 5 and 10 mm superior, inferior, left, right, anterior and posterior. For every isocenter position the resultant dose distribution and rectal DVH was calculated without altering the beam configuration or beam profiles. Results: The effects of patient‐beam positioning uncertainties are found to be significant when the isocenter is shifted along the A‐P direction. The rectal V95% for the unshifted plans was smallest for the IMRT plan. However, a 5mm posterior shift increases the V75% (55.5Gy) by 11.5% for the IMRT plan. This compares with increases of 10.1% and 10.6%, for the 3‐field and 4‐field plans respectively. In the higher dose region V90% (67Gy) increases by 10.4%, 9.7% and 9.4% for the IMRT, 3‐field and 4‐field plans respectively. Conclusion: Rectal DVHs for the IMRT plan seem to be more sensitive to patient‐beam positioning uncertainties along the A‐P direction in the medium dose region compared to the conformal plans. An acceptable plan, determined using dose constraints can become an unacceptable plan when geometric uncertainties are taken into account.
SU‐FF‐T‐172: Dose Mapping for Interrupted Tomotherapy Treatments
Authors: Rassiah P , Papanikolaou N , Fuss M
Published: July 2006

Purpose: To map out the dose delivered to a patient, when treatment is interrupted and not completed, several times while undergoing tomotherapy. Method and Materials: The patient data whose treatment had been interrupted 5 out of 25 fractions of 2 Gys was extracted. Two plans were used for patient treatment, plan 01 with field width of 1.06 cm and plan 02 with 2.49 cm field width. The data includes dose per fraction, total dose, total treatment time, tt, duration of partial treatment, tp; field width, FW; couch speed and coordinates of the first CT slice with the target delineated. The treated distance from the first CT slice with target delineated is given by, Distance treated = ((tp − 10.0s) × couch speed) − FW 10s is the time needed for machine warm up and the couch does not move during this time. A field width distance needs to be subtracted to account for the start of treatment when the proximal edge of the beam hits the superior most slice that contains the delineated target. Since 100 % of the prescribed dose is delivered in each treated slice, the prescribed dose per fraction can be subtracted from the untreated area, hence reducing the total dose in the untreated area for every untreated fraction. Results: The composite plan is then produced based on the subtraction of dose from various untreated sections. Conclusion: This method serves as a simple and fast way of approximating the composite dose to a patient with single or multiple interrupted treatment fractions, which are not completed. However, a more accurate computer based calculation that takes, into account the scatter dose, leakage dose and the partial delivery of the modulated beams may be needed to provide more accurate doses mainly at the junction of interruption.
SU‐FF‐T‐173: Dose Per Monitor Unit Determination for Proton Therapy Treatment Portals with and Without the Range Compensator
Authors: Fontenot J , Newhauser W
Published: July 2006

Purpose: To determine whether dose per monitor unit values (D/MU) for small‐field proton therapy treatment portals can be more reliably measured with or without the field‐specific range compensator present. Methods and Materials: Treatments of 14 geometric models representative of typical neurosurgery patients were simulated using a Monte Carlo model of the M. D. Anderson Proton Therapy Center‐Houston double scattering nozzle. Simulations of field‐specific D/MU calibration measurements were carried out in a water phantom with and without the range compensator present. D/MU values from each calibration technique were compared to values from the patient treatment simulation. For each case, D/MU values were scored with metrics that characterized the accuracy, uncertainty, the standard deviation of accuracy and uncertainty, worst agreement, and maximum uncertainty. The metrics were combined by defining the following figures of merit (FOM), which ranged in value from 0 to 1 (0 being worst, 1 being best): total FOM (a composite of all metrics), clinical FOM (accuracy and uncertainty metrics), variability FOM (standard deviations of accuracy and uncertainty metrics), and worst‐case FOM (worst agreement and maximum uncertainty metrics). The two D/MU calibration techniques were compared based on the FOMs. Results: the total fom when measuring without the range compensator was 0.85 and 0.49 with the compensator. The clinical, variability, and worst‐case foms were 0.85, 0.92, and 0.79, respectively, without the range compensator, compared to 0.51 (clinical), 0.48 (variability), and 0.46 (worst‐case) with the range compensator. The superiority of calibrating without the compensator was mainly attributable to the fact that the dose distributions were more similar to those in the patients. Additionally, determining d/mu values without the compensator is conceptually simpler and more convenient. Conclusion: For the 14 cases considered in this work, measuring D/MU without the range compensator provided more reliable values of D/MU than measuring with the range compensator.
SU‐FF‐T‐174: Dose Rate Dependence of Film Dosimetry in Radiation Treatment: Study of Reciprocity Law
Authors: Srivastava S , Das I
Published: July 2006

Purpose: Film has become an important tool for dose verification in individualized IMRT treatment fields. The optical density (OD) is related to dose rate also known as reciprocity law; (D= Dr*t). However for modern films (EDR and XV) reciprocity law has not been investigated which is presented in this study. Method and Materials: Using a Varian linear accelerator, dose rate dependence was studied for Kodak films (XV and EDR). The dose rate on this unit could be varied in the range of 80–400 MU/min for both (6 MV and 15 MV) photons beams. A large dose rate range; 5 cGy/min −1100 cGy/min was achieved in conjunction with distance (1–4 meters) and machine dose rate. This was verified using ion chamber. At each dose rate, films were exposed in a solid phantom at a depth of dmax for 300 cGy and 50 cGy for EDR and XV films, respectively. Calibration curves (dose vs. OD) were also established during this experiment in a standard condition. The measured dose through film and ion chamber were compared and analyzed. Results: Reciprocity law holds good in the dose rate range of 20–400 cGy/min for both energies but deviates at low and high dose rates. The effect is more pronounced at dose rate beyond 400 cGy/min where deviation up to 7.5% was noted for both the films. At low dose rate, the deviation is −3.5% for both films and energies. Conclusion: Low and high dose regions are created in the same time of exposure in IMRT and hence reciprocity law becomes critical for film dosimetry. The reciprocity law failure is due to the interaction of ion pairs to form latent image, which could be suppressed at extreme dose rates. The dosimetric impact is noted to be up to 7.5% for both films.
SU‐FF‐T‐175: Dose Rate Optimization for Intensity‐Modulated Arc Therapy
Authors: Goldman P , Chen J , Wong E , Battista J
Published: July 2006

Purpose: We use multiple dynamic arcs with different MLC shapes at different gantry angles for Intensity‐modulated arc therapy (IMAT). This study is to develop a fast and simple algorithm to optimize the dose rate as a function of gantry angle for IMAT for linear accelerators with dose rate variation capability for a dynamic arc. Method and Materials: The dose rate as a function of gantry angle for a dynamic arc is expressed by an analytical expression that favors beam directions with lower dose deposited to critical structures and higher dose deposited to the target volume. Three parameters are included in the analytical expression for further optimization of the dose rate base on an objective function. The optimized plan is transferred to a forward treatment planning system for final dose calculation. The method is applied to a phantom and a few clinical cases using 6 MV and 18 MV beams for a Varian linear accelerator. Results: The method has been demonstrated with a test phantom and a few clinical radiotherapy cases. We have showed that dose distributions and dose volume histograms have been improved with the dose rate optimization for IMAT. It gives more uniform dose to the target volume and lower dose to the critical structure. Conclusions: We have developed a fast and simple technique to optimize the dose rate as a function of gantry angle for intensity‐modulated arc therapy, while retaining its advantages of an intuitive treatment planning process and efficient radiation delivery.
SU‐FF‐T‐176: Dose Verification in Moving Targets During Helical Tomotherapy Beam Delivery
Authors: Zeidan O , Langen K , Meeks S , Willoughby T , Wagner T , Kupelian P
Published: July 2006

Purpose: To compare dose distributions from stationary and moving targets subject to simulated intra‐fraction motion during treatment deliveries on a helical tomotherapy unit. Method and Materials: We have constructed an in house platform that moves in the superior‐inferior direction in a controlled manner. Using a stationary CT scan of a thorax phantom, a fictitious target volume was defined and a helical tomotherapy plan was generated. The phantom was placed on top of the moving platform and set in motion at a frequency of 16.7 cycles/min and 20 mm peak‐to‐trough excursion. An IMRT plan was delivered while the phantom was in motion and when it was still. Ion chamber readings were recorded at the center of the target. Different phases were artificially introduced by starting the platform motion at different times before the start of irradiation. In each delivery a film was placed in the coronal plane inside the phantom to register 2D dose. Results: Profiles along the direction of motion from the co‐registered images of moving and stationary deliveries have shown dose differences as large as 70% in the penumbra region. Gamma function maps for different dose and distance to agreement criteria revealed the regions at which the gamma criteria fails and was shown to be a strong function of initial motion phase. Ion chamber point measurements recorded insignificant differences between still and moving deliveries. Conclusions: We have shown that there is a significant difference in 2D dose distributions in the penumbra region both in the inferior‐superior and transverse directions between stationary and moving deliveries that simulate breathing motion. Our results indicate that the differences are highly dependent on the breathing motion initial phase. The phantom motion had little effect on the delivered dose at the central target region.
SU‐FF‐T‐178: Dosimetric and Technical Aspects of Intraoperative I‐125 Brachytherapy for Stage I Non‐Small Cell Lung Cancer
Authors: Johnson M , Colonias A , Parda D , Miller L , Miften M
Published: July 2006

Purpose: Initial outcome data from our institution have shown that sublobar resection in combination with I‐125 brachtherapy is associated with recurrence rates of 2.0%, compared to 18.6% with sublobar resection alone. The objective of this work is to present the technical and dosimetric aspects required to execute this procedure from the radiation oncology perspective, as well as to analyze the dose distributions of patients treated with this technique. Method and Materials: I‐125 seeds in vicryl suture were embedded into vicryl mesh and surgically inserted providing a 2.0 cm margin on each side of the resection staple line. A nomogram was developed to determine the suture spacing in the vicryl mesh, as a function of seed activity in order to deliver 120 Gy at a distance of 0.5 cm above and below the seed array. Postoperative dosimetry consists of CT‐based planning and dose volume analysis. Results: Dose distributions, DVH data, mean dose and NTCP values for lung were analyzed in a group of patients. The mean doses ranged from 3.72 Gy to 9.10 Gy. NTCP values were below 1%. DVH data shows that a small volume of ipsilateral lung was irradiated for all patients. A comparison of brachytherapy with external beam therapy was investigated for one patient. Brachtherapy was judged to be superior to external beam therapy. Results showed that most of the ipsilateral lung volume received a lower dose when treated with an implant to 120 Gy compared to external beam therapy treated to 60 Gy. Conclusions: Lung brachytherapy with I‐125 at the time of sublobar resection is a precise and conformal option of dose delivery for stage I NSCLC patients with compromised physiologic reserve. Patient related toxicity has not been linked to this procedure. This simple technique provides significant lung sparing when compared to standard external beam therapy.
SU‐FF‐T‐180: Dosimetric Characteristics of Tm‐170 as a Radionuclide for Its Possible Use in Brachytherapy
Authors: Ballester F , Meyer B , Granero D , Perez‐Calatayud J , Casal E , Cases R
Published: July 2006

In clinical brachytherapy several types of photon sources are used, mainly Cs‐137, Ir‐192, I‐125, and Pd‐103. The Tm‐170 is a promising radionuclide for use in brachytherapy because of the low mean‐energy (46.75 keV or 66.39 keV if the lines below 10 keV are removed) and the possible high specific activity (2.21×1014 Bq/g for a half life of 128.6 days). Tm‐170 is produced in a nuclear reactor by neutron absorption of the natural Tm‐169 and decays mainly via β‐emission. The maximum energies of the β‐rays are 0.290 and 0.323 MeV. These β particles are thus absorbed in the source core and in the encapsulation cover producing bremsstrahlung that contributes significantly to the dose. These facts must be taken into account to design Tm‐170 sources in order to calculate source and encapsulation thicknesses. The purpose of this study is to determine by means of the Monte Carlo method the dosimetric characteristics for a Tm‐170 point source and for a hypothetical spherical source with an active core of Tm‐170 encapsulated by a titanium or stainless‐steel cover. Different active radius and cover thicknesses have been considered. The Monte Carlo GEANT4 code was used in this study to obtain the radial dose function of the sources studied. The radial dose function obtained is similar to the radial dose function of Ir‐192 or Cs‐137 sources. This fact and the low energy of the photons emitted by Tm‐170 makes this radionuclide very interesting for its use in HDR or PDR brachytherapy. This study may be interesting for manufacturing future Tm‐170 sources.
SU‐FF‐T‐182: Dosimetric Comparison of An Elekta “Synergy S” Beam Modulator & Radionics MMLC Using Monte Carlo Simulations & Measurements
Authors: Esnaashari N , Heydarian M , Allahverdi M , Jaffray D
Published: July 2006

Purpose: To compare dosimetric parameters of a new Elekta “Synergy S” dedicated stereotactic radiosurgery MLC, namely the beam modulator (BM), with Radionics mini‐multileaf collimator (MMLC). Methods and Materials: The Beam Modulator maximum opening is 16cm×21cm and consists of 40 pairs of Tungsten leaves of 4mm thickness at the isocentre, with no back up jaws. Radionics MMLC has a maximum field size of 9.6cm×12cm and 3.75mm leaf thickness at the isocentre. Leakage and transmission, percentage depth doses (PDD) and dose profiles were measured and calculated for different field sizes and depths and for different source to surface distances (SSD). Kodak XV films, photon diode detector (diameter of active area 2mm), CC13 Wellhofer ion chamber (cavity volume 0.13 cm3) and Wellhofer water tank were used for measurements. BEAMnrc code was used for the Monte Carlo (MC) simulations. All the data are for a 6MV photon beam. Results: It is shown that the BM beams are slightly more energetic so that PDD at 10cm depth is 2% more for a 10.4cm × 9.6cm field, compared to Radionics MMLC. Dose profile results are generally comparable, except for the penumbra which is sharper for Radionics MMLC, especially in the leaf travel direction by up to 1.1 mm. Maximum and average leakage was 1.7 and 1.1 for BM and 1.2 and 0.9% for MMLC, respectively. MC calculation and measurement results for PDD and profiles agreed well to better than 1% and or 0.5mm. The uncertainty in simulation was less than 0.5%. Conclusion: Elekta “Synergy S” beam modulator and Radionics MMLC have successfully been modeled for the first time using the BEAMnrc MC simulations. The MC results showed an excellent agreement with the measurements. BM has a wider penumbra, mainly due to the larger isocentric distance and rounded leaf ends.
SU‐FF‐T‐187: Dosimetric Evaluation of MammoSite Breast Treatments
Authors: Kim H , Beriwal S , Heron D , Huq M , Yue N
Published: July 2006

Purpose: This study was performed to assess the volumetric dosimetry of single point optimization with single dwell position technique for MammoSite brachytherapy treatment and compare the results with accepted dosimetric constraints in the ongoing NSABP B39/RTOG 0413 partial breast irradiation protocol. Method and Materials: 74 patients received MammoSite breast treatments with single dwell position, single point optimization technique using orthogonal film based planning. CT‐images were transferred to the PLATO treatment planning system version 14.2.6 and retrospectively planned for volumetric calculations without changing dwell time and position. The D90, V100, V150 and V200 and dose homogeneity index (DHI) were calculated. For comparison 9 randomly selected patients were planned with 3D volumetric optimization using multiple dwell positions and compared with 2D plan. Results: The mean D90 and V100 were 96% and 88% respectively. The mean V150 and V200 were 26 cc and 3.4 cc. The mean DHI was .68. All except three patients met the dosimetric criteria and constraint required in protocols. Two patients had D90 of 88.5% and 89% instead of ⩾90% and another patient had V200 of 18cc instead of ⩽10cc. The D90 and V100 for single vs. multiple dwell position were 93% vs. 98% (p= .0003) and 86% vs. 97% (.0004) respectively. The V150 and V200 were 23.5 cc vs. 34 cc (p=.0006) and 2.9 cc vs., 7.2 cc (p< .0001) respectively. The DHI was .71 vs. .61 (p=.0006). Conclusion: Single dwell position with single point optimization gives acceptable coverage and meets the requirements outlined in the NSABP B39/RTOG 0413 protocol in majority of patients. The multiple dwell position technique can generate plans with significantly better dose coverage of target but at the cost of increased dose inhomogeneity and increased hot spot. Correlations of outcomes with dosimtery will better help us assess these different treatment plans.
SU‐FF‐T‐188: Dosimetric Evaluation of Parallel Opposed GRID Radiation Therapy for Deep‐Seated Bulky Tumors
Authors: Meigooni A , Gnaster M , Dou K , Soleimani‐Meigooni N , Dini S , Kudrimoti M
Published: July 2006

Purpose: Grid radiation therapy, using single field of megavoltage x‐ray beam has been proven to be an effective method for management of bulky (>8 cm in diameter) malignant tumors. However, the effectiveness of this treatment modality for deep seated tumor is limited by the dose to the overlying normal tissue. In this investigation the use of parallel opposed beam is being evaluated for treatment of deep seated bulky tumors, using two different grid patterns. Method and Materials: Dosimetric characteristics of single field and parallel opposed radiation field were experimentally determined with film in Solid Water phantom material and using ion chamber in water. These measurements were performed with 6 and 18 MV x‐ray beams from a Varian Clinac 2100EX linear accelerator. Two different Grid block patterns, fabricated by Radiation Products Design, Inc. were utilized in these investigations. The GRID blocks molded into a cerrobend block of 7.5 cm thickness were manufactured with hole diameters of 5.9 mm and 8.5 mm. Results: Dose profiles and percentage depth doses of two GRID blocks for the parallel opposed beam were compared with a single radiation field. The results of these investigations indicated that with a 5.9 mm GRID block, dose to the overlaying normal tissue at dmax reduced from 200% for a single field to 100%, for the same tumor dose, with equally weighted parallel opposed fields. Similarly, for the 8.5 mm GRID block, dose to normal tissue at dmax were reduced from 185% to 100%. Conclusion: Parallel opposed GRID therapy resulted in a substantial decrease in dose delivered to the normal tissues for the same prescribed tumor dose. Thus, parallel opposed GRID radiotherapy is a viable option for treatment of deep‐seated bulky tumor. With this technique, the tumor dose can be increased for a higher therapeutic result.
SU‐FF‐T‐189: Dosimetric Figures‐Of‐Merit Based Comparison of Three IMRT Modalities: Helical Tomotherapy, Sequential Tomotherapy and DMLC‐IMRT
Authors: Duan J , Shen S , Popple R , Wu X , Pareek P , Brezovich I
Published: July 2006

Purpose: Intensity modulated radiation therapy (IMRT) can be delivered using several different techniques. The present study compares dosimetric characteristics of three commercial IMRT planning and delivery systems, i.e., helical tomotherapy (TOMO), sequential tomotherapy with MIMiC (MIMiC) and dynamic multileaf collimated IMRT (DMLC‐IMRT). Method and Materials: IMRT plans for three common clinical cases, head‐and‐neck (HN), brain and prostate cancers, were generated using respective RTOG protocols 9406, 9803 and H0022 as guidelines. For each case, multiple (more than four) plans were produced for each technique using different strategies by adjusting constraints and/or parameters such as beam width, modulation factor, the number of fields, etc. Dosimetric indices were used as figures‐of‐merit to evaluate the plans. These indices include coverage index (CI), overflow index (OI), sparing index (SI), and overall conformal index (COIN), where COIN=CI×OI×SI. The plans for each technique were compared among themselves to determine the best plan of each technique which in turn was used for the comparison of different techniques. Results: For the HN case, the COINs for TOMO, MIMiC and DMLC‐IMRT were 0.333, 0.064 and 0.305, respectively. TOMO had highest SI, but DMLC‐IMRT had highest CI and OI. MIMiC plan performance was poorer than TOMO and DMLC‐IMRT for the HN case. The COINs for TOMO, MIMiC and DMLC‐IMRT were 0.471, 0.616 and 0.685 for the brain case, and 0.304, 0.487 and 0.554 for the prostate case, respectively. DMLC‐IMRT had the highest COIN for both the brain and prostate cases. TOMO had best coverage for the brain case while DMLC‐IMRT had best coverage for the prostate case. Conclusion: IMRT plan optimization is a process to balance and compromise between target coverage and normal tissue sparing. The three techniques may result in different plans for similar constraints. Using different strategies, all three techniques produced clinically acceptable treatment plans for all cases.
SU‐FF‐T‐191: Dosimetric Improvement in Treatment of Advanced‐Stage Nasopharyngeal Carcinoma Using Split Organ Delineation Approach and Multiple Virtual Organs Generation Approach in Intensity‐Modulated Radiation Therapy Dose Optimization
Authors: Chau M , Leung S , Kam K , Cheung K , Kwan W , Yu K , Chiu K , Chan T
Published: July 2006

Purpose: To evaluate the efficacy of improving target coverage and sparing of the organs‐at‐risk (OARs) using split organ delineation approach (SODA) and multiple virtual organs generation approach (MVOGA) in planning of intensity‐modulated radiation therapy (IMRT) for advanced‐stage nasopharyngeal carcinoma (NPC) Method and Materials: Twenty NPC patients with T3‐4 tumors were selected. For each patient, a reference (REF) IMRT plan was generated with optimized target coverage and sparing of OARs. An investigative plan (INV) was also generated with the same planning protocol as the REF plan, with the exceptions that first, the contours of the parotid glands and temporal lobes were split into target‐overlapping and non‐overlapping regions, and second, multiple virtual organs were created to represent the normal tissues (such as segments of mucosa and muscles that are not conventionally designated as OARs). Each of the split and virtual organs was assigned with independent dose‐volume constraints. The REF and INV plans were compared with respect to the conformity index (CI), and dose‐endpoints of the OARs using paired‐t test. Results: The INV plan was superior to the REF plans in terms of the CI of the GTVs (0.55 vs 0.50, p = 0.017), CI of the PTVs (0.79 vs 0.70, p =0.000), and the minimum dose of the PTVs (47.2Gy vs 44.8Gy, p=0.008). For the OARs, there was significant reduction in dose in the INV plan in terms of the mean dose (max 4.6Gy, p=0.003) to the parotid glands, the maximum dose (max 10Gy, p<0.024) and the mean dose (max 6Gy, p<0.000) to the virtual organs. Conclusion: Using a split organ delineation approach and multiple virtual organs generation approach in IMRT treatment planning, further dosimetric improvement in target coverage in the regions that overlapped with organs‐at‐risk can be achieved, together with sparing of the parotid glands and normal surrounding tissues.
SU‐FF‐T‐193: Dosimetric Study and In‐Vivo Dose Verification for Conformal Avoidance Helical Tomotherapy of Anal Adenocarcinoma
Authors: Han C , Chen Y , Liu A , Schultheiss T , Wong J
Published: July 2006

Purpose: Recent studies have shown that for patients with anal canal malignancies, conformal avoidance intensity‐modulated radiotherapy (IMRT) could provide better dose homogeneity and sparing of organs‐at‐risk (OARs) than conventional 3‐D radiotherapy techniques. As a new IMRT technique, helical tomotherapy could achieve better dose modulation, and was expected to deliver adequate dose to surface lesion using tangential beams. This study aims to have dosimetric comparison between conformal avoidance helical tomoterapy plans and step‐and‐shoot IMRT plans for patients of anal adenocarcinoma, and to test the efficacy of helical tomotherapy for skin dose delivery. Method and Materials: We retrospectively generated conformal avoidance step‐and‐shoot IMRT (sIMRT) plans and helical tomotherapy plans for two anal cancer patients, one male and one female, with PTV volume being 5,622 cm3 and 5,280 cm3, respectively. The CORVUS Treatment Planning System was used to generate 7‐field sIMRT plans. Helical tomotherapy plans used a jaw width of 2.5 cm. In‐vivo skin dose measurements were performed using diodes placed on surface lesion and skin of external genitalia during conformal avoidance helical tomotherapy treatments of a recent anal cancer patient. Results: Compared to the sIMRT plans, the helical tomotherapy plans showed significant improvement of dose homogeneity for the PTVs, sharper dose drop‐off outside the PTVs, and significantly less radiation to femoral heads and external genitalia in terms of maximum dose and average dose. In‐vivo dose measurements showed adequate dose delivery to the surface lesion, and verified reduction of radiation to the skin of external genitalia. The maximum deviation of diode measurement from plan dose is 5.5%. Conclusion: Helical tomotherapy plans showed better dose homogeneity and conformity to the PTV and better sparing of OARs in conformal avoidance treatments of anal cancer compared to sIMRT plans. In‐vivo dose measurements confirmed the ability of helical tomotherapy for adequate skin irradiation.