bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐GG‐T‐356: Benchmark of Geant4 and MCNPX for Proton Radiation Therapy (E<70 MeV): Total Dose and Secondary Particle Production
Authors: Zhang M , Perez‐Andujar A , Hecht A , Mackie T
Published: July 2008

Purpose: The validity of the Monte Carlo (MC) for proton dose calculation has been proved by the fact that the simulation matches measurement. The secondary particles generated by hadronic interactions, e.g. neutron, alpha, can give high effective dose even if the production rate is low. This will potentially spoil the gain of using proton radiotherapy. Challenged by the measurement, attempts have been made to use MC to predict the secondary particle dose. Different code using different physics models or cross section data may lead to quite different spectrum. The comparison should be made among those MC codes to document the difference. Method and Materials: The ocular‐beamline from the Harvard Cyclotron Laboratory (Cambridge, USA) as well as measured data was used. Two MC codes, Geant4 (4.9.1) and MCNPX, were compared. Since the physics process can be customized in Geant4, different physics module combinations were generated. Proton nozzle, scoring region, material, and lateral spread out of the initial beam were kept same in all the simulations. In the first step, different simulations were tuned differently by changing the energy spread of the initial beam and the thickness of the range shifter to match the measurement. Then, the neutron flux as well as the dose from different particles within the phantom was compared between the simulations. Results: All the simulations match the proton beam ion chamber measurements providing different energy spreads and range shifter thickness. The neutron flux and specific particle dose within the phantom varies between different simulations in the preliminary result, which likely represents differences between physics module. Conclusion: Both Geant4 and MCNPX are good for proton dose calculation. However, for the secondary particle dose, the preliminary result shows that quantitative evaluation is suspect. Conflict of Interest: Thomas Mackie has a commercial relationship with proton radiotherapy.
SU‐GG‐T‐358: Monte Carlo Characterization of a Medical Linear Accelerator for Small Animal Irradiation
Authors: Lárraga‐Gutiérrez JM , Galván E , Moreno‐Jímenez S , Celis MA , Díaz‐Ruíz A , Ríos C
Published: July 2008

Purpose: Characterize a medical linear accelerator using Monte Carlo methods to investigate the precision and exactitude of dose delivery on small animal irradiation, particularly Wistar rat species. Method and Materials: A dedicated 6.0 MV linear accelerator (Novalis®, BrainLAB, Germany) for stereotactic radiosurgery (SRS) was simulated using BEAMnrc. A phase space (PS) data file was generated. The Monte Carlo (MC) calculations were tuned and validated to match depth and off‐axis dose profile data measured using a shielded diode detector (PFD3G, IBA‐dosimetry, Germany) for a 15.0 mm circular collimator. The animal model was based on a computed tomography (CT) scan of a Wistar rat for medullar trauma lesion model. An in‐house mask fixation system compatible with the treatment planning system (TPS) was developed to immobilize the rat. The CT scan images were imported to DOSXYZnrc using ctcreate tool and to the TPS via DICOM network. Dose distribution was calculated by the TPS using 2 non‐coplanar circular (NCAs). The NCAs were simulated using 20 static beams. The MC dose calculations were exported for analysis and compared with TPS dose calculations. On the other hand, absolute dose measurements were performed using the PFD3G inter calibrated using a Farmer type ionization chamber. Results: Absolute dose measurements showed that dose difference between TPS and treatment delivery is less than 4.3% on the selected points. Difference between TPS and MC dose calculations showed an over‐estimation by the TPS up to 15.0%. However, in the spinal cord lesion there is a good agreement between TPS and MC calculated data. Conclusion: Dose delivered by the dedicated linear accelerator is reliable to perform spinal cord irradiation in trauma lesion models. These preliminary results must be improved including an accurate source model of an arc radiation beam instead of the 20 discrete static beams.
SU‐GG‐T‐453: Verify the Reliability of Optical Guided Stereotactic Radiosurgery Target Localization Using Cone‐Beam CT
Authors: Zhu J
Published: July 2008

Purpose: The Optical Guided System (OGS) Stereotactic Radiosurgery target localization is different from the traditional Stereotactic Radiosurgery (SRS) target localization. Instead of mechanically aligning the laser to the marked isocenter on the SRS localization frame or box, the OGS accomplishes localization digitally by matching the target coordinates read from the treatment planning system under the guidance of the OGS software (SonArray). The OGS also offers advantages of inter‐arc (i.e. among couch rotations for arc beams) localization adjustment and real‐time patient position monitoring. These new features provide the potential of higher accuracy of target localization. However, the procedure of using the new features loses the intuition and its quality is fully depended on the system's reliability. The purpose of the study is to verify the reliability of OGS in target localization and adjustment. Method and Materials: A comprehensive SRS head phantom with a cubic insert is used in the study. The CT images of the phantom are taken and transferred to SRS treatment planning software to define the pseudo target and isocenter. The planned images and isocenter are then transferred to the OGS for initial target localization and Cone‐beam CT acquisition. The SRS phantom is displaced to the indicated translations and rotations guided by OGS. Cone‐beam CT are taken and compared to the initial Cone‐beam images to measure the displacements. The agreement of the measured phantom displacements to the indicated phantom displacements tells the reliability of OGS in target localization and adjustment. Results: For each phantom translation or rotation, the displacement captured by Cone beam CT matches the indicated displacement accurately. Conclusion: The OGS system is reliable in providing new features for higher accuracy of Stereotactic Radiosurgery target localization.
SU‐GG‐T‐465: Sequential Optimization Script to Generate HDR‐Like Dose Distribution for Hypofractionated Prostate Treatment with CyberKnife System
Authors: Lessard E
Published: July 2008

Purpose: To develop a Sequential Optimization script that generates HDR‐like dose distribution for hypofractionated prostate treatment with CyberKnife® Robotic Radiosurgery System and evaluate its consistency over different patients. Method and Materials: Sequential Optimization is the latest optimization algorithm available for CyberKnife treatment planning. The optimization is performed sequentially, as a series of individual optimization steps. Each step corresponds to a specific clinical objective. First, dose constraints to organs at risk are defined that cannot be violated by during the optimization. Additional constraints are defined to the maximum Monitor Units for each beam, for each robot position, and for the entire treatment plan. Then the objectives of each optimization step are specified. The result of each step represents the optimal value for this objective that is achievable within the existing constraints. Each result is retained as an additional constraint for all subsequent steps. In this manner the treatment plan is built up one clinical objective at a time, with a gradual improvement observed at each step. A sequence of optimization steps (script) was created that generates HDR‐like dose distribution for prostate treatment delivering 38 Gy in 4 fractions. Treatment plans were generated with the same script and with the Iris Variable Aperture Collimator for 10 prostates. Results: Prostate volumes ranged from 25–70 cc. Prostate V100, V125 and V150 are ∼95%, 50% and 15%. Urethra Dmax and D10 are <4560 and 4180 cGy. Rectal wall Dmax and D10 are <3800 and 2560 cGy. Bladder Dmax and D10 are <4560 and 4180 cGy. The number of beams is less than 250. Conclusion: A script was created that consistently reproduces dose distribution similar to an HDR treatment. It was demonstrated that a script can be defined for particular clinical case and used as a starting point reducing planning time.

Research sponsored by Accuray Incorporated.
SU‐GG‐T‐476: Clinical Impact of Applying Heterogeneity Correction to Dose Calculations for Esophageal Sites
Authors: Dirksen B , Flynn R , Svare C , Goodin F , Frigo S
Published: July 2008

Purpose: Many studies of the effects of tissue heterogeneity corrections exist for lung sites, but relatively few have focused on the esophagus. In this study, we characterize the differences between heterogeneity corrected and uncorrected esophageal cancer treatment plans in terms of physical and equivalent uniform dose (EUD). The aim is to provide guidance on prescription formulation when corrections are utilized, in order to connect past observed clinical outcomes to future planning calculation methods. Method and Materials: Seven patients with esophageal tumors superior to the diaphragm and inferior to the carina were evaluated. All plans used a four field 3D conformal technique, with a dose of 50.4 Gy prescribed to cover, at minimum, 93% of the PTV in 28 fractions. Three treatment plan variations were developed for each patient: (1) with heterogeneity correction, (2) without, and (3) with heterogeneity correction, but prescribed such that the monitor units were the same as the uncorrected plan. The corrected plan was taken to reflect the actual delivered dose and utilized as a reference for comparison. Results: The mean dose to the PTV was on average 2.6% higher in the corrected homo plan compared to the uncorrected plan (range: 0.9% to 5.8%). The standard deviation of the mean dose difference was 1.6%. The standard deviation of the PTV dose increased on average by 39.7% and volume of lung receiving 20 Gy increased by an average of 2.2%. The EUD of the PTV increased by an average of 2.2 percent (range: 0.0% to 5.7%). Conclusion: Using a homogeneous model will result in an increase in the EUD to the PTV as well as greater dose inhomogeneity within the PTV, a greater lung volume receiving at least 20 Gy, and a higher uncertainty in the PTV mean dose.
SU‐GG‐T‐484: Optimization and Normalization of Hypofractionated Lung Treatment Plans
Authors: Tyagi N , Rice R , Wang J , Pawlicki T , Jiang S , Sandhu A , Mundt A
Published: July 2008

Purpose: For small solitary lesions in the lung, we have adopted a protocol for hypofractionated treatment using multiple conformal or IMRT beams. Since the lesion is near unit density and the surrounding lung tissue is low density, the loss of scatter in the lung tissue can reduce the dose to the PTV, when, in reality, the CTV or ITV may be adequately covered with margin. We have performed calculations using several ways to create the PTV and to normalize the treatment plan to provide adequate coverage of the CTV or ITV with margin while minimizing hot spots which result from normalizing to the low density PTV. Method and Materials: Gated CT scans were obtained on 2 patients and target volumes were drawn on axial series representing several individual phases of the breathing cycle. An ITV was then created and used for planning. In addition, a PTV was created by adding a clinically relevant margin around the ITV. The resulting treatment plan was then used to calculate the dose delivered to the CTV for each of the phases of the breathing cycle using different normalization methods. ITV coverage was also studied by moving the ITV inside the PTV to simulate setup error. Results: 2 patients were planned using the above mentioned technique. Properly selected normalization results in adequate CTV and ITV coverage through all phases of the breathing cycle. Effect of setup error also results in adequate ITV coverage as the high dose region moves with the ITV. Conclusion: Small isolated lesions in the lung can be treated with hypofractionation, but care should be taken in selecting the normalization technique to minimize the creation of unnecessary hotspots. Coverage of the near unit density lesions is not predicted by the low density PTV coverage.
SU‐GG‐T‐488: Dosimetric Parameter Comparison of the Electronic Tissue Compensator Technique with the Conventional Physical Wedge Technique for the Whole Breast Treatment
Authors: Su M , Ayzenberg V , Li W
Published: July 2008

Purpose: To compare a dosimetric benefit between electronic tissue compensator (ETC) technique and the conventional physical wedge (PW) technique for a whole breast (WB) tangential field irradiation treatment. Method and Materials: 17 breast cases, with chest wall separation 16 to 26 cm at normalization point slice, were included in this study. Eclipse Planning System (V7.3) was used to generate a plan. The plan remained with a same beam setup, beam weighting, normalization point, and energy (6MV or 6MV mixed with 18MV) in both technique for each case. The breast tissue was outlined as a planning target volume (PTV). For a PTV dose coverage information, PTV encompassed by 95% isodose line (V95) and PTV received the minimum dose (Dmin) were studied. To examine a high dose region, a PTV received at least 105% prescription dose (V105) was computed. To study dose inhomogeneity (DI) in the PTV, a ratio of the difference of Dmax and Dmin to the prescription dose was estimated. In additional, a total MU from each plan was recorded to not only measure the linac beam on time, but also as a factor for the machine scatter and leakage evaluation. Results: By comparing with the PW plan, (1) The PTV coverage, V95, showed quite similar (difference within 0.5%) in both techniques and Dmin was lower on a percentage of 1.2±9.1 from ETC plan; (2) The hotspot, V105, was reduced by a percentage of 2.9±2.2 in ETC plan. (3) DI was decreasing by a percentage of 1.3±12.3 in ETC plan. The total MU reduced by about 8.9±17.1 percent in ETC plan. Conclusion: ETC technique presented a dosimetric benefit for the WB tangential field treatment. And these benefits were subjected to case‐by‐case study. A careful comparison is needed for the treatment plan selection.
SU‐GG‐T‐492: Dosimetric Comparison of Prostate Radiotherapy Treatment Techniques
Authors: Buckey C , Stathakis S , Swanson G , Su F , Papanikolaou N
Published: July 2008

Purpose: There is a significant difference in complexity between the planning and delivery of 3D conformal therapy as compared to IMRT. This study compares and evaluates radiation therapy plans for prostate cases using both 3D conformal plans and IMRT plans, to ascertain if there are dosimetric advantages to IMRT given the current constraints. Method and Materials: A single physician outlined the prostate (GTV), rectum, bladder, femoral heads and penile bulb of 10 consecutive prostate patients. IMRT plans, using the current RTOG constraints from RTOG study 0415, and 3D conformal treatment plans (3DCRT) were created for each patient. The 3DCRT plans consisted of 6 beams, with opposed lateral and paired oblique beams, using 6 MV and 18 MV photons, created with the Pinnacle treatment planning system (TPS). The IMRT plans were generated using the Pinnacle system, the HiArt Tomotherapy system, and the Corvus TPS. In total, 6 plans were created for each patient. Results: By comparing the plans in terms of dose statistics and dose volume histograms we observed that the 3DCRT was able to meet the RTOG dose constraints. The dose to critical structures was comparable between 3DCRT and IMRT plans, independent of the TPS. Dose homogeneity of the PTV was better achieved with the IMRT plans. The 3DCRT plan could be further improved if a better choice of beam angles was made. Conclusion: The plans were evaluated for: (i) ability to deliver 95% of the dose to the PTV without significant variation, and (ii) for dose to the rectum and bladder. The 3D conformal plans were able to meet the RTOG criteria, suggesting that the added time and expense of IMRT planning for these cases was not justified and that the true power of IMRT is not being realized.
SU‐GG‐T‐493: Dosimetric Impact of Beam Energy and Number of Photon Fields On Prostate Dose Distributions: Inverse Planning and Validation Study
Authors: Ahmad M , Lund M , Halpin H , Deng J , Chen Z , Mani S , Moran M , Nath R
Published: July 2008

Purpose: To examine the quality of various IMRT techniques based on treatment plans that utilize different photon beam energies and beam arrangements for radiotherapy of prostate carcinomas. Method and Materials: For this study, a sample patient diagnosed with prostate cancer was selected. The patient underwent computerized tomography (CT) scans for inverse planning. These CT images were used to outline the planning target volume (PTV) and several organs at risk (OAR) including bladder, rectum, and femurs for defining dose constraints during the inverse planning process. Using the Pinnacle3 planning system (V. 8.0d), 5‐ and 7‐field IMRT plans were generated using (a) 6 MV, (b) 10 MV, and (c) 15 MV photons. These six plans were scored using dose‐volume‐histograms (DVH) to infer quality index parameters, namely, dose uniformity and conformity in the PTV, dose to OAR, and integral dose in the irradiated normal tissue. Each IMRT beam of the selected treatment plan was transferred on to a homogeneous flat phantom for planar‐dose calculations at 10‐cm depth. These planar dose distributions were measured using MapCHECK and film dosimetry to superimpose on the planned dose distributions for comparison. Results: Our planning studies have shown that: (1) 5‐F vs. 7‐F IMRT plans for all beam energies predict less doses to the normal tissue surrounding the prostate target volume below 3500 cGy level, (2) As expected, superior dose distributions are achieved with all IMRT techniques for all photon beam energies in the 6–15 MV range, (3) Dose uniformity compares fairly well between 106.5 and 108.3% for all plans irrespective of photon energy and beam arrangements. Detailed dosimetric differences between plans will be discussed. Conclusion: We have successfully performed an inverse planning study to examine the effect of photon beam energy and the number of these beams on the quality of IMRT plans to treat prostate patients.
SU‐GG‐T‐495: Proton Treatment Planning of Complex Cases with Patch‐Fields
Authors: Ly A , Nookala P , Watt D , Patyal B
Published: July 2008

Purpose: To illustrate the use of patch field arrangements to treat geometrically complex tumors with proton beams. Method and Materials: One very important property of a high‐energy proton beam is the ease with which it can be stopped at a desired depth in a medium and the sharp distal dose fall‐off near the end of range. This leads to the creation of straightforward beam arrangements to treat very complex tumor shapes. One variation of these beam arrangements is the patch field arrangement. These patch field arrangements are a powerful tool in proton therapy planning. A geometrically complex tumor, e.g., one wrapped around a critical structure, can be treated by a patch field arrangement in which one beam, called a shoot‐through beam, irradiates a part of the tumor. Patching another beam from a different direction and distally stopping this beam on the lateral edge of the shoot‐through beam irradiates the tumor not treated by the shoot‐through beam. We will show how proton patch fields can be used to treat complex clinical cases. Results: We present two plans showing the use of proton patch fields to treat two clinical cases. These cases demonstrate the simplicity of the patch field beam arrangements and the ease with which critical structures can be conformally spared and tumors can be fully irradiated. In addition, these beam arrangements are quite simple to set up clinically. Conclusion: In any clinical practice, one is frequently faced with difficult cases in which tumors, because of their proximity to critical structures, can neither be fully resected nor can they be safely treated with conventional radiation. Proton irradiation, because of its superior dosimetric characteristics, offers treatment options for such cases.
SU‐GG‐T‐502: A Fast Algorithm to Predict Dose Distribution Perturbations in An MRI‐Linac Hybrid System
Authors: Kirkby C , Stanescu T , Murray B , Rathee S , Carlone M , Fallone B
Published: July 2008

Purpose: The hybrid MRI‐linac unit under construction at our institute consists of a unique rotating bi‐planar (RBP) geometry where a linac is coupled to the open end of a 0.2 T permanent magnet and the entire structure rotates about the patient as a unit. Monte Carlo (MC) simulations that incorporate the influence of the magnetic field in this geometry have demonstrated small perturbations to patient specific dose distributions. This work takes advantage of the constant direction of the net Lorentz force in the RBP geometry to derive an electron density‐scaled dose shifting algorithm that will quickly and accurately incorporate magnetic field effects into the treatment planning process. Method and Materials: MC simulations of the projection of a 6MV photon beam onto a slab phantom with a central slab of variable electron density were performed using DOSXYZnrc with macros invoked to account for the changes in electron velocity due to the Lorentz force for each transport step. Dose profiles at 0.2 T were shifted in the direction of the Lorentz force until a minimum difference with 0.0 T profiles was observed. From this we derived an electron density scaled‐shift function. This function was applied to a four field brain plan calculated by the treatment planning system and the results were compared to full MC simulations. Results: The shift correction reasonably reproduced magnetic field effects on dose profiles for relative electron densities down to 0.3. The shift correction accurately accounted for dose perturbations within the patient's body for the brain plan. Conclusion: Magnetic field induced dose perturbations for the MRI‐linac system at our centre result in a shift of dose in the net Lorentz force direction, which can be quickly accounted for using a shift of the dose distribution based on the relative electron density of the local medium.
SU‐GG‐T‐510: Dose Smearing in GRID Therapy Due to Tumor Motion
Authors: Naqvi S , Mohiuddin M , Ha J , Regine W
Published: July 2008

Purpose: Clinical and biological evidence suggests that GRID therapy is successful in debulking large tumors because of the high heterogeneity in the dose distribution. In this paper we show that the characteristic peaks and valleys are significantly smeared out by motion, possibly affecting clinical outcomes. Method and Materials: Using a Monte Carlo engine that emulates phantom motion, we calculate dose for a hexagonal GRID, with 1.3 cm wide holes separated by 2.1 cm at isocenter. We study motion in the z‐direction, parallel to rows of holes, or in the perpendicular x‐direction. The displacement‐time waveform follows a cosn pattern, with n assigned 1 or 6 to account for a range of inhale‐exhale asymmetry. Dose calculations are performed for 6 MV x‐rays. Results: Near dmax, the static valley dose is 0.12 D0, where D0 is the peak static dose. For a sinusoidal z‐motion with amplitude a, the minimum peak (Dp‐min) and maximum valley (Dv‐max) occur at a ≈ 0.8 cm with values 0.67 D0 and 0.60 D0 respectively. As a is increased, contributions from neighboring holes make the peaks and valleys vary periodically with a. Less degradation is seen with asymmetric cos6 type motion, with and . For both waveforms, 0.7 cm amplitude lowers the peaks by 20% of D0, whereas 0.4 cm amplitude raises the valleys by this amount. The corresponding amplitudes for x‐motion are 0.7 and 1.3 cm. Thus, motion effects may be alleviated by properly choosing the grid orientation. GRIDs with smaller hole separations yield motion effects at smaller amplitudes. Conclusion: The results correlate with the clinical finding that tumors in abdominal and thoracic areas respond poorly compared to those in static areas such as skin and head & neck. Thus motion compensation may be necessary to maintain the efficacy of GRID therapy.
SU‐GG‐T‐520: Inhomogeneity Corrected/Non‐Corrected Plan Evaluation for Simple and Conformal Plans For Radiation Therapy of Lung Cancer
Authors: Harrison A , Podder T , Werner‐Wasik M , Xiao Y , Yu Y , Galvin J
Published: July 2008

Purpose: To investigate the effects on dose to critical structures for density corrected planning on simple (APPA and off‐cord) and conformal (4–5 field) lung plans compared to historical experience assuming homogeneous density. Method and Materials: Ten patients were planned using both techniques with XIO CMS software. The conformal plans were normalized to percentage coverage of target volumes and the simple plans were calculated to midplane. All plans were calculated using homogeneous density as the standard. After planning, CT density corrections were implemented and monitor units were matched to the homogeneous plan; the plans were not re‐optimized. Dose volume histograms were reviewed for critical structures of cord, “hot spot” D5(PTV), and V20(lung). Plan uniformity was assessed by the slope of the PTV dose curve using the equation [(D5–D95)/Dmean]. Results: The V20 increased with corrections: 23.7±1.7 to 24.6±7.2 for complex plans; 25.2±10.8 to 26.1±11.3 for simple plans. The conformal plans degraded in uniformity by a factor twice that of the simple plans and D5 increased accordingly. Cord dose increased slightly in conformal plans (33.65Gy±12.0Gy to 34.38±12.2Gy) and remains similar in simple plans (37.83±16.5Gy to 38.11±16.8Gy). The range of cord dose variations in corrected conformal plans was from −0.22–3.24Gy; one increase raised the cord V1% from 45.95Gy to 48.84Gy. Additionally, since the simple plans historically were assumed to approximate zero cord dose for the off‐cord obliques, the calculated/delivered cord dose is much higher. The effect of density corrections increased notably in the conformal plans where more beams transverse the lung. Conclusion: Although the effect of homogeneity corrections are well documented, the outcome on plan uniformity and critical structure tolerances should be further examined when transitioning from simple to multi‐field beam arrangements including IMRT. This study provides clinical dosimetric reference for adoption of heterogeneity corrected planning.
SU‐GG‐T‐523: Benchmarking of An MMLC for Intensity‐Modulated Proton Radiotherapy, with Emphasis On Secondary Neutron Dose
Authors: Daartz J , Bangert M , Engelsman M , Bussière M , Oelfke U , Kooy H
Published: July 2008

Purpose. We discuss the results of a feasibility study for multileaf collimator driven intensity modulated proton radiotherapy. The main focus is directed towards secondary neutron dose when using a miniature multileaf collimator. We also present a dosimetric comparison with conformal brass apertures. Method and Materials. Mechanical properties of an Integra Radionics MMLC were tested. The dosimetry of the MMLC in a fixed intra‐cranial proton beamline was assessed by comparing depth and lateral dose profiles, as well as the conformity of 2D dose distributions. Neutron dose to the patient is a consequence of protons impinging on beam modifying devices, such as scatterers and collimators. Neutron dose was measured for both brass apertures and MMLC shaping at different distances from the aperture edge for varying proton energies, airgaps and field sizes. Activation of the MMLC leaves was quantified. Results. Mechanical reproducibility and centering of the MMLC leaves are excellent. As expected we observe differences in dose distributions shaped by brass aperture and MMLC with regards to lateral fall‐off, conformity and the field size dependence of the Bragg peak‐to‐entrance ratio. These differences are clinically negligible. Neutron dose depends strongly on particle energy, airgap, distance from isocenter and collimator material. It is increased for the tungsten MMLC, but the required increased airgap reduces the difference between MMLC and brass aperture. Because of the design of our intra‐cranial fixed beamline the neutron dose is a factor of 10 to 20 smaller than in our passive‐scattered IBA gantry system. Conclusion. Extensive measurements proved that secondary neutrons do not prevent the implementation of intensity modulated proton therapy in our fixed beamline. Our results will furthermore be used as basis for the development of an MMLC dedicated for proton therapy.
SU‐GG‐T‐537: Implementation of a Sector‐Integration Method for Calculating Electron Cutout Factors at Extended Distances
Authors: Vossler M , Liew M
Published: July 2008

Purpose: To develop a spreadsheet that enables a physicist or dosimetrist to quickly determine electron‐beam cutout factors for irregularly‐shaped apertures at extended distances. Method and Materials: Cutouts with circular apertures from 10cm to 1cm diameter were fabricated to fit inside a 10×10cm electron applicator. The cutout factor for each was measured with 9 MeV electrons at two depths (dmax and R90) and five SSD's (100, 102, 105, 110, and 120cm). A polynomial fit to the data yielded the cutout factor as a function of aperture radius for each SSD‐depth combination. A spreadsheet was written that estimates the cutout factor for an irregularly‐shaped cutout by dividing the cutout into sectors, calculating the cutout factor for each sector, and taking the average. Results: Changes in the cutout factor as a function of SSD were noted. The spreadsheet was tested using 10 electron cutouts from actual patient treatments. For each cutout, the cutout factor was measured for all five SSD's at both depths. The spreadsheet was used to estimate the cutout factor for each cutout at each SSD and depth. Except for cases where the cutout was especially narrow and concave, the spreadsheet estimated the cutout factor to within 1% of the measured value. Conclusion: The spreadsheet correctly estimated the cutout factor for most situations, regardless of depth or distance, and took only a few minutes to use. Using this spreadsheet reduces the need for the physicist to measure the cutout factor directly, which can be a considerable time savings. It also removes any need to use less‐accurate approximations.
SU‐GG‐T‐538: Dosimetric Effect of Energy Spread of Proton Beams
Authors: Luo W , Ma C , Yin F , Fan J
Published: July 2008

Purpose: To investigate how proton beam energy spreads dosimetrically affect proton plans. Method and Materials: Scanning proton beams were simulated using the Monte Carlo method. A computer program was developed to calculate the weights to produce the SOBP that will be used for treatment planning. The prostate proton plans (PT and IMPT) with different beam orientations were generated with the Monte Carlo simulation. The energy spread ranged from 0 to 20 MeV. The plans were compared to show the effects of energy spread. Results: For the one‐beam (AP) PT plans, the increase of energy spread increased the target heterogeneity by a factor of 4, and the mean doses to the normal tissues by up to 28%, but decreased V65 for the rectum by 60%. For the two‐beam (lateral) plans, the energy spread increase reduced the target heterogeneity by up to 20%, the mean doses to the bladder and rectum by about 2%, and V65 for both the rectum and bladder by up to 4%, while increased the mean doses to the whole body and the femoral heads by up to 8%. For 4‐beam (box) plans, all those parameters were increased with the energy spread except that V65 and V40 for the rectum were reduced by up to 20%. All IMPT plans have consistently reduced the mean doses to the normal tissues by up to 60% and V40 and V65 except the one‐beam plan that increased V65 for the rectum. Conclusion: The dosimetric effect of energy spread could result in either increasing or decreasing normal tissue doses depending on the beam geometry. But almost all IMPT plans reduced the doses to normal tissues compared to IMRT. Therefore, the energy spread of proton beams can be effectively used for proton therapy with the appropriate use of beam orientation and intensity modulation.
MO‐A‐342‐01: Activity Quantitation and Dose Estimation From Nuclear Medicine Imaging
Authors: Williams L
Published: July 2008

Because of resistance to chemotherapy and multiple sites of disease, internal emitters are being more frequently used in treating advanced cancer patients. This radiotherapy is predicated upon injection of beta‐ or alpha‐labeled antibodies and other agents that target tumor markers. Most clinical success has occurred in the case of B‐cell lymphoma and hepatic lesions. For associated treatment planning, estimate of radiation dose (D) to tumors and normal tissues requires application of the equality where S is a rectangular matrix and Ã is a set of total decays for each source tissue. While S may be determined via Monte Carlo (MC) methods, Ã requires that the observer integrate time‐activity curves for each source. Quantitation of activity at depth in patients has been a continuing problem in nuclear medicine. Multiple methods have been developed ranging from direct counting (surface lesions), through geometric mean estimates (GM) to hybrid techniques (SPECT/CT) involving nuclear and CT scans. In the last strategy, attenuation may be taken directly from the CT data set. Quantitative SPECT (QSPECT) studies typically involve such hybrid imaging techniques and correct for attenuation, scatter, collimator geometry and partial volume effects. Accuracy of these methods, as measured by phantom studies, varies from +/−; 30% (GM) to as little as +/− 5% for QSPECT. While PET/CT may also be considered, the limited number of positron emitters causes this application to be more problematic. Time considerations make extensive use of QSPECT difficult. Standard clinical procedure follows Koral et al (Cancer Biother Radiopharm. 2000; 15; 347–355) in combining GM at the requisite multiple time points with an overlap of QSPECT at one time point. The latter study is then used to normalize the geometric mean values and improve quantitation of activity. Finally, we must note that two types of dose estimate are being done: phantom (Type I) and patient‐specific (Type II). In the former case, a standard geometry is applied assuming; e.g., the OLINDA program. Here, the patient or volunteer activity integral must be corrected for relative mass differences between phantom and patient. With Type II, the patient's own organ geometry is used to generate an S matrix. This may be done directly with MC techniques or a phantom value may be corrected by the target organ mass differences between the phantom and patient. If these corrections are not made, errors in S may be on the order of several‐fold. Uncertainty in resultant D values is estimated by combining errors in S and Ã.

Educational Objectives:

1. Know how to estimate internal emitter doses via .

2. Understand various methods to quantify activity at depth in patients.

3. Realize that both phantom and patient dose estimates are needed.

4. Understand the size of errors involved in dose estimation.
MO‐A‐352‐01: Fluoroscopic Imaging Equipment
Authors: Rauch P
Published: July 2008

Modern fluoroscopy imaging equipment is designed to simultaneously monitor x‐ray beam attenuation, detector exposure rate, contrast‐to‐noise ratio, x‐ray generator parameters, and x‐ray tube loading. Knowledge of system design is essential to understanding equipment operation; and understanding equipment operation can facilitate optimization of performance. Under pre‐programmed control, fluoroscopy systems automatically make adjustments to the parameters controlling x‐ray production, beam filtration, detector signal output, digital image processing, and image presentation. The result is that reduction of patient dose does not necessarily require a loss of image quality.

Optimizing performance begins with the determination of which system parameters are dynamically controlled, and the identification of the trigger points for changes in these parameters. Secondly, an understanding of how system parameters affect each other, affect image quality, and affect patient dose can lead to better choices in how the system should be set up for specific applications. This presentation will systematically describe the functions of the major system components including automatic exposure rate control, variable pulse‐rate fluoroscopy, generator operating curves, spectral beam filtration, and other factors that affect the ability of the imaging system to deliver optimum diagnostic images at moderate patient dose.

Educational Objectives:

1. Provide an understanding of modern fluoroscopic imaging system component design and functionality.

2. Describe specific ways in which equipment settings can enhance or detract from optimum performance.
MO‐B‐350‐01: Functional/Molecular Imaging: PET for Treatment Planning and Response Assessment
Authors: Jeraj R , Machtay M
Published: July 2008

Purpose: Positron Emission Tomography (PET) images show physiological and biological information through the in vivo distribution of radioactive, positron‐emitting agents. PET imaging shows focal and distributed regions of cancer and its metastases. Initial PET uses in oncology include diagnosis and staging, which are important for determining treatment decisions. Current PET uses now include target definition for radiation planning followed by PET‐based assessment of treatment. Hybrid PET‐CT devices are beginning to be used as radiation treatment simulators. Method and Materials: F‐18‐labled Fluoro‐deoxyglucose (F‐18 FDG) is the most commonly used PET imaging agent. FDG shows regions of active glucose metabolism, such as local cancer, metastases, non‐cancer inflammation, and normal glucose use. Non‐FDG PET agents can be more specific in cell targeting (binding), and can image different aspects of tumor biology, like hypoxia (FMISO, CuATSM) and cell proliferation (FLT). Results: PET imaging has coarse spatial resolution compared to CT and MR. PET's clinical use is valued because of its great sensitivity for cancer detection. Voxel intensity and image fidelity depend on equipment design, patient size, anatomic site, and imaging study parameters. PET‐CT units enable CT‐based attenuation corrections and inclusion of CT information in a registered PET‐CT dataset. The Standardized Uptake Value (SUV) is a normalized intensity measure for quantitative indication of disease, and can be used to identify disease or possibly for target delineation, with certain limitations. While FDG remains the most promising agent for tumor detection, other, biologically more specific agents (e.g., FMISO, CuATSM for hypoxia imaging or FLT for cell proliferation imaging) might be more appropriate for target delineation and treatment assessment. Conclusion: This course reviews PET‐CT hybrid scanning devices, uses of FDG and non‐FDG PET for oncology imaging, and quantitative aspects for PET‐based radiation target definition and treatment assessment. Example images demonstrate the potential contributions and limitations of FDG and non‐FDG PET oncology imaging. This review course is intended for both imaging and radiation oncology physicists.

Educational Objectives:

1. Describe FDG PET imaging and oncologic indications.

2. Review the uses and limitations of PET images in radiation treatment.

3. Describe the SUV and other threshold parameters for target delineation.

4. Review non‐FDG PET imaging of tumor biology.

5. Discuss quantitative aspects of PET imaging for treatment assessment.
MO‐B‐AUD A‐01: Quality Assurance in Stereotactic Radiosurgery and Fractionated Stereotactic Radiotherapy
Authors: Shepard D , Solberg T
Published: July 2008

In the 50‐plus years since it was first introduced, stereotactic radiosurgery, high‐dose irradiation of cranial neoplasms delivered in a single fraction, has become a standard of care in the treatment brain tumors, vascular malformations, functional disorders, and pain. Modern radiosurgery can be performed non‐invasively and on an outpatient basis, yet with an extremely high degree of accuracy. Within the past ten years, the field of radiosurgery has seen numerous technological enhancements, including: the development of dedicated devices for stereotactic delivery, the use of relocatable frames to facilitate fractionated delivery, the development of image guided and “frameless” approaches, and the application to extracranial tumor sites. Each of these developments is accompanied by its own challenges in assuring targeting and dosimetric accuracy. In this presentation we review the technologies for stereotactic localization and treatment of cranial targets with particular emphasis on the quality assurance aspects associated with establishing and maintaining a clinical radiosurgery program.

Educational Objectives:

1. Differentiate how radiation is delivered for Gamma Knife and Linac‐based (conventional and robotic) stereotactic radiosurgery.

2. Define the treatment planning parameters for Gamma Knife and Linac‐based stereotactic radiosurgery.

3. Discuss measures for assuring accuracy in stereotactic localization and dose delivery for Gamma Knife and Linac‐based stereotactic radiosurgery.
MO‐B‐351‐01: Dual Energy CT: Physics Principles
Authors: Pelc N
Published: July 2008

Recent advances in CT scanner technologies have generated a great deal of interest in dual energy CT. The basic research on the fundamental principles, however, dates back to the 1970's, and that work in turn was based in part on even earlier work on x‐ray absorptiometry. The purpose of this presentation is to discuss the basic physics principles of multi‐energy x‐ray methods for material characterization, and the strengths and limitations of the techniques and the various implementations.

A single energy CT image cannot possibly fully characterize materials. At best it is an image of the linear attenuation coefficient at one energy, and it is well‐known that this does not uniquely identify the tissue. The x‐ray attenuation of materials with different atomic numbers has different energy dependence, and the concept behind multi‐energy imaging is to measure the x‐ray attenuation properties at different energies to more fully characterize the material. At first glance one might assume that measurements at N energies would enable the separation of the object into N component materials. However, an important limitation to the ability of x‐ray attenuation measurements to fully characterize materials is the fact that in the diagnostic energy range the attenuation of all materials is dominated by Compton scattering and photoelectric absorption, and, unless there is an absorption edge in the energy range used, these x‐ray interaction mechanisms have the same energy dependence for all materials. Under these conditions, there are really only two “basis functions” and multi‐energy CT essentially measures the effective atomic number and the electron density of the material. This leaves us with considerable residual ambiguity as to what the material is. Also, measurement noise limits the ability to measure small changes in atomic number.

The main requirement for dual energy CT is to measure projection data using two x‐ray spectra. There are several implementations available. These include systems that obtain measurements at multiple kVp and/or filtration. There are also methods in which the x‐ray energy discrimination is performed at the detector. These include layered detectors and photon counting systems with energy analysis. These various approaches to obtaining energy dependent measurements have different dose efficiency and different sensitivity to subject motion.

There are two main ways to process the multi‐energy data. The simplest method reconstructs CT images from the data with each spectrum and performs the multi‐energy analysis on the values in the reconstructed images. The energy dependent processing can alternatively be applied to the measured projections prior to reconstruction. This latter method is somewhat more complicated but provides additional benefits, such as an implicit beam hardening correction.

Research sponsored by GE Healthcare.
MO‐B‐351‐02: Dual‐Energy CT: Dual‐KV Techniques and Clinical Applications
Authors: McCollough C
Published: July 2008

In CT imaging, materials having different chemical compositions can be represented by the same, or very similar, CT numbers, making the differentiation and classification of different types of tissues extremely challenging. In dual energy CT, an additional attenuation measurement is obtained at a second energy, allowing the differentiation of the two materials. Previously implemented in the 1980s, dual‐kV techniques are again available on clinical CT systems, accomplished with either slow or fast tube potential switching or dual‐source methods. The fundamental principles of dual‐kV techniques and their relative strengths and weaknesses will be reviewed. Clinical applications of dual‐kV CT imaging will be described, including: 1) automatic removal of bony anatomy, including calcified plaque, from a CT data set; 2) semi‐quantitative indication of the perfused blood volume in lung parenchyma or the myocardium; 3) removal of the iodine signal from contrast‐enhanced CT data, which may allow for the elimination of the non‐contrast scan phase in some exams; and 4) characterization of tissue by its chemical composition, as in the discrimination of uric acid from calcium‐containing renal stones.

Educational Objectives:

1. The technical approaches to dual‐kV, dual‐energy CT currently implemented or under investigation on commercial CT systems.

2. The technical strengths and weaknesses of each approach.

3. What clinical applications are currently in use or under investigation.
MO‐B‐352‐02: Multi‐Modality Imaging: Physics and Technology Considerations in Diagnostic and Image‐Guided Procedures
Authors: Siewerdsen J
Published: July 2008

Medical imaging technologies that combine the capabilities of two or more imaging modalities are becoming increasingly important in the diagnosis, staging, treatment, and monitoring of disease. In particular, the combination of morphological imaging modalities (e.g., x‐ray, CT, ultrasound, and MR) with functional or molecular imaging modalities (e.g., optical, PET, SPECT, and functional MR) offers synergistic advances. This session provides an overview of technical and physical aspects of such developments, reviews the most prevalent technologies under consideration, addresses the challenges and limitations of such technologies, and discusses the opportunities for future research in multi‐modality imaging. Example multi‐modality approaches include x‐ray / ultrasound, CT / PET, MR / PET, and Optical / CT. Applications include pre‐clinical imaging, diagnosis and staging, image guidance, and treatment response monitoring.

Educational Objectives:

1. An understanding of the technical and physical factors of multi‐modality image quality, registration, and accuracy.

2. An understanding of the challenges and potential opportunities associated with various multi‐modality imaging technologies.

3. An understanding of the various clinical applications enabled by such technologies.