bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐14: A High MU Efficiency Dynamic MLC Leaf Sequencing Algorithm
Authors: Thongphiew D , Wu Q , Chankong V , Song H , Yin F
Published: July 2007

Purpose: MU efficiency is a major figure of merit for dMLC leaf sequencing. However, leaf sequencing is often generated with focus on other constraints and the MU efficiency is taken as a byproduct. This work is to develop a new dMLC leaf sequencing algorithm (MinMU) that explicitly minimize the total MU. Method & Materials: At a constant dose rate, the maximum MU efficiency is achieved by minimizing, through optimization, an objective function which is essentially the total beam on time. The constraints considered are: maximum leaf speed, maximum separation between leaves and under dosing caused by tongue‐and‐groove effect. These mixed integer constraints are converted to linear ones by enumerating and eliminating redundant scenarios, for a given intensity map. These steps also help eliminating the infeasible solutions. The optimization was implemented for the Varian Millenium 120‐leaf MLC and was solved with the CPLEX software. Its performance was tested against published algorithms using phantom and real patient treatment fluence maps. Results: The fluence maps in the phantom study were generated with various grid sizes. The MU increases at a much slower rate for the MinMU algorithm than for the published algorithms with finer grid size. The MU efficiency advantage of the MinMU is more pronounced for more complex and larger fluences.

Among the 18 fluence maps that were taken from two real head‐and‐neck cancer treatment plans, the MinMU algoithm uses more MU for 2 fluence maps, same MU for 2, and less MU for the rest 14 fluence maps than the commercial treatment planning software. Overall, the MinMU used 10% and 7% less MU for the two plans, respectively. Also, the MinMU reproduced the original optimal fluence with minimal difference. Conclusion: A new dMLC leaf sequencing algorithm is developed. Its high MU efficiency potential is demonstrated.
SU‐FF‐T‐18: A Method for Determining the Water Kerma in a Graphite Phantom for X‐Rays with Up to 50 KV Tube Voltages
Authors: Schneider T , Selbach HJ , Kramer HM
Published: July 2007

Purpose: The Physikalisch‐Technische Bundesantalt (PTB) is developing a primary standard for the realization unit of the absorbed dose to water for low‐energy photon‐emitting brachytherapy seeds. As a first step towards this aim, an extrapolation chamber is used for the determination of the water kerma inside the phantom. Method and Materials: The water kerma in a graphite phantom has been measured by means of an extrapolation chamber and using a novel Monte Carlo based evaluation method. In essence it consists of a measurement of the air kerma in the phantom under secondary electron equilibrium, i.e. for large plate separations. The air kerma is converted to the water kerma for the case of zero plate separation. In order to verify this method a measurement of the air kerma free in air was made which was converted to the water kerma in the graphite phantom again by Monte Carlo calculations. Results: The comparison of the water kerma values in a graphite phantom, which are obtained by in phantom and free in air measurements shows an agreement of both methods within their combined uncertainty. The uncertainty of the presented method is approximately 1% (k=1). Conclusion: The good agreement found in the comparison supports the new evaluation method. In future work it is planned to calibrate a suitable secondary standard in terms of water kerma by means of the extrapolation chamber and using this device in the radiation field of I‐125 seeds.
SU‐FF‐T‐25: A Monte‐Carlo Based Dose Engine for Proton Radiotherapy Treatment Planning
Authors: Newhauser W , Fontenot J , Zheng Y , Taddei P , Mirkovic D , Titt U , Zhu X , Sahoo N , Schaffner B , Langenegger A , Koch N , Zhang X , Mohan R
Published: July 2007

Purpose: To develop a fast Monte Carlo (MC) dose engine for proton radiation treatment planning calculations and research studies. Method and Materials: We developed a complete MC simulation system for calculating the dose distributions in patients. The system uses a commercial treatment planning system with an analytical dose algorithm to design the treatment plans. A DICOM‐RT‐ION interface was developed to allow automated data transfer between the treatment planning system and the MC system. The MC model included all major components of a passively‐scattered proton treatment unit and a CT‐based patient model. A suite of programs converted the prescriptive data (e.g., range, modulation width, field size) and patient CT data into a MC input file, including coordinate system transformations for arbitrary treatment beam orientations. The radiation transport calculations are performed with the MCNPX Monte Carlo system running on a cluster of 512 64‐bit CPUs. For each treatment field, multiple simulation output files were postprocessed and the resulting MC dose matrix was written to the DICOM‐RT‐ION plan. The plan was then imported into the commercial planning system for visualization. Results: The MC simulations and pencil beam dose distributions are in good agreement for a two‐field prostate plan and a three‐field lung plan. The prostate plan required 1.6 hours and the lung plan required 14 hours using 512 CPUs to achieve < 2 % statistical uncertainty in the total dose at isocenter. The computing time was directly related to the number of voxels in the patient model. Timing studies revealed that the simulation speed for this system scales almost linearly with the inverse number of CPUs. Conclusion: The results of this study strongly suggest that it is feasible to implement a fast and easy‐to‐use MC treatment planning dose engine with currently available computing technologies and resources.
SU‐FF‐T‐29: A New Quantitative Quality Assurance Evaluation Parameter (gamma Plus) and Its Clinical Assessment
Authors: Xu J , Shi C , Jiang S , Papanikolaou N
Published: July 2007

Purpose: To extend existing gamma index by including biological effect dose information and propose a new index called gamma plus. Method and Materials: A new quality assurance index, gamma plus, is proposed based on the theoretical concept of the gamma evaluation presented by the Low et al. The dose difference including the biological dose information (Biological Uniform Dose BUD was used in this study) is used instead of the physics dose difference when performing the gamma plus calculation. An in‐house software was developed to compare the dose distribution based on the gamma plus concept. A test pattern for two‐dimensional dose comparison was done using the developed software platform. The gamma plus was also tested on planar dose exported from Pinnacle Treatment Planning System (TPS) and film dose distributions acquired in a solid water phantom for one clinical case. Results: The preliminary result showed that the gamma plus index can exhibit the similar result as the gamma index. At the same time, the gamma plus index showed differences from the gamma index for the test pattern analysis since it considers biological dose information. For the clinical case, it is observed that the gamma plus index varies for different treatment parameters (e.g. dose per fraction), which indicates that treatment outcome is considered in the gamma plus index. Conclusion: The gamma evaluation is widely accepted for the quality control of IMRT treatment. The gamma plus proposed in this study is based on the gamma index theory and also considers the biological effect. Like gamma evaluation, gamma plus index combines the dose difference and the spatial shift. Furthermore, gamma plus index includes biological effective dose information, which can help to understand the biological effect difference for the quality assurance result.
SU‐FF‐T‐34: A Possible New Proton Therapy Technique — Proton Knife
Authors: Lu L , Chen D
Published: July 2007

Purpose: We propose a possible new proton therapy technique — “Proton Knife” that may enhance the superiority of proton therapy to other external beam therapy techniques. Method and Materials: Proton beam therapy is considered to be superior to X‐ray beam radiation therapy because of proton's sharp Bragg peak, though clinical studies of proton therapy verse other kinds of radiotherapy are still debatable and not conclusive. Compromise between capital cost and clinical effect becomes a major issue for proton therapy. We propose a preliminary new strategy to take advantage of the proton Bragg peak, which greatly enhances Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP). The feasibility of this technique will be carried out through Monte‐Carlo simulation. Results: A flow chart describing this technique is presented and the feasibility will be discussed. Conclusion: A preliminary technique of “Proton Knife” is proposed and discussed. Conflict of Interest: The authors of this abstract here claim that any actual or potential conflicts of interest do not exist in this abstract.
SU‐FF‐T‐38: A Recursive Differential Equation Approach Towards the Problem of Estimating the Probability of M Surviving Clonogens After the Ith Irradiation
Authors: Stavreva N , Stavrev P , Fallone BG
Published: July 2007

Purpose: A model is developed that describes the tumor dynamics of a repopulating tumor during a fractionated external radio treatment. Method and Materials: An iterative algorithm is constructed for the calculation of the probability distribution of the surviving number of tumor cells for any point in time during the treatment and for post treatment times. This is accomplished through the assumption, that cell repopulation takes place only during the intervals between the fractions of irradiation. Thus, the processes of cell repopulation and of radiation‐induced cell kill are considered decoupled and are therefore modeled separately. Binomial statistics is applied to describe the radiation induced cell damage. Natural death is ignored. Tumor repopulation is modeled through the construction of differential equations for the probability that a certain number of cells exist at a certain point in time. The differential equations are solved in a straightforward manner. Results: The probability of zero survivals is compared numerically to the Zaider‐Minerbo tumor control probability (TCP) model for an arbitrarily chosen regime of treatment. An excellent agreement is found to exist between the two expressions. The Poisson approximation of the probability distribution of the number of cells is also studied and it is compared to the proposed method for several different combinations of the model parameter values. Conclusion: It is concluded that the Poisson approximation works well only for slowly repopulating tumors and for rather high cell radiosensitivity (i.e. low cell survival probability after a fraction of radiation). Therefore, the use of the exact method for the description of the radiation response of repopulating tumors is recommended.
SU‐FF‐T‐40: A Robust Proton Treatment Planning Strategy to Account the Diaphragm Motion for the Esophagus Cancer Using 4D CT
Authors: Pan X , Zhang X , Li Y , Mohan R
Published: July 2007

Purpose: The purpose of this work is to design a treatment plan so that the target will be adequately covered on each phase of four‐dimensional computed tomography (4D CT) for patients with large diaphragm motion when the same treatment plan was recalculated in all 10 phases of the 4D CT. Method: We evaluate two planning strategies based on (1) average CT and (2) inspiration CT. For both strategies, smearing, proximal and distal margins were adjusted to improve the target dose coverage for 10 phases of 4D CT. For an esophagus patient with large diaphragm motion (∼5cm), a treatment plan was designed based on strategies (1) and (2). The same treatment plan was recalculated in all 10 phases of the 4D CT. Result: For strategy (1), internal target volume (ITV) prescription dose coverage at inspiration/expiration phase decreased/increased from 99.3%/91.5% to 94.6%/99.6% when the smearing margin was increased from 0.5 cm to 3.5 cm. Adjusting the distal and proximal margin had not effect on the ITV coverage. For strategy (2), using the smearing margin 3.5 cm and an increased distal margin for one of the three beams, the ITV prescription dose coverage is at least 99.5% for all ten phases of 4D CT. Conclusion: By designing a proton treatment plan on the inspiration phase with slightly adjusted planning parameters, it is possible to have the target adequately covered for the esophagus cancer patient with large diaphragm motion.
SU‐FF‐T‐42: A Simple Geometric Algorithm to Predict Optimal Starting Gantry Angles Using Equiangular‐Spaced Beams for IMRT of Prostate Cancer
Authors: Potrebko P , McCurdy B
Published: July 2007

Purpose: A fast, geometric beam angle optimization (BAO) algorithm for intensity‐modulated radiation therapy (IMRT) was implemented on ten localized prostate cancer patients on the Radiation Therapy Oncology Group 0126 protocol. Method and Materials: Fifteen segmental IMRT plans per patient were generated in Pinnacle3 using 5 equiangular‐spaced beams with 5° increments of the starting gantry angles. Constant target coverage was ensured for all plans in order to isolate the variation in the rectal dose metrics as a function of starting gantry angle. A geometric BAO algorithm computed the beam intersection volume (BIV) within the rectum using 5 equiangular‐spaced beams as a function of starting gantry angle for comparison to the rectal V 75 Gy and V 70 Gy. Results: The variations in rectal V 75 Gy and V 70 Gy as a function of starting gantry angle using 5 equiangular‐spaced beams were statistically significant (p < 0.001) with dosimetric importance. The class solution ‘W’ pattern in the rectal V 75 Gy and V 70 Gy (with two separate minima centered near 20° and 50°) was reproduced by the 5 BIV within the rectum. A strong correlation (Pearson's correlation coefficient > 0.70) was found between the rectal 5 BIV and the rectal V 75 Gy and V 70 Gy. The geometric BAO algorithm predicted the location of the two dosimetric minima in rectal V 75 Gy and V 70 Gy (optimal starting gantry angles) to within 5°. Conclusions: This BAO method is the first geometric algorithm capable of predicting an optimal IMRT dose distribution with typical computation times of only a few minutes. Given the clinically infeasible computation times of many dosimetric BAO algorithms, this robust geometric BIV algorithm has the potential to facilitate beam angle selection for prostate IMRT in clinical practice.
SU‐FF‐T‐45: A Simulation Study of Intra‐Fraction Motion Effects On the Proton Discrete Scanning Treatment
Authors: Mo X , Mackie T , Kissick M
Published: July 2007

Purpose: Spot scanned beam delivery can be used to implement intensity modulated proton therapy (IMPT) to better conform dose distributions to the target volume. Unlike passive scattering beam delivery techniques, the interplay of the infra‐fraction organ motion and the discrete spots scanning can introduce ‘hot’ and ‘cold’ spots. We investigated the variation of computer simulated delivered dose, and the improvements for motion effects by scanning with multiple beam widths so that regions that needed higher resolution had smaller beam widths. Method and Materials: The simulation was done for one‐dimensional spots scanned with target motion. The motion pattern was recorded from an actual patient breathing normally. The analysis was made for a range of beam widths and number of repaintings. The beam widths were chosen to be narrow at the edge and wider in the center of the target in order to provide the best efficiency. Results: The variation in delivered dose can be 100% more than planned when there is no repainting. After 10 repaintings, the variation of dose distribution is reduced to 5% or 20% depending on beam width. Beam width modulation approaches high uniformity faster as the number of repaintings increases without the compensation of conformity. Conclusion: The dosimetric effect of the intra‐fraction target motion is a function of the beam width and number of repaintings. The simulation showed that the interplay produces large inhomogeneities. A wider beam can improve the uniformity, with the side effect of a larger penumbra. Multiple repainting and fractionation also smooth out these variations, with the cost of increased total treatment time. Beam width modulation has shown a good balance of these two factors and is expected to be a delivery technique to achieve both high uniformity and conformity as long as sufficient numbers of fractions or repainting.
SU‐FF‐T‐46: A Software Approach to Speed Up Collapsed Cone Convolution Superposition Dose Calculation On Multi‐Processor Personal Computers
Authors: Corbit C , Narayanan S
Published: July 2007

Purpose: A new software technique for speeding up dose calculation for photon beams on high end personal computers is presented. Method and Materials: The software exploits inherent parallelism in the dose calculation process using multi‐threading, thus providing a reduction in computation time that is proportional to the number of processor cores. In order to multi‐thread the TERMA calculation, a thread‐safe function calculates the TERMA contribution from a single beamlet. Multiple threads, typically matching the number of processor cores are started and run in loops, calculating the TERMA contribution for any un‐calculated beamlet. The collapsed cone convolution/superposition algorithm then works in a similar manner, with the thread function calculating the dose contribution from a single cone direction. The threads are run in loops until all directions for all beams have been computed. For both cases, the results are accumulated on a per‐beam basis. The summing is done in the thread functions and a lock is required to synchronize access to the per‐beam result arrays. The thread granularity is such that lock contention is not an issue and the observed speedup when increasing the number of threads with processor cores is near optimal. The recursive feature of deposition point of view dose computation is exploited to further speed up the computation process. Results: A full dose calculation of a seven beam head and neck case with a voxel size of 3mm × 3mm × 3mm was computed in 35 seconds on a dual‐core, dual‐processor computer. A four field breast plan took 91 seconds to compute. No compromise in dose calculation accuracy was observed. Conclusion: Multithreading significantly reduces the dose computation time for convolution/superposition dose calculation. Dose calculation for very large patient geometries can be performed in three minutes. With the advent of four‐core processors, real‐time dose calculation on PC's is a possibility.
SU‐FF‐T‐53: A Theoretical Study of Boussinesq Convection in a Water Phantom
Authors: Tosh R , Chen‐Mayer H
Published: July 2007

Purpose: To model natural convection in measurements of absorbed dose from water calorimeters operated at room temperature and to evaluate the efficacy of digital filtering for removing its effects. Method and Materials: Finite‐element calculations were conducted to simulate the response of a water phantom at room temperature (295K) to internal heating similar to that produced by a typical, radiotherapy‐level beam. Phantom dimensions of 30cm depth and diameter and an internal heat source measuring 10cm × 10cm at 5 cm depth and exhibiting a depth‐dose profile were chosen (per TG‐51). The internal heat source was given a square‐wave time dependence to simulate conditions of repeated opening/closing of a shutter, and spatial distributions of both temperature and velocity were obtained as functions of time and (via FFT) frequency. Simulations were carried out both with and without buoyant forces specified, with shutter periods up to 1800 seconds, so that attenuation and phase distortion due to natural convection could be isolated. Results: Natural convection was observed at all shutter periods studied, increasing in intensity with increasing shutter period. In the time domain, convection and conduction produce cooling effects that cause similar nonlinearities in measured temperature waveforms at smaller shutter periods, while at longer shutter periods, convection is marked by temporal oscillations. In the frequency domain, convection is readily identified in the temperature signals at all shutter periods by modulation products at even harmonics of the shutter fundamental frequency. This artifact is consistent with a perturbative treatment of the Boussinesq Navier‐Stokes equations. Conclusion: With the frequency‐domain results, we believe we have found a unique experimental signature for natural convection in water calorimetry. The simple oscillatory behavior of the phenomenon suggests that its effects may be eliminated by means of digital filtering.
SU‐FF‐T‐54: A Three‐Phase Strategy for a Partial Implementation of Biologically Guided Radiation Therapy
Authors: Li X , Stewart R , Semenenko V , Qi S , Chen G , Ahunbay E , Schultz C
Published: July 2007

Purpose: Rapid advances in imaging and delivery technologies and our understanding of radiation responses herald the coming of the long‐sought goal of practicing patient‐specific biologically‐guided radiation therapy (BGRT). In this work, we propose a three‐phase, incremental approach for the clinical implementation of BGRT. Method and Materials: BGRT, beginning with patient‐specific radiobiological parameters that may be extracted from biological images, can be implemented in three phases. Phase I includes planning population‐based treatments with empirical knowledge of tumor and normal‐tissue responses followed by plan evaluation with dose‐volume parameters as the primary criteria and outcome predictions as secondary criteria. Phase II involves the use of biological images and/or tissue assays to identify tumor region of radioresistance, use of outcome modeling to estimate a dose boost needed to overcome the radioresistance, development of a treatment plan to deliver the boost which does not reduce the dose to other tumor regions and maintains the desired dose to all critical organs, and plan evaluation with outcome prediction models. In Phase III, individualized‐BGRT plans derived using the validated outcome prediction models developed in Phases I and II are iteratively refined based on spatial and temporal varied patient‐specific information. In all three phases, treatments will be delivered with image guidance. The 3D dose actually delivered to an individual patient will be documented. Patients will be periodically evaluated with imaging and/or assays. The acquired follow‐up tumor and normal‐tissue response data will be used to refine outcome prediction models. Results: A three‐phase approach for a partial implementation of BRGT is proposed. An example of using physiological MRI to extract patient‐specific radiobiological parameters required for BGRT is presented. Conclusion: The full implementation of BGRT in the clinic will require new technologies and additional research. However, even the partial implementation of BGRT treatment planning has the potential to improve clinical outcomes.
SU‐FF‐T‐58: Accuracy in Prostate Delineation From CBCT Fo RT Planning
Authors: Lerma F , Li H , Liu B
Published: July 2007

Purpose: Evaluate the accuracy in prostate contours from CBCT scans for RT planning. Method and Materials: One FBCT and five CBCT scans are taken per each of five prostate cancer patients. The prostate is delineated on all scans three times by each of three observers. The intersection and union contour of each scan by each observer are computed. The prostate contours from the FBCT scan are mapped onto the daily CBCT scans. Three margin strategies (3 or 5 mm expansions about the intersection and 2 mm expansions about the union of the prostate contours) are used to create three PTV's. The three PTV's from each CBCT are used to create 3D plans. Geometrical overlap (FBCT contours to CBCT contours), FBCT prostate contour coverage by the CBCT plans, rectal and bladder dose indices are evaluated on each CBCT. Results: The average overlap of the FBCT prostate with CBCT PTV's (with 2 to 5 mm margins) range from 92% to 98% with average coverage of the FBCT prostate contours of 93% to 98% of the prescription dose. FBCT prostate contours may extend in some regions up to 5 mm at the base and apex of the unexpanded CBCT prostate for some cases (possibly due to interfraction prostate deformation). However, 3D plans using PTV's resulting from 3mm (L/R and A/P) and 5mm (S/I) margin expansions cover the FBCT prostate contours with more than 95% of the dose. Conclusion: 3DCRT, CBCT‐based planning of localized prostate carcinoma should include a 2–5 mm expansion of the prostate contour to correct for contouring uncertainties, based on comparisons to FBCT‐based planning. This result includes the influence of transient prostate deformations between imaging sessions and CBCT imaging uncertainties, but does not include intra‐ and inter‐fractional motion considerations.
SU‐FF‐T‐59: Accuracy of Heterogeneity Correction Algorithms Used with Step and Shoot and Sliding Window IMRT
Authors: Schmitter M , de Boer S , Liu J , Podgorsak M
Published: July 2007

Purpose: To evaluate the accuracy of tissue heterogeneity dose correction algorithms for IMRT delivery using step and shoot and sliding window techniques. Method and Materials: A solid water phantom incorporating a lung equivalent heterogeneity layer along with several planning targets was constructed. The phantom was CT scanned and contouring of targets was done. Several anatomical dose constraints and penalty functions were defined. A commercial treatment planning system was used to evaluate the optimal fluence for several IMRT field setups. Each fluence was then run through a leaf motion calculator to provide deliverable fluences using both step and shoot and sliding window techniques. Heterogeneity correction algorithms were used in both the dose optimization process and the final dose calculations. GafChromic EBT film was strategically placed within the phantom. The phantom was irradiated with each set of IMRT treatment fields and each film was then processed. The measured dose distribution was compared to the predicated distributions. Results: Preliminary studies with static fields (1×10 cm2 through 10×10 cm2) showed that heterogeneity algorithms became less accurate with decreasing field width. The largest difference between the predicted and the delivered dose on the beam axis inside the lung equivalent material was 28% found for the EqTAR algorithm for the 1×10 cm2 field size. Not surprisingly, calculated dose distributions were most accurate for step and shoot delivery where field widths are typically larger than those used with sliding window delivery. Conclusion: Current heterogeneity correction algorithms were found to become less accurate with decreasing field widths. The accuracy of IMRT dose delivery in regions of tissue heterogeneity is therefore dependent on the delivery technique. Since sliding window techniques typically irradiate with narrower fields, heterogeneity algorithms are more accurate for step and shoot delivery where fewer but larger fields are typically used.
SU‐FF‐T‐60: Accuracy of Varian Eclipse Dynamic Conformal Arc Dose Calculation
Authors: Duggan D , Ding G , Coffey C , English M , Clark L
Published: July 2007

Purpose: Dynamic conformal arc (DCA) treatments, in which the aperture is continuously modified to conform to the beams‐eye‐view of the target as the gantry rotates, are potentially the fastest and most efficient method of delivering a conformal treatment, while still achieving results comparable to IMRT. DCA could make stereotactic body radiation therapy (SBRT) more practical and cost‐effective. The purpose of this study is to demonstrate the accuracy of the DCA algorithm of the Varian Eclipse radiation treatment planning system (RTPS). Method and Materials: DCA plans were made for patients who had already received SBRT for Stage I NSCLC or IMRT for prostate cancer. The accuracy of the dose calculation was tested by two methods: The plan was mapped onto a cylindrical plastic phantom, the dose distribution in the phantom was calculated using the same monitor units. Then an ionization chamber was placed in the center of the phantom, the phantom was set up with the chamber at isocenter and the treatment was performed. In the second method, a DCA plan was mapped to a two‐dimensional (2D) ionization chamber array, with the plane of the array at isocenter. The dose distribution was calculated in the plane of the array. This phantom was also set up with the isocenter in the center of the chamber array and the treatment was performed. Results: The point doses measured with an ionization chamber in a cylindrical phantom agreed within ±2% with those predicted by the Eclipse RTPS. The gamma factor for the comparison between the measured and calculated doses in the plane of the 2D chamber array was less than 1 for 95% of the pixels. Conclusion: Our results indicate that the Eclipse DCA can accurately predict dose distributions.

Research sponsored by Varian Medical Systems.
SU‐FF‐T‐62: Alternative IMRT QA Technique During Prostate Patient Treatment
Authors: Xu M , Sethi A , Gao M , Ke Y , Rusu I , Glasgow G , Fan John
Published: July 2007

Purpose: The objective of this study is to propose an alternative QA technique that addresses concerns of conventional IMRT QA. Methods and Materials: During IMRT patient treatments, delivered doses were measured using film and MOSFET detectors. Film was placed under the patient sandwiched between the patient and 1‐cm bolus; MOSFET detectors were placed on the anterior skin near the central‐axis under 0.5‐cm bolus. IMRT QA was also performed by exporting the patient plan into a 30×30×30 cm3 cubic phantom. In addition to the central axis dose, relative dose distributions were measured using XV films placed near the anterior and posterior surface of the QA phantom. Results: The results of in‐vivo measurement with film placed under the patient indicate good agreement between planned and measured doses with gamma index of 0.8. Anterior surface doses measured with MOSFET detectors however deviated from planning dose around 10–15%. For comparison, the same patient plan was also exported into the phantom for IMRT QA. The ion chamber measurement at the phantom's center showed good agreement with TPS (measured dose = 197±1 cGy). Results of surface doses measured with film in phantom agree with planned doses. This is consistent with in‐vivo measurement of dose distribution during patient treatment. The surface dose measured with film placed under the patient samples doses at different depths from various beams. The quality of agreement seen with films is consistent with the ion chamber data at the central axis. Thus, film based surface dose measurement is a good manifestation of central axis dose. Conclusion: An in‐vivo IMRT dose measurement is proposed as an alternative QA procedure. Based upon our measured data, proposed method is an accurate and reliable verification of patient dose. The technique is quick, efficient, and directly measures actual dose received by patient during IMRT delivery.
SU‐FF‐T‐66: An Assessment of the Use of Skin Flashes in Helical Tomotherapy Using Phantom and In‐Vivo Dosimetry
Authors: Tournel K , Verellen D , Linthout N , Duchateau M , Reynders T , Fierens Y , Voordeckers M , Storme G
Published: July 2007

Background and Purpose: In helical tomotherapy the nature of the optimizing and planning systems allows the delivery of dose on the skin using a build‐up compensating technique (skin‐flash). However, positioning errors or changes in the patient's contour can influence the correct dosage in these regions. This work studies the behavior of skin‐flash regions using phantom and in‐vivo dosimetry. Material and Methods: The dosimetric accuracy of the tomotherapy planning system in skin‐flash regions is checked using film and TLD on phantom. Positioning errors of 5mm to 10mm in both directions are induced and the effect on the skin dose is measured. Further a volume decrease is simulated using bolus material and the results are compared. Results Results show that the tomotherapy planning system can does adequately calculates dose on skin regions within 2SD using TLD‐measurements. Film measurements show drops of dose of 2.8% and 26% for resp. a 5mm and 10mm mispositioning of the phantom towards air and a dose increase of 9% for a 5mm shift towards tissue. These measurements are confirmed by TLD measurements on phantom. A simulated volume reduction shows a similar behavior with a 2.6% and 19.4% drop in dose, measured with TLDs. Conclusion: The tomotherapy system allows adequate planning and delivery of dose using skin flashes. However, exact positioning is crucial to deliver the dose at the exact location and re‐evaluation could be in order when patients will reduce in volume during their treatment.
SU‐FF‐T‐73: An Intra‐Operative Dosimetry System for Prostate Brachytherapy Using Dual‐Modality Imaging
Authors: Cho P , Tutar I , Narayanan S , Lam S , Gong L , Pathak S , Kim Y , Wallner K
Published: July 2007

Purpose: (1) To develop an intra‐operative dose evaluation and optimization system for prostate brachytherapy using complementary imaging modalities (fluoroscopy for seed localization and ultrasound for prostate delineation). (2) To evaluate the usefulness of the intra‐operative dosimetry system. Method and Materials: Several algorithms were developed to realize the concept of fluoroscopy‐ultrasound based dosimetry. These include automated seed detection, 3D seed reconstruction, patient motion correction, computer‐assisted prostate contouring, and seeds‐prostate registration algorithms. In order to accelerate the seed reconstruction from multiple x‐ray projections, the dimensionality of seed matching process was reduced by strategically forming search restriction sub‐images. Problems of incomplete data due to clustering and superposition of projected seeds were solved by a pseudo‐matching technique. Patient movements between fluoroscopic image acquisitions were corrected through analysis of epipolar‐planes. Registration of seeds and prostate volume was accomplished by establishing a sparse (small subset of seeds detected on ultrasound) to full (seeds reconstructed from fluoroscopy) data set correspondence. Results: Intra‐operative dosimetry was performed on 25 patients implanted with Pd‐ 103 seeds. In nine patients, no additional seeds were implanted after intraoperative dose evaluation. In 16 patients, remedial seed were added based on the intra‐operative evaluation. Subsequently, V100 values improved from 86±8% to 93±4% (p=0.005). For all 25 patients, the post‐implant dosimetry results using our system were compared with Day 0 CT‐study. The V100 and D90 values from the fluoroscopy‐ultrasound based dosimetry were 95±4% and 120±24% while the CT‐based dosimetry computed 95±4% and 122±24%, respectively. Conclusion: We have developed and implemented an intra‐operative dosimetry system that combines the strengths of fluoroscopy and ultrasound. A clinical study has successfully demonstrated its intended utility in intra‐operative setting by significantly improving the quality of seed implant in prostate brachytherapy.
SU‐FF‐T‐81: Anatomy‐Based Inverse Planning Simulated Annealing (IPSA) Optimization in HDR Prostate Brachytherapy
Authors: Jacob D , Raben A , Sarkar A , Grimm J , Simpson L
Published: July 2007

Purpose: Independent validation of IPSA algorithm in obtaining superior target coverage and reduced dose to organs at risk, by comparing with the results of other optimization techniques in HDR brachytherapy. Method and Materials: In a recent prostate HDR brachytherapy protocol study (RTOG‐0321) we used an anatomy based inverse optimization technique known as Inverse Planning Simulated Annealing (IPSA), incorporated as a beta version in the Plato TPS Version 14.2.3 (Nucletron Corp., Veenendaal, The Netherlands) in an effort to satisfy the dose constraints set by the protocol for Prostate, Bladder, Rectum and Urethra. Between the period of June 2005 and November 2006, 20 patients received HDR boost dose of 19 Gy in two fractions to the prostate in addition to an external beam dose of 45 Gy with IMRT technique. 3D Brachytherapy treatment plans were generated using Plato Version 14.2.3. Dosimetry was obtained for the following optimization schemes in Plato, for the same dose constraints for all the patients treated during this time period: Anatomy based IPSA optimization, Geometric optimization on volume and Dose Point optimization on volume. Dose volume histograms were generated for PTV and organs at risk for each optimization method, from which V75 volumes for Rectum, Bladder, and V125 volume for Urethra were determined. Also dose homogeneity index as well as conformal index for the PTV was calculated for each optimization technique. Results: Despite suboptimal needle placement in some implants, using the same dose constraints on all patients, IPSA algorithm was able to comply with the tight RTOG dose constraints for 90% of the patients in this study, whereas it was only 30 % for DPO and only 5% for GO. Conclusion: Anatomy based IPSA optimization proved to be the overall best and fast technique for reducing the dose to organs at risk without compromising target coverage.
SU‐FF‐T‐86: Assessment of Carbon Fiber and Synthetic Fiber Radiation Therapy Tabletops for Kilovoltage Cone Beam CT Imaging
Authors: Ye J , Wong T , Cao D , Afghan M , Riley J , Shepard D
Published: July 2007

Purpose: The Calypso® System uses electromagnetic fields to localize and continuously track implanted Beacon® transponders. For sub‐millimeter localization accuracy, a non‐conductive surface is required. We studied kilovoltage (kV) cone‐beam CT (CBCT) image quality of a non‐conductive synthetic fiber composite tabletop (Qfix Systems, kVue IGRT™). Material and Method: CBCT images were acquired with 3 tissue phantoms (CatPhan®) through carbon fiber tabletop (Medical Intelligence) and synthetic fiber tabletop (Kevlar®). Image resolution, low contrast sensitivity, 3D uniformity, circular symmetry and spatial uniformity with carbon and synthetic fiber tabletops were compared. Multiple protocols were used to evaluate image registration accuracy. Impact of side rails incorporated in the synthetic fiber model was evaluated. Results: Both tabletops demonstrated better than 2% of low contrast sensitivity, 3D uniformity was within 2%. Circular symmetry and spatial uniformity were within 1mm. Carbon tabletop and synthetic fiber tabletop with rails outside the kV field achieved equal resolution of 8 1p/cm while the synthetic fiber tabletop with rails inside the field provided a resolution of 7 1p/cm. Compared with the carbon fiber, image pixel values for six materials with relative electron densities ranging from 0.28 to 1.69 were found to be within 4% and 11% for the Kevlar with rails inside and outside of the field respectively. Phantom images of head‐and‐neck, chest, and pelvis were registered to reference images via bony and gray‐value auto‐registration methods. Accuracy measured within 2 mm for both designs. Conclusion: CBCT imaging characteristics using synthetic and carbon fiber tabletops are comparable. Synthetic fiber tabletops are compatible with electromagnetic localization systems and meet image quality performance criteria.
SU‐FF‐T‐88: Augmenting Lyman NTCP by Adding the Influence of Non‐Dose Variables Via Decision Trees: Application to Lung Radiation‐Pneumonitis Prediction
Authors: Das S , Zhou S , Yin F , Zhang J , Marks L
Published: July 2007

Purpose: Develop a model that augments the dose‐based parametric Lyman NTCP formulation using non‐parametric decision trees that can additionally account for the influence of non‐dose variables. Apply and test the model to predict Grade 2+ lung radiotherapy (RT) induced pneumonitis. Methods and Materials: The model was created by sequentially combining weighted sub‐models (prediction units) using a recent statistical “boosting” technique. Boosting progressively improves model accuracy with the addition of each prediction unit. Each prediction unit was composed of the sum of the Lyman NTCP (LNTCP) metric and a decision tree. Thus, shortcomings in the more rigidly parametric dose‐based LNTCP metric were compensated by decision trees, which selected specific dose and non‐dose features to further separate the injured‐uninjured populations. The database used to create the model consisted of 234 lung cancer patients treated with RT, of whom 43 were diagnosed with pneumonitis at followup. A realistic estimate of model predictive capability was obtained via 10‐fold cross‐validation. To facilitate dissemination, a simplified version of the complex model structure was extracted from the cross‐validation results. Results: The cross‐validated model Receiver Operating Characteristics area of 0.72 was significantly better than the corresponding LNTCP area of 0.63 (p=0.005). Important variables extracted by the simplified model were: LNTCP, sex, histology, chemotherapy schedule, and treatment schedule. For a given patient radiotherapy treatment plan, injury prediction was highest and lowest for the combinations [pre‐RT chemotherapy, once‐daily treatment, female sex] and [no pre‐RT chemotherapy, non‐squamous‐cell histology], respectively. Application of the simplified model to two example patients reveals that non‐dose variables can heavily influence radiation‐pneumonitis prediction (e.g., 3%–64% for various settings of non‐dose variables). Conclusions: Radiation‐pneumonitis prediction was significantly improved by adding the influence of non‐dose variables to Lyman NTCP. The simplified model has an easily interpretable architecture that combines Lyman NTCP with a single decision tree.
SU‐FF‐T‐91: Automated Selection of Tradeoff Parameters in IMRT Prioritized Prescription Treatment Planning
Authors: Clark V , Chen Y , Wilkens J , Deasy J
Published: July 2007

Purpose: Prioritized prescription optimization is a method of IMRT treatment planning which solves the optimization problem step‐wise, first by determining the best performance of a high‐priority objective and then converting it to a constraint as lower priority goals are optimized in turn. This system is fully automated; however, there is a variable value we call ‘slip’ factor which determines the amount of degradation allowed after each priority is addressed. The purpose of this work is to investigate an approach to automating selection of the slip factor. Method and Materials: We propose a new version of prioritized prescription optimization that, after each step, estimates the desired value of the slip factor for the next step. We estimate, to first order, the rate of change of the objective function with respect to small perturbations in the slip. We then apply the Sensitivity Theorem to analyze the effect of the slip factor. In summary, after solving the first problem and obtaining the solution to the first step, wI, we plug the value of wI into a closed‐form expression of the Lagrange multiplier vector λ(0) to get λ(0), and then use the last element of λ(0) to compute the first‐order approximation model for the optimal objective value with variable slip. Results: We use the Sensitivity Theorem and Lagrange multipliers to derive a first‐order approximation model for the optimal objective function value for a given slip factor. On this basis, we propose to automatically select slip values based on the expected gain in the objective function. Conclusion: We have developed a mathematical theory and framework which allows us to automate the selection of the slip factor in prioritized prescription optimization, based on a local linear approximation. This proposal is the subject of ongoing tests.
SU‐FF‐T‐95: Biologically Guided Determination of Individualized Dose Distributions
Authors: Park JH , Li XA , Stewart RD
Published: July 2007

Purpose: A biologically motivated scheme to guide the determination of individualized dose distributions is developed from the Poisson tumor control probability (TCP) and linear‐quadratic (LQ) survival model. The method combines clinical experience, in the form of a reference treatment, with possibly patient‐specific information to help circumvent well‐known issues associated with outcome modeling. To illustrate the approach, isoeffect doses for populations of prostate cancer patients are compared. Method and Materials: Monte Carlo methods are used to sample prostate cancer radiosensitivity parameters derived from clinical data in ways that mimic two treatment planning scenarios. In the first scenario, the dose distribution needed to achieve the same TCP in all patients is estimated. In the second scenario, the distribution of doses needed to achieve the same distribution of TCP values in the patient population as a reference treatment (37 daily fractions of 2 Gy) is determined. Results: For 28 to 40 daily fractions, uncertainties associated with population‐averaged radiosensitivity parameters correspond to about a 20% uncertainty in the fraction size needed to achieve the same TCP. However, the fraction size needed to achieve the same distribution of clinical outcomes as 37 daily fractions of 2 Gy can be estimated to within 3% for 5 to 50 daily fractions. Conclusion: To achieve the same TCP in all prostate cancer patients requires very accurate (a priori) estimates of radiosensitivity parameters. However, the fraction size needed to achieve the same distribution of TCP values among a patient population with an alternate fractionation schedule can be accurately estimated without the need for patient‐specific information. When combined with information from predictive assays or functional and biological imaging, the proposed method has the potential to determine patient‐specific dose distributions for use in commercially available inverse planning software.
SU‐FF‐T‐96: Characteristics of Narrow Field Photon Beam Measurements for a Micro‐MLC Based Radiosurgery System
Authors: Yenice K , Wu T , Tu H , Reft C
Published: July 2007

Purpose: To determine the response of various detectors in the commissioning of narrow field megavoltage photon beams for a micro‐MLC based radiosurgery system. Method and Materials: We commissioned both a standard and a high dose rate 6 MV photon beams from a Trilogy unit (Varian Medical systems, Palo Alto CA) for the pencil beam algorithm using a micro‐MLC based Radiosurgery system (BrainLAB AG Germany).

The loss of lateral equilibrium, high dose gradients, and spectral changes within high energy narrow photon beams imposes restrictions on detector composition and size used for relative measurements. We investigated the response of two p‐type Si‐diodes (Scanditronix photon and stereotactic diodes), a diamond (PTW) detector, a cylindrical (Scanditronix RK chamber) and a micro‐ion‐chamber (Exradin #a16) for depth dose and relative output measurements. Results: Percent depth dose measurement with photon diode (guarded) yielded consistently higher measurements by 1–2% compared to ionization chamber results for larger depths with increasing field sizes. Stereotactic diode measurements of depth doses for field sizes ⩾18×18 mm2 yielded 1–2% lower values compared to ionization chamber results. For field sizes ⩽12×12mm2 photon diode depth dose measurements were higher by 1–5% at depths > 5cm compared to those measured by stereotactic diode. The relative output factors measured by photon diode, diamond detector and both ion chambers were within 1% agreement with each other for all field sizes ⩾ 30×30mm2. The stereotactic diode measurements were lower by ∼2.5% for all but the smallest field size of 6×6 mm2 compared to other detectors. Relative output factors for the high dose rate beam were higher by 2–4% for fields < 18×18mm2 compared to the standard 6 MV photon beam. Conclusion: Measurement of relative output factors and depth doses for megavoltage narrow photon fields require multiple detectors to correctly estimate the true response over the range of measurements.