bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐412: Dosimetic Characterization of a Compact Proton Therapy Unit with Monte Carlo Simulations
Authors: Zheng Y , Klein E , Low D
Published: April 2010

Purpose: A compact proton therapy unit is under development, so measured dosimetry data does not yet exist. The beamline was modeled and profiles were generated using Monte Carlo methods to evaluate the dosimetric characteristics of the proposed proton unit. Method and Materials: MCNPX code was used to model the proton therapy unit and simulate beam profiles. The range modulator, scatterers, range shifter, applicator, aperture and other components in the beam shaping system were modeled according to the design data provided by the manufacturer. A Gaussian distributed source was assumed. Spread out Bragg Peaks (SOBP) were simulated by combining the simulation results for various steps of the range modulator wheel. Simulations were performed in a water phantom as well as in air. Up to 108 proton histories were tracked per simulation to achieve < 2% statistical uncertainties. Beam characteristics such as flatness, symmetry and penumbra were evaluated against machine specifications. Results: The 14 combinations of range modulator wheel and scatterers were modeled for this unit. For each combination, beam profiles were generated including: lateral fluence profiles in air with an open aperture, lateral fluence profiles in air with a half beam block, fluence in air along the beam axis, lateral dose profiles in water, and percentage depth dose in water for both unmodulated beam and SOBPs. The generated data met expectations in most cases. Subsequently, a commercial treatment planning system was configured using the simulated beam data. Conclusion: The Monte Carlo tool allowed the generation of dose profiles and evaluation of beam characteristics for a compact proton therapy unit prior to its construction. The simulated data provided feedback for machine design optimization and valuable reference data for future acceptance testing and commissioning.
SU‐FF‐T‐413: Density Scaling of the Proton Energy Spectrum for Analytic Reconstruction of the SOBP
Authors: Tafo A Guemnie , Fourkal E , Veltchev I , Ma C
Published: April 2010

Purpose: To investigate a method to derive the proton energy spectrum required for clinical spread out Bragg peaks in heterogeneous patient geometry for divergent proton beams with small source‐to‐surface distances (SSD) for compact proton therapy gantries. Method and Materials: Previous work using a simple analytical model based on Boltzmann kinetic equations permitted to derive a SOBP for broad proton beams in homogeneous media at large SSDs. The calculated results agreed well with Fluka Monte Carlo simulations. We revised this formula for broad proton beams with smaller SSDs designed for compact proton treatment gantries under development. This study further improved the analytic model to generate deliverable SOBPs for broad proton beams with small SSDs in heterogeneous patient geometry. Fluka Monte Carlo simulations were performed to validate our analytical calculations. Results: Using the same spectrum for different density materials leads to a distortion of the SOBP for divergent beams in heterogeneous media. One has to recalculate the spectrum according to the density. We showed that the spectrum expression for the SOBP reconstruction can be given by: where μ is the density dependent factor. Analytic calculations with the revised model and direct Monte Carlo simulations are in good (within 2%) agreement. Conclusion: We demonstrated the possibility of analytically calculating the proton spectrum to achieve the SOBP for broad divergent proton beams in low‐density media. This model is accurate and efficient for proton therapy treatment planning.
SU‐FF‐T‐415: Impact of Dm Vs. Dw in Electron Beam Monte Carlo Based Treatment Planning for Breast
Authors: Gil E , Vandervoort E , Clark B , Cygler JE
Published: April 2010

Purpose: One of the issues arising when using Monte Carlo based treatment planning systems is “dose‐to‐water (Dw)” vs. “dose‐to‐medium (Dm)” calculations. The two are conceptually different and the method used has an impact on the reported doses to tumor and different organs. A systematic analysis of clinical data is needed to help understand the differences between the Dm and Dw approaches and for more accurate evaluation of treatment outcomes. Method and Materials: The analysis included plans for phantoms containing soft bone and lung heterogeneities as well as 54 breast cancer patients treated with electron beams. Comparisons between Dm and Dw plans were performed by means of dose volume histograms and isodose distributions. Results: The plans for phantoms show very small differences between the Dm and Dw approaches, consistent with medium/water stopping power ratios (lung/water=0.985, soft bone/ water=0.980). Differences between Dm and Dw were also observed in clinical plans, with magnitudes depending not only on the beam energy but also on the location of the tumor and organs at risk. The largest difference was found for a treatment of the chest wall with 13 MeV electrons following a complete mastectomy. The dose to the sternum was 49.9 Gy and 50.7 Gy for Dm and Dw approach, respectively. This amounts to 1.52% difference in the maximum dose delivered to that volume. For the same patient, the dose received by the internal mammary nodes was 46.9 Gy and 46.2.4 Gy for Dm and Dw plans, respectively, corresponding to a 1.27% difference. Conclusion: For breast patients, the plans calculated for Dm and Dw showed only small differences consistent with the ratios of medium/water stopping powers and depended on beam energy, location of the tumor and organs at risk.
SU‐FF‐T‐419: Improved Electron Monte Carlo Dose Calculation for Low Energy Electron Beams Using EMC
Authors: Fix MK , Frei D , Volken W , Neuenschwander H , Born EJ , Manser P
Published: April 2010

Purpose: The electron Monte Carlo (eMC) dose calculation algorithm in Eclipse (Varian Medical Systems) is based on the macro MC method and is able to predict dose distributions for high energy electron beams with high accuracy. However, there are limitations for low energy electron beams. This work aims to improve the accuracy of the dose calculation using eMC for 4 and 6 MeV electron beams of Varian linear accelerators. Method and Materials: Improvements implemented into the eMC include: (1) improved determination of the initial electron energy spectrum by increased resolution of mono‐energetic depth dose curves used during beam configuration; (2) inclusion of all the scrapers of the applicator in the beam model; (3) reduction of the size of the sphere within the macro MC transport when the energy of the incident electron is below certain thresholds. The impact of these changes in eMC is investigated by comparing calculated dose distributions for 4 and 6 MeV electron beams with applicators ranging from 6×6 to 25×25 cm2 of a Varian Clinac 2300C/D with the corresponding measurements. Results: Dose differences between calculated and measured absolute depth dose curves are reduced from 6% to less than 1.5% for both energies and all applicators considered. Using the original eMC implementation, absolute dose profiles at depths of 1 cm, dmax and R50 in water lead to dose differences of up to 8% for applicators larger than 15×15 cm2. Those differences are now reduced to about 2% for all dose profiles investigated when the improved version of eMC is used. Conclusion: In this work several enhancements were made in the eMC algorithm leading to significant improvements in the accuracy of the dose calculation for 4 and 6 MeV electron beams of Varian linear accelerators. Conflict of Interest: This work was supported by Varian Medical Systems.
SU‐FF‐T‐440: Validation of Accuray Multi Plan Monte Carlo Treatment Plans
Authors: Mardirossian G , Muniruzzaman M , Lee C , Jin H , Fuller D
Published: April 2010

Purpose: To validate Accuray Monte Carlo treatment plans in various heterogeneous phantom systems. Materials and Methods: Monte Carlo Dose Calculation algorithm is considered to be the gold standard as compared to the ray tracing algorithm especially in rapidly changing heterogeneous density structures as well as in obliquely incident beams and superficial targets. Three anthropomorphic phantoms loaded with radiochromic EBT films were used for the validation of the dose predicted by Monte Carlo calculations algorithm. Plans performed in MultiPlan (MP 3.1) system simulated lesions in head phantom, lung phantom and a breast phantom. The lung phantom was custom fitted to accommodate small stereotactic A16 chamber for absolute dose verification. Using fixed cones, plans were first obtained and optimized then, calculated in Monte Carlo using the same beam orientation and weighting with %0.5 calculation uncertainty. Six exposed EBT films crossing the target at various axes were used for relative and absolute dose comparison with the calculated dose. Results: FilmQA statistical analysis performed with 3% dose difference index and 1mm distance to agreement (DTA) criteria revealed an average pixel passing (Mean, SD) of 83(11)% and 94(10)% respectively with an average pass of gamma index 88(4)%. Consideration of small target volume and high dose gradient within the evaluated region, the high DTA is indicative of good dose prediction. Both film dosimetry and chamber dose value predictions were in agreement to within <2%. Conclusion: Both EBT film and small stereotactic chamber measurements confirmed Accuray Multiplan Monte Carlo algorithm to be highly accurate for heterogeneous and shallow skin depth dose prediction. This technique can be used to validate Accuray treatment plans.
SU‐FF‐T‐448: Siemens Primus Linear Accelerator Commissioning for IMRT Verification Using PMCEPT Monte Carlo Code and GAFCHRONICTM EBT Film
Authors: Kum O , Han Y , Cheong K , Ju S , Jeong H
Published: April 2010

Purpose: To implement a dose calculation system that accurately predicts the dose delivered by Siemens Primus 6 MV linear accelerator and to check the IMRT treatment planning system calculation with an independent calculation system. Method and Materials: The BEAMnrc and the PMCEPT Monte Carlo code were used to simulate Siemens Primus 6 MV photon beams. The BEAMnrc was used to generate phase space (phsp) files at 100 cm source‐to‐surface distance for several beams, defined by the jaw and MLCs. Symmetric square beams ranging from 1 × 1 to 20 × 20 cm2 were calculated to simulate pyramid density shape. The left hand side non symmetric rectangular beams of 3 × 15, 4.5 × 15, and 6 × 15 cm2 and the right hand side non symmetric rectangular beams of 6 × 15, 4.5 ×15, and 3 × 15 cm2 were calculated to simulate inverse‐pyramid density shape. The PHSP files were subsequently used as the input files for the PMCEPT code. The commissioning was verified against EBT film, ion chamber, diode detector measurements for a solid water phantom. Results: A solid water phantom of 30 × 30 × 30 cm3 with cell size of 0.1 × 0.1 × 0.2 cm3 was used for the PMCEPT simulations. For each beam, the MC calculated central axis depth dose and lateral profiles were compared with those of experiments. The agreement between calculated and measured dose distributions was within 2%. For both pyramid and inverse‐pyramid density shapes, superposed‐lateral profiles show agreement better than 2%, even in the region of penumbra where gradient varies sharply. Conclusions: Our results suggest that the PMCEPT code can be used to calculate IMRT dose distributions as well as to calibrate detectors with great accuracy. Moreover, it can be used as a double check system for IMRT QA and treatment planning.
SU‐FF‐T‐451: Determination of Optical Properties in a Heterogeneous Turbid Media Using a Cylindrical Diffusing Fiber
Authors: Dimofte A , Zhu T , Finlay J , Wang K
Published: April 2010

Purpose: For interstitial photodynamic therapy (PDT), cylindrical diffusing fibers (CDF) are often used to deliver light. This study examines the feasibility and accuracy of using the CDF to characterize the absorption (μa) and reduced scattering (μs′) coefficients of heterogeneous turbid media. Method and Materials: Measurements were done in prostate simulating phantoms. Liquid and solid phantoms were used (homogeneous and inhomogeneous, with optical properties of ⌈s′=7.5cm−1 and ⌈a=0.1cm−1). The inhomogeneous phantom had three tumor simulating inhomogeneities (⌈s′=15cm−1 and ⌈a=0.1, 0.3 and 0.9 cm−1). Linear light sources of varying lengths were placed inside the phantom through the atheter. In‐air measurements were performed to characterize the intensity profile of each linear fiber. Fluence rate was measured using a 0.5mm scattering tip isotropic detector that was moved along each catheter using a motorized probe. To ensure that the optimization technique was accurate, we measured the optical properties of each phantom using a well established method. Results: Optical properties were measured in a homogeneous liquid phantom by measuring the fluence rate along a 2mm pint source and compared against results using linear source method. The absorption coefficients were then determined using a method in which ⌈s′ was kept fixed. Using the determination of ⌈s′ from the homogeneous fit, we were able to determine the absorption coefficients with a standard (maximum)deviation of 5.6 % (9.8%) while using the fixed ⌈'s′ method, the standard (maximum) deviation were 6.9% (19%). Conclusion: We showed that it is possible to accurately determine the optical properties inhomogeneities (⌈a) using linear sources to a relative accuracy of better than 10%. Sensitivities for determination of ⌈s′ is quite poor using linear sources. Linear source method is more sensitive to detector optical heterogeneities. The effect of intensity profile on determination of the absorption coefficient ⌈a is relatively small — this is important for forward calculation.
SU‐FF‐T‐458: Complementing Targeted Radionuclide Therapy with External Beam Radiotherapy: Planning Study
Authors: Grudzinski J , Bowen S , Tome W , Weichert J , Jeraj R
Published: April 2010

Purpose: Combining intensity modulated radiotherapy (IMRT) with targeted radionuclide therapy (TRT) can reduce local radiation toxicity. TRT systemically distributes the toxicity but its non‐uniform tumor doses are difficult to complement with conventional IMRT, which warrants more sophisticated voxel‐based prescriptions. The purpose of this work was to investigate feasibility of combined TRT/IMRT therapy to achieve uniform tumor coverage. Methods and Materials: A NSCLC case with highly heterogeneous tumor radionuclide uptake was investigated. The voxel‐based TRT dose was estimated using STRATOS (Systemic Targeted Radiotherapy Optimization Solution), a research plug‐in within Pinnacle™. The voxel‐based IMRT dose distributions were obtained via the in‐house treatment planning software simulating helical tomotherapy delivery. The total tumor dose was kept at 70 Gy in the study. A parametric experiment with varying maximum TRT doses was performed. A supplementing IMRT prescription was defined as the difference between the total tumor dose and the TRT dose. The PTV uniformity of the combined therapy was measured using DVHs as well as evaluating tumor dose profiles. Results: In this particular case, the TRT doses were highly non‐uniform, also requiring highly non‐uniform IMRT dose distributions. Regardless, the resulting combined TRT/IMRT plans were highly uniform. DVH analysis demonstrated that a lower TRT dose provides a more uniform tumor dose which can be attributed to a higher fraction of IMRT. The volume percentage within 5% of the prescription was 67, 61, and 55% for the maximum TRT dose of 50, 60, and 70 Gy, respectively. The DVHs also show the difficulty in conforming to the prescription in low density regions which is due to lack of dose build‐up that inhibits modulation of steep dose gradients. Conclusions: Preliminary results suggest that it is feasible for helical tomotherapy to complement TRT in providing uniform tumor dose coverage.
SU‐FF‐T‐461: Dosimetric Evaluation of a 120‐MLC Plan Delivered On a 80‐MLC Linear Accelerator
Authors: Corona I , Esquivel C , Gutiérrez A , Stathakis S , Shi C , Papanikolaou N
Published: April 2010

Purpose: To evaluate a 120‐multileaf collimator IMRT plan delivered on a 80‐multileaf collimator linear accelerator. Materials and Method: In practice, it is possible to deliver a conventional treatment plan generated for the planned linear accelerator on another machine with matching characteristics. This study reviewed four intensity modulated radiotherapy treatment plans (IMRT) using the Pinnacle3 Treatment Planning System generated for a Varian 2100C/D 120‐Millenium MLC linac. Using the Varian MLC Shaper application, each field was reconfigured for a 80‐MLC Millennium machine. The new fields were uploaded to the Pinnacle treatment planning system via RadCalc software. Each plan was recalculated to deliver the same dose to the same treatment volume using the same number of monitor units for the entire treatment regime. The isodose distribution and dose volume histogram (DVH) of both plans were compared. Results: Evaluation of 120‐ and 80‐MLC plans was based on DVHs of the planned target volume and critical structures and isodose line distribution. Generally, the lower isodose lines were similar for both plans. The higher isodose values differed. Hotspots were significantly larger for the 80‐MLC plans for the planned target volumes. Conclusion: The results indicated that delivery of a 120‐MLC IMRT plan delivered on an 80‐MLC linac can vary. Investigation of each individual plan must be evaluated before the transfer delivery of treatment on the 80‐MLC linac. In cases were a patient is transferred to another machine for a small number of fractions of the total dose regime, changes in dose distribution may be insignificant. Further investigation of evaluating delivery of dose with both and 120 and 80‐MLC units is forthcoming.
SU‐FF‐T‐465: Study On Secondary Cancer Risk Induced by Intensity‐Modulated Radiotherapy and Passive Proton Beam Therapy
Authors: Ahn Sung Hwan , Kim Jingsung , Cheong Minho , Shin Dong Ho , Park Sung Yong , Lee Se Byeong , Yoon Myounggeun , Kim Woojin , Park Seokhwan
Published: April 2010

Purpose: Secondary cancer risk induced by radiation therapies, intensity‐modulated radiotherapy (IMRT) and passive proton beam therapy, was estimated and compared for prostate and head & neck patients. Method and Materials: The concept of an organ equivalent dose (OED) for radiation‐induced cancer was applied to dose distributions of patients to estimate organ specific radiation‐induced cancer risk. IMRT with 6 MV photons and proton beam therapy were planned for 5 prostate and 5 head & neck cancer patients. The treatment beams were delivered to humanoid phantom and secondary doses during irradiation were measured at various points from 20 cm to 60 cm apart from the beam isocenter on humanoid phantom using ion chamber and CR‐39 detectors for IMRT and passive proton beam therapy, respectively. Results: The average secondary doses for prostate cancer patients in passive proton beam therapy, which is neutron dose equivalent to the proton absorbed dose (H(10)/D) measured from 20 to 60 cm from the isocenter, ranged from 0.4 mSv/Gy to 0.1 mSv/Gy. The average secondary doses in IMRT for prostate patients, ranged between 3 mSv/Gy and 1 mSv/Gy whose values are approximately order of magnitude higher than the proton therapy. Although the average secondary doses for head & neck cancer patients in proton therapy were less than the average secondary doses in IMRT, the difference was not significant as the prostate cases. Conclusion: By a comparison between passive proton beam therapy and IMRT for diseases of prostate and head & neck, it was shown that the estimated secondary cancer risk using scattering mode in proton beam therapy is either significantly lower than the cases in IMRT treatment or, at least, does not exceed the risk induced by conventional IMRT treatment.
SU‐FF‐T‐466: Optimization Models for Strategic Patient Scheduling Problems in Proton Therapy Delivery
Authors: Men C , Salari E , Li Z , Mendenhall N , Palta J , Romeijn E
Published: April 2010

Purpose: To develop a strategic‐level capacity assessment and planning model for radiotherapy treatment delivery at the University of Florida Proton Therapy Institute (UFPTI). Method and Materials: A Mixed‐Integer Programming (MIP) model has been proposed which maximizes the daily average number of fractions delivered while ensuring a given mix of patient disease site and treatment complexity is maintained, and respecting operational and technical restrictions. In addition, an alternative model has been formulated to maximize pediatrics treatment delivery while maintaining a minimum daily average number of fractions and ensuring operational and technical restrictions. The proposed models have been implemented using CPLEX software. Results: The model has been used to assess the treatment capacity of the facility with on‐going operational parameters. It has also been utilized as a sensitivity analysis tool to evaluate the current treatment policies and to explore potential improvements in patient treatment capacity. Various capacity expansion scenarios have been tested and analyzed using the new model to verify their effectiveness in practice. Conclusion: This work presents a new decision support model designed to assess and plan the treatment capacity of a radiation therapy facility. It also allows for studying the capacity response to different changes in operational parameters. With the help of the proposed model, the required set of operational parameters can be determined so that a certain patient throughput, as well as a desired mix of disease sites and complexity, is obtainable.
SU‐FF‐T‐469: Energy Spectra Reconstruction From Beta Emitters: A Study of the Sr‐90/Y‐90 Case
Authors: Gueye P , Keppel C , Munden A
Published: April 2010

Purpose: Reconstruction of the energy spectrum of a radioactive source for low dose (∼1Gy) radiation biology experiments. Method and Materials: Electrons originated from a 25 μCi radioactive source were recently used in radio‐biology experiments to study the response of cancer cells to low dose irradiation (∼1Gy). The beam was separated into individual energies using a dipole magnet, the number of exited electrons measured by a scintillating fiber based detector and the dose obtained using double‐layered EBT films. The dipole was constructed from two 5.08 × 5.08 × 2.54 cm3 permanent magnets separated by a distance of 2 cm and having a maximum field of about 5 kG at the center. The beam entered the magnet through a 1 cm diameter, 0.5 cm thick leaded glass collimator. The detector was an array of 16 blue shifted 1 mm thick scintillating fibers. A realistic Geant4 simulation was used to account for the energy loss while electrons propagate throughout the system. The incident energy spectrum was generated from the NIST‐logft calculation. Results: The data was first compared with measurements using a pancake frisker detector for an overall general assessment of the data quality, giving an agreement to within 15%. The average deviation of the data/simulation ratio is ±2.5% and there is a . A comparison between the measured and simulated mean energies gives an agreement to within 1σ (less than 0.3%). Conclusion: The simple system developed for this work can serve as a mean for an efficient way to accurately reconstruct the energy distribution of charged particle emitters as well as to assess expected dose delivery to biological systems. Such data can also be used to benchmark low energy physics processes implemented in simulation tools for medical physics purpose.
SU‐FF‐T‐480: Minimum Multi‐Plan IMRT Framework Knowledge Base for Predicting Organ‐At‐Risk Dose‐Volume Levels and Complications
Authors: Zhang H , Meyer R , Shi L , D'Souza W
Published: April 2010

Purpose: To determine the minimum number of treatment plans required to build accurate representations of the IMRT plan surface in order to predict organ‐at‐risk (OAR) dose‐volume (DV) levels and complications as a function of input DV constraint settings. Method and Materials: 125 and 256 plans respectively were generated as the full knowledge base for a representative head and neck case and prostate case by varying input DV constraints on OARs (parotids, cord, bladder, bowel and rectum). Knowledge subsets of various sizes were used to determine the minimum sizes needed to obtain accurate plan surface models and accurate complication predictions via Machine Learning algorithms. Predictions of saliva flow rate were obtained via a sequential minimal optimization algorithm for training a support vector regression model. Binary classification via optimized decision trees was used to predict rectal bleeding. To build the plan surface for each of the two test cases, 100%, 50%, 25%, and 12.5% of the full knowledge base of plans were used (6.25% was also used for pelvis case). For each fractional value 50 random samples of the corresponding size were used as training sets. Results: The doses received at various volume levels can be modeled with less than 2% mean relative errors with 30 plans (95% confidence interval (C.I.) on largest error was [1.66%, 1.72%]). Saliva flow rate can be predicted with 1.91% mean relative error and a 95% C.I. [1.87%, 1.94%] with 30 plans. Grade 2 rectal bleeding complications can be predicted with mean correct prediction of 92.64% and a 95% C.I. [92.18%, 93.09%] using 64 plans. Conclusion: Our results indicate that only a small number of plans (30–64) are needed in order to model the plan surface and to obtain models that yield accurate prediction of OAR DV levels and complications as a function of DV constraints.
SU‐FF‐T‐486: The Impact of Prolonged Fraction Delivery Time in Advanced Radiotherapy Techniques On Cell Survival and DNA Damage
Authors: Khachonkham S , Chawapun N , Uaapisitwong S , Kamnerdsupapon P
Published: April 2010

Purpose: Currently, the advanced radiotherapy techniques such as Stereotactic radiotherapy (SRT) and Intensity modulated radiotherapy (IMRT) have been implemented in radiotherapy centers to improve the efficiency of cancer treatment. Due to their complexity, these techniques require the longer fraction delivery time comparing to conventional technique. This study aimed to prove a hypothesis that prolonged treatment time had a significant impact on cell sterilizing effect due to an increasing in DNA damage repair. Method and Materials: HeLa cells were irradiated by 6 MV linear accelerator using 3 different protocols simulated the advanced radiotherapy techniques. The absorbed dose of 200 cGy were given in 15, 30 and 45 min. Colony formation assay was used to determine cell survival while DNA damages were detected by Comet assay. The results were then compared to that of conventional technique with 5 min delivery time and analyzed by student T‐test. The Linear‐Quadratic model with α/β ratio and repair half‐time taken into account was used to calculate the dose compensating for prolonged time. Results: It revealed that, the relative percentage DNA in tail of cervical cancer cell line observed in comet assay were significantly decreased to 0.724 – 0.905 (p < 0.05) while relative surviving fraction increased to 1.02 – 1.17 to that of conventional technique. Although the surviving fraction tended to increase with increasing delivery time, only the surviving fraction at 30 min delivery time was statistical different. The absorbed dose of 204 cGy calculated by LQ model should be used to compensate for this impact. Conclusions: Prolonging treatment time allowed sublethal damage repair to take place during irradiation demonstrated by decreasing in DNA damage and subsequently increasing in cell survival. Therefore, biological effective dose should be compensated in advanced radiotherapy techniques which delivery time longer or close to repair half‐time of tumor cell.
SU‐FF‐T‐492: Mechanistic Formulation of a Lineal‐Quadratic‐Linear (LQL) Model: Split‐Dose Experiments and Exponentially Decaying Sources
Authors: Guerrero M , Carlone M
Published: April 2010

Purpose: In recent years, several models were proposed that modify the standard linear‐quadratic (LQ) model to make the predicted survival curve linear at high doses. Most of these models are purely phenomenological and can only be applied in the particular case of acute doses per fraction. We consider a mechanistic formulation of a LQL model in the case of split‐dose experiments and exponentially decaying sources. This model provides a comprehensive description of radiation response for arbitrary dose‐rate and fractionation with only one additional parameter. Method and Materials: We use a compartmental formulation of the LQL model from the literature. We analytically solve the model's differential equations for the case of a split‐dose experiment and for an exponentially decaying source. We compare the solutions of the survival fraction with the standard LQ equations and with the Lethal‐Potentially Lethal (LPL) model. Results: In the case of the split‐dose experiment, the LQL model predicts a recovery ratio (RR) as a function of dose per fraction that deviates from the square law of the standard LQ. The survival fraction as a function of time between fractions follows a similar exponential law as the LQ but adds a multiplicative factor to the LQ parameter β. The LQL solution for the split‐dose experiment is very close to the LPL prediction. For the decaying source, the differences between the LQL and the LQ solutions are negligible when the half‐life of the source is much larger than the characteristic repair time, which is the clinically relevant case. Conclusion: The compartmental formulation of the LQL model can be used for arbitrary dose‐rates and provides a comprehensive description of dose response. When the survival fraction for acute doses is linear for high dose, a deviation of the square law formula of the recovery ratio for split‐doses is also predicted.
SU‐FF‐T‐500: A Digital Monte Carlo Tumor Phantom to Study the Collective Cell Responses to Radiation Therapy
Authors: Sheng K
Published: April 2010

Purpose: To evaluate the response of a solid tumor to fractionated radiation therapy, a digital Monte Carlo tumor phantom is created to model the collective behavior of tumor cells based on their individual radiobiological characteristics. Methods and materials: Tumors with total number of 2×106 cells each were created as ones in two dimensional matrices. Varying tumor shapes were described by a geometrical factor defined as the ratio between the circumference and the area. Hypoxic tumor cells were defined as cells with a minimum cellular distance of 400 cells to the tumor surface. The digital tumors underwent regression with the fractionated radiation therapy. The viability of an individual tumor cells was determined by Monte Carlo simulation and its microenvironment. Hypoxic tumor cells were more radioresistive but did not proliferate. The treatment continued for different tumors until all cells were eliminated. Ten histories were run for each differently shaped tumor. Results: For a given shaped tumor, the logarithm survival followed the same slope for the first 15–17 fractions, where the survival started to separate, resulted in a relatively large statistical spread in the total number of fractions to sterilize the tumor among the 10 histories. For a round tumor, the number of fractions required to eliminate all cells ranged from 24 to 32, with an average of 26.3 and standard deviation of 2.71. The numbers needed to sterilize the tumor are nonlinearly decreased as the geometrical factor increased. Conclusions: A Monte Carlo tumor phantom was used to simulate collective biological responses to radiation therapy. Results indicate large statistic fluctuation in the total dose required to completely sterilize a solid tumor. It was also indicated that on average, tumors with higher surface to volume ratio require less radiation dose to kill.
MO‐A‐303A‐01: MR Acceptance Testing and Quality Control: Report of AAPM MR Subcommittee TG1
Authors: Jackson E
Published: April 2010

The MR Subcommittee of the AAPM Imaging Physics Committee (IPC) has recently completed a revision of the previous AAPM TG1 and TG6 reports on MR acceptance testing and quality assurance. The report of the MR Subcommittee has been approved by the IPC and will be available electronically (http://www.aapm.org/pubs/reports/). In this session of the MR continuing education course, the contents of the report will be presented. Recommendations regarding testing of magnetic and radiofrequency (RF) shielding will be followed by recommendations on specific acceptance tests of magnetic field homogeneity, the RF subsystem (calibration, stability), the gradient subsystem (calibration, eddy current compensation), combined subsystems (slice thickness and spacing), global system performance (SNR, uniformity, high and low contrast resolution), and advanced tests (MR spectroscopy, echo planar imaging).

Objectives:

1. Understand the requirements for MR site and system acceptance tests.

2. Review recommended MR site and system acceptance testing procedures, including phantom selection, data acquisition, data analysis, and reporting.

3. Review recommended action criteria for both standard imaging options and advanced imaging and spectroscopy options.
MO‐B‐210A‐01: Digital Dose Reporting
Authors: Balter S
Published: April 2010

This course will review the availability of digital radiation dose reporting tools An increasing fraction of imaging equipment is capable of measuring its own X‐ray output. Dose and related information is present in many DICOM image headers. Depending on the make/model/software data is in some mix of public and private fields. This information usually reports only the dose associated with the images in the file.

DICOM now provides tools for communicating dosimetric and related data in a comprehensive manner. A first‐generation structured report (RDSR), optimized for fluoroscopically guided procedures, was released as part of the 2007 DICOM Standard. Similar structures for mammography and computed tomography are in the 2008 Standard. The RDSR is a DICOM object that is created and managed separately from the creation and storage of images. Even if images are discarded (e.g. fluoroscopy, rejected radiographs), the RDSR will record all of the radiation used during a procedure.

All of the data in a RDSR is in public fields, each identified by a DICOM tag or unique concept name. The RDSR always contains patient and examination data, total dose data for the entire procedure. For interventional procedures, the RDSR also contains technical, geometric, and dosimetric data for each individual irradiation. RDSRs are designed to be distributed on a network and captured by free‐standing dose‐management ACTORS as well as by RIS and PACS. The IHE Radiation Exposure Monitoring (REM) Profile gives additional guidance and supplies use cases for RDSR handling. In addition RDSRs can also be stored within an imaging system and manually downloaded.

Learning Objectives

1. Understand the availability of dose information in DICOM image headers.

2. Understand the functionality of the new DICOM RDSR and associated components.

3. Understand how to use the RDSR and other dose reporting tools as part of an institutional quality program.
MO‐B‐213A‐01: Magnitude of Radiation Exposure to US Population (NCRP Report ♯160) with Focus On CT Dose
Authors: Mahesh M
Published: April 2010

National Council of Radiation of Protection (NCRP) recently released a long awaiting report (NCRP report ♯ 160) on the radiation exposure of the US population from various sources including medical exposure. This new report replaces the current NCRP report ♯ 93 published in 1987.

According to the newly released NCRP report ♯ 160 (March 2009), the medical radiation exposure to US population has increased by nearly 6 times compared to the previous NCRP publication (NCRP 93). The previous pie‐chart published in 1987 (NCRP report ♯93) indicated contribution of 3 mSv from background radiation, 0.53 mSv from medical exposure and 0.07 mSv from other sources (consumer products, occupational and other sources) exposure while the new pie‐chart published in March 2009 (NCRP report ♯160) indicates contribution of 3.1 mSv from background radiation and 3.0 mSv from medical and 0.1 mSv from all other sources (consumer products, occupational and other sources).

The largest contributor to the collective dose to US population is from CT and Nuclear Medicine. CT scanning has increased nearly 10–11% annually in the US in the past two decade. The number of CT procedures has increased from 3 million CT scans in 1980 to more than 69 million CT scans in 2007.

The purpose of this talk is to provide an overview of the newly released NCRP report ♯160. As a member of the NCRP scientific committee 6‐2 that developed the above report, the author will also focus on the medical radiation exposure including CT.

Learning Objectives:

1. To become familiar with the NCRP report number 160.

2. To learn about the various sources and magnitude of radiation exposure including medical radiation exposure to US population

3. To familiarize with the types and distribution of medical imaging procedures in US.

4. To examine the various aspects related to CT use and CT dose.
MO‐D‐BRB‐07: Retrospective RapidArc Dose Reconstruction Based On MLC Dynamic and Delivery Log Files Recorded During Treatment
Authors: Lee L , Qian J , Xing L , Chau R , Cheung K , Kam K , Yu K , Leung S , Chan A
Published: April 2010

Purpose: To develop a methodology for retrospectively reconstructing the dose delivered to head‐and‐neck (HN) patients in RapidArc treatment based on dynamic log‐files which record the actual leaf positions, gantry angles, and delivered monitor units (MUs) during the RapidArc delivery. Method and Materials: After a RapidArc treatment was finished, two dynamic log‐files were retrieved from the linear accelerator: (1) MLC log‐file which recorded the actual leaf positions and respective gantry angles every 50 ms and (2) delivery log‐file which recorded the actual delivered MUs and gantry angles at the control points defined in the RapidArc plan. Through the common parameter of gantry angle recorded for both dynamic log files, the actual delivery status such as leaf positions, delivered dose indices, and gantry angles for every control points were re‐constituted. This data was compiled and converted into a DICOM radiotherapy plan (RP) file using in‐house developed software written in MatLab code (Mathworks, Natick, MA). The DICOM RP file was then imported into Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA) and the actual delivered dose was reconstructed on the on‐treatment CBCT acquired for the patient. Results: A retrospective dose reconstruction procedure has been established for RapidArc and applied to a phantom and two dummy HN cases. For the case in which the tumor shrinkage is minimal, the reconstructed and planned doses were consistent to within 3–5% in high dose region. The DVHs of the target and other organs do not have significant differences. However, large dosimetric changes (10–15%) were observed for the case with tumor shrinkage, indicating the need for re‐planning or adaptive measure to be taken. Conclusion: RapidArc dose reconstruction provides a pragmatic way to probe the actual dose delivery at a particular fraction and represents an indispensable step toward adaptive radiotherapy for this highly conformal treatment.
MO‐D‐BRC‐02: Clinical Significance of In‐Vivo Proton Range Detection and Potential of MRI Scanning After Proton Therapy
Authors: Hong T
Published: April 2010

Highly conformal radiation therapy has the potential advantage of limiting radiation exposure to normal tissue and improving the therapeutic ratio of radiation therapy. However, with improving technologies, much greater emphasis has been placed on the accuracy of both treatment delivery and localization. Shrinking PTV margins based on our confidence in the perceived accuracy of our treatment delivery runs the risk of geographic miss and subsequent marginal recurrences. Proton therapy is particularly prone to this risk. Range uncertainties exist at the distal edge of the Bragg peak. Also, proton range and depth of penetration is highly dependent on tissue density. Hence, for moving targets with changing densities within the field, like at the lung bases or dome of the liver, there is greater uncertainty in the dose that was actually delivered to the tumor and normal tissue. For these reasons, verification of dose delivered in patients receiving proton therapy is of critical importance.

Post‐treatment imaging may be a useful way to verify proton range in‐vivo. Radiation may produce changes in imaging characteristics within organs that may be used to verify that the target has been adequately treated. In this lecture we will review some of the issues that complicate proton planning. Furthermore, we will demonstrate the utility of MRI scanning after proton therapy to document target coverage for both static and dynamic tumor systems.

Learning Objectives:

1. Discuss the clinical data regarding marginal misses with highly conformal therapy

2. Demonstrate the complexities of 4‐dimensional proton planning for moving targets, using liver as a model

3. Demonstrate the utility of post‐treatment MRI scanning as validation of the 4D‐proton treatment planning process and delivery for liver, as well as other clinical situations.
MO‐D‐303A‐04: Models and Resources for Intergrated Teaching and Learning of Medical Imaging Physics and Technology
Authors: Sprawls P
Published: April 2010

Purpose: Develop a model of an educational system that supports the effective and safe utilization of contemporary medical imaging technology on a global basis and to produce open resources, based on the model, that will enrich the local learning environments in radiology residency programs and related activities professional development activities. Method and Materials: The results of extensive research on the educational process, relating to both learning and teaching, identify the characteristics that determine the effectiveness and efficiency of learning activities. This is used to guide the development of models that use digital technology, including the internet, to enhance human performance of both learners (residents, students, etc) and medical physicists as learning facilitators (teachers). Results: A resource containing high‐quality visuals and images to increase the effectiveness of class/conference presentations and discussions, modules for “point and time of need learning” and review, online textbook materials, and interactive simulations is available on the web at: http://www.sprawls.org/resources/. Conclusion: The resource is being used in virtually all radiology residency programs, either/both as a support for classroom/conference discussions or for individual resident study and review. Residents report that the visuals, images, and easy to understand discussions in the modules are very helpful to their learning. On a global basis, several thousand medical imaging learners and professionals in over 80 countries are using the resource each week.
MO‐D‐303A‐05: Investigation of On‐Line Learning Methods and Software to Augment CAMPEP Residency Training Programs
Authors: Musser S , Mayo C
Published: April 2010

Purpose: It has been estimated that the number of CAMPEP accredited therapeutic medical physics residency programs needs to increase from 23 to approximately 100 if the ABR 2014 requirement is to be met. One hurdle for new programs is development and ongoing provision of didactic training needed to meet AAPM report 90 guidelines. A significant amount of FTE time is required to create and run courses in a traditional class room context. One solution is to create didactic training as web based modules that can be used to supplement mentored programs. This paper examines feasibility and one set of tools for creating web based training programs. Method and Materials: Educational literature was examined for the strengths and weaknesses of web based learning and to gain insight into effective techniques. Several commercial products were examined for their ease of use in a clinical environment and adaptability to medical physics needs. On‐line learning modules were created for several topics. Results: Recent studies show 1) exam scores were not impaired by use of a web based lecture series, 2) student satisfaction ratios did not differ significantly between web based and live lectures, and 3) student satisfaction ratings did not differ significantly between PowerPoint slides with audio narration and video stream. Participation by the resident in creating content for online learning modules enhanced the resident's mastery of the didactic material as well as providing training in educational practices. Conclusion: Creation and use of on‐line learning modules are an effective component of a didactic program. They fit naturally with TG133 based structured mentorship affiliated with a primary CAMPEP accredited residency. The technology makes possible creation of a library of modules by experts in particular fields that could be used in all programs.
MO‐D‐304A‐04: Cone‐Beam CT Lag Correction Models: Effect of Optimized Parameter Selection
Authors: Mail N , Moseley D , Siewerdsen J , Jaffray D
Published: April 2010

Purpose: Image lag degrades image quality in cone‐beam CT (CBCT). This work investigates the magnitude of lag artifacts and develops an optimized lag coefficient model to correct lag artifacts in CBCT images to improve guidance of radiotherapy(Elekta Synergy XVI). Method and Materials: Image lag and its relationship with various parameters including signal strength and frame number was investigated for a PerkinElmer (RID1640) flat‐panel imager. A new lag correction model referred to as the “average‐optimized lag coefficients model” (ALCM) is developed to correct CBCT images. The optimization of lag coefficients was purely based on quantitative improvement in lag corrected CBCT images. Each projection was corrected for lag effects by subtracting previous projections weighted by the magnitude of image lag. The quantification/measurement of lag coefficients for four different techniques including RESF(Rising‐Edge‐Step‐Response‐Function), IRF(Impulse‐Response‐Function), FESF(Rising‐Edge‐Step‐Response‐Function) and ALCM(Average‐Optimized‐Lag‐Coefficient‐Model) for the same detector. These models are applied/tested in correcting CBCT images of two customs made phantoms referred to as Ellipse_Lucite (MTF and skinline) and Irregular_Lucite (CNR). Results: Experimental results illustrate that the nth frame lag of the imager for all four model shows different behavior with frame number. The RCTN at 5 mm depth after lag correction was measured in CT♯ as 4.38±1.01, 10.51±1.35, 8.35±1.31 and 2.121±0.81 for RESF, IRF, FESF and ALCM, respectively. Similarly, the spatial frequency/cm for MTF(50%) before and after lag correction for RESF, IRF, FESF and ALCM was measured as 6.3±0.24, 5.7±0.23, 5.8±0.22 and 6.5±0.24, respectively. CNR for ALCMwas almost two times higher than nominal. Conclusion: Lag artifacts can be reduced by correction of the projection images using the ALCM model. Lag correction is most important for high contrast and irregularly shaped objects. The performance metrics suggest a significant improvement for RESF and ALCM and strongly support their use for lag correction in cone‐beam CT.

Research sponsored by Elekta.