bluuebunny/crossref_metadata_2025 huggingface dataset:
Automated gamma knife radiosurgery treatment planning with image registration, data‐mining, and Nelder‐Mead simplex optimization
Authors: Lee Kuan J. , Barber David C. , Walton Lee
Published: July 2006

Gamma knife treatments are usually planned manually, requiring much expertise and time. We describe a new, fully automatic method of treatment planning. The treatment volume to be planned is first compared with a database of past treatments to find volumes closely matching in size and shape. The treatment parameters of the closest matches are used as starting points for the new treatment plan. Further optimization is performed with the Nelder‐Mead simplex method: the coordinates and weight of the isocenters are allowed to vary until a maximally conformal plan specific to the new treatment volume is found. The method was tested on a randomly selected set of 10 acoustic neuromas and 10 meningiomas. Typically, matching a new volume took under 30 seconds. The time for simplex optimization, on a Xeon processor, ranged from under a minute for small volumes ( cubic mm, 2–3 isocenters), to several tens of hours for large volumes . In 8/10 acoustic neuromas and 8/10 meningiomas, the automatic method found plans with conformation number equal or better than that of the manual plan. In 4/10 acoustic neuromas and 5/10 meningiomas, both overtreatment and undertreatment ratios were equal or better in automated plans. In conclusion, data‐mining of past treatments can be used to derive starting parameters for treatment planning. These parameters can then be computer optimized to give good plans automatically.
SU‐FF‐I‐08: Results of An Optical Fiber‐Based Dosimetry System for Use in Computed Tomography Characterization
Authors: Klein D , Peakheart D , Gaza R , Rong X , McKeever S
Published: July 2006

Purpose: Modern multi‐detector CT and cone‐beam CT offer wide beams, making the concept of CT dose index (CDTI) no longer valid for CT dosimetry. A real‐time OSL dosimetry system has been developed and is evaluated for CT dosimetry in this study. Comparisons with a pencil ionization chamber were made. Methods and Materials: The system utilizes the optically stimulated luminescence (OSL) of KBr:Eu. The size of the KBr:Eu single crystal dosimeter equals approximately 1 mm3. The dosimeter was affixed to the terminal end of a plastic fiber cable and placed in the center hole of a plastic cylindrical phantom. The distal end of the fiber cable was attached to OSL reader, containing a 658 nm red laser, and photo‐multiplying tube (PMT), and associated optics/ electronics. CT slices of 1 s duration were performed over a range of energies (80–140 kVp) and tube currents (60–350 mA), as well as slice thickness (5 and 10 mm) using a GE LightSpeed Ultra scanner. Gantry tilt dependence was investigated over a range of 40.5° (22° superior to 18.5° inferior). OSL data was obtained before, during, and after the scan at the rate of 10Hz. Results: Performance was determined in part by normalizing both the initial OSL intensity and the background‐subtracted integral OSL to exposure reported previously by an ionization chamber. Good correlation between exposure and OSL data was found. Initial intensity and background‐subtracted OSL normalized to exposure show coefficients of variation of ∼%5 or less. Significant deviation was observed between the ∼10 OSL measurements taken for each slice, presumably as a result of absorption of x‐rays by the patient table. Conclusions: Initial tests have shown that this OSL dosimetry system possesses great potential for faster CT characterization. This system may prove a valuable alternative to CTDI.
SU‐FF‐T‐84: An Alternative Calibration Method for Solid Modulator IMRT
Authors: Cashon K , Warner C
Published: July 2006

Purpose: Solid compensators for intensity modulation have become very popular. The purpose of this presentation is to investigate the differences, for three separate measuring tools, in calibration using open fields and calibration with an attenuating material in the beam. Doses for both open fields and attenuated fields were confirmed using farmer chamber measurements. It is crucial to understand the differences in detector response between open and attenuated beams in order to make measurements of modulator based IMRT more accurate. Method and Materials: Three different types of measuring devices were investigated for this analysis: GafChromic EBT film, Kodak EDR2 film, and the SunNuclear Mapcheck system, each compared against ion chamber readings. Each system was irradiated with a 4 × 4 cm2 field size for various doses. The same dose measurements were made for each system with a 3cm brass modulator in the wedge slot. After the films were scanned, light transmission value readings were obtained. For the Sun Nuclear Mapcheck system, direct readings of the central axis diode were obtained, again using the open field and the brass modulator. Measurements were made with a Farmer chamber to confirm that the doses from open and attenuated fields were equal. Results: The percent error differences in measurements using open versus compensated fields for EBT, EDR2, and MapCheck are 0.63%, 2.88%, and 1.01% respectively, while the Farmer chamber was only 0.07%. Conclusion: The small value in the Farmer chamber indicated that our calculation was accurate. The EDR2 film showed a high difference in readings compared to that for the EBT film and the MapCheck system. Due to these differences we conclude that one should use a modulator in the beam while calibrating a system for the use of modulator based IMRT. Conflict of Interest: Chris Warner works for .decimal.
SU‐FF‐T‐86: An Automatic Field‐Matching Technique to Treat Multiple Targets with a Single IMRT Plan
Authors: Jiang Z , Schrum A , Zhang G , Earl M , Shepard D , Yu C , Linder J , Cheston S , Xia P
Published: July 2006

Purpose: To treat multiple targets with a single IMRT plan with automatic field matching and different sets of angles for each target. In the treatment of head‐and‐neck (HN) malignancies with IMRT for example, the traditional approach is to deliver 7–9 IMRT fields matched with a static half‐beam blocked supraclavicular field. However, significant cold and hot spots are frequently observed near the field junction. We have developed a technique to generate a single IMRT plan that eliminates the need for beam matching and reduces excess irradiation of normal tissue. Method and Materials: Direct aperture optimization (DAO) [1, 2] is an inverse planning technique where the MLC delivery constraints are incorporated into the plan optimization. By defining the initial apertures prior to optimization, the IMRT fields are limited in the search space for the MLC leaves, which served as a seeding solution. The fields are restricted so as to prevent them from exceeding the beam's eye view of their assigned targets. With this approach a single IMRT plan can be generated for multiple targets with different sets of gantry angles and automatic field matching. Results: Using DAO and defining the initial MLC aperture technique can produce a single IMRT plan for multiple targets without field matching. In the case of HN, 7 to 9 fields were assigned to the primary tumor, upper neck, and a portion of the lower neck nodes. An anterior and a posterior field were assigned to the mediastinum and a portion of the lower neck nodes. The resulting single isocenter IMRT plans were delivered without the need to junction fields. Conclusions: By using different beam arrangements, a single IMRT HN plan can be generated to treat multiple targets with needing to match fields.
SU‐FF‐T‐90: An Improved Irradiation Setup for An Accurate Measurement of the Dose Rate Constant of Low‐Energy Brachytherapy Sources Using Micro‐TLD Cubes
Authors: Chen Z , Deng J , Bongiorni P , Nath R
Published: July 2006

Purpose: To develop an improved irradiation setup for the measurement of dose‐rate‐constant for interstitial brachytherapy sources using micro‐TLD cubes by placing a TLD cube at the mid‐point of two sources of nearly equal air kerma strength separated by a distance of 2 cm in a water equivalent solid phantom. Method and Materials: The impact of geometric uncertainties associated with micro‐TLD cubes on measuring the dose‐rate‐constant of low‐energy brachytherapy sources was investigated quantitatively. Integral dose deposited to the volume occupied by a TLD cube in water was calculated by using the Berger formula for the conventional one‐source and the proposed two‐source irradiation setups. The effects of TLD size, shape‐asymmetry (due to mechanical damage and manufacturing defect), and positioning uncertainty on the dose measured by TLD was quantified by comparing the integral TLD dose to expected point‐dose at the measurement point. Results: For a perfect TLD cube centered at the reference point, the integral dose to TLD was nearly identical to the expected point‐dose, independent of TLD orientations. Displacements of TLD by 0.1 to 0.5 mm in the source‐TLD direction resulted in dose errors of 2.3 to 10% in the one‐source setup and 0.1 to 1% in the two‐source setup. The dose errors were < 1% when the displacements of < 0.5 mm was in the perpendicular direction for both setups. The shape asymmetry measured by volume deficit produced similar dose errors, from 2.5% to over 9%, in both setups for volume deficits of 3 to 10%. Conclusion: To keep geometry‐induced uncertainty in dose‐rate‐constant measurement < 2%, the TLD cube must have 1) nearly perfect symmetry with volume deficits < 2%, 2) positional uncertainty < 0.1 mm in source‐TLD direction and < 0.5 mm in perpendicular direction. The requirement for TLD positioning is greatly reduced in the proposed two‐source experimental setup.
SU‐FF‐T‐94: Analytical Calculations of Dosimetry Data for a 6‐MV Narrow Radiosurgery Beam with Cones
Authors: Yang J , Pino R , Ding X , Nizin P
Published: July 2006

Purpose: To analytically derive narrow beam dosimetry data for a 6 MV radiosurgery beam with cones from broad beam measurements. Method and Materials: currently radiosurgery dosimetry calculation is commonly done using a radiosugery treatment planning system. Such a planning system is typically commissioned using output factors, TMR, and off‐axis ratio measured in water for narrow beams. The experimental measurements in water for small radiosurgery beams are difficult and with large uncertainty due to lack of lateral electronic equilibrium and finite detector size. In this work, an analytical model was used to calculate the cone factors, TMR, and off‐axis ratio (OAR) for various cone sizes for a 6 MV radiosurgery unit. The model is parameterized with the measured broad beam cax beam data where lateral dose equilibrium exists and accurate measurement can be performed without much of difficulties. The calculated results were benchmarked with the experimental measurements. The cone size ranged from 4 mm to 20 mm in diameter in this study. Results: Compared the calculated dosimetry data with those from measurements, the agreement for cone factor was within 1 %, and within 4 %for TMR beyond dmax. The agreements for OAR are within typical experiment uncertainties. Conclusion: The analytical method described here can be used to calculate narrow cone beam dosimetry data in place of the arduous in‐water narrow beam measurements. Alternatively, it can be used to validate narrow beam dosimetry measurements if the direct measurements are prefered.
SU‐FF‐T‐103: Application of the Strydom Analytical Model to the Remote Monitoring of Electron Beam Dosimetry Parameters by Thermoluminescent Dosimetry
Authors: Weathers R , BenComo J , Stovall M , Ibbott G
Published: July 2006

Purpose: To provide a practical methodology and a reference percentage depth dose (PDD) data set for streamlining the Radiation Dosimetry Services (RDS) TLD verification of electron beam output and PDD. Methods and Materials: PDD data from 29 Siemens Primus and 19 Varian Clinac 21EX accelerators were retrieved from the databases of RDS and the Radiological Physics Center (RPC) and were simultaneously fitted to the nonlinear Strydom analytical model. The NLIN (SAS 9.1) procedure was used to fit the models. The PDD data sets used in the models were independently measured by many physicists from many institutions and verified by TLD and RPC onsite measurements. The resulting reference PDD data sets were incorporated into the RDS TLD analysis software for electron beam monitoring. Results: The parameters of the model were estimated for 5 to 21 MeV electron beams for Siemens and Varian machines. Individual PDD data fell within ± 1.5 mm of the reference PDD data for all depths for all beams analyzed. The curves exhihited similar shapes and were found to agree within ± 1.0 mm at all depths when all the curves for a particular energy were made to coincide at the depth of 50%. The validated reference model's predictions shows good agreement with other published results. Conclusions: The shape of the PDD curves was found to depend only on beam energy, and was independent of model for newer machines. These results allowed RDS to streamline its process of evaluating TLD results for electron beams and to acquire additional information about an institution's PDD data to facilitate the identification of problems. The reference PDD data sets can serve as a resource to the medical physics community. This work was supported by PHS grant CA 10953 awarded by the NCI, DHHS and Radiation Dosimetry Services.
SU‐FF‐T‐105: Assessing Influence of Motion in Non‐Gated and Gated DMLC‐IMRT Delivery Using Measured Fluence Fields for Dose Reconstruction
Authors: Verellen D , Tournel K , Linthout N , Storme G
Published: July 2006

Purpose: Recalculating dose distributions using measured IMRT fluence fields imported into the treatment planning system (TPS) to evaluate breathing synchronized irradiation. Method and Materials: DMLC‐IMRT fluence patterns acquired on radiographic film, generated in non‐gated and gated mode, have been imported into the TPS. The effect of dose blurring and the efficacy of a breathing synchronized irradiation technique have been evaluated using radiographic film mounted to a phantom simulating a breathing pattern of 16 cycles/min and 4cm amplitude. Two situations have been investigated: (a) A spherical lesion located close to the diaphragm assessing the influence of motion on the dose to the target volume and the gastro‐intestinal tract. (b) A mediastinal lesion requiring complicated fluence patterns. Results: Disturbed dose reconstructions have been observed in case of the non‐gated delivery with the phantom in motion (both orthogonal and parallel to the leaf direction), whereas the measurements from the static and gated deliveries showed good agreement with the theoretical dose distribution. These findings were confirmed by dose‐volume histograms, tumor control probabilities (comparable for the original, static and gated measurements; reduced with a factor 2 for the in‐motion‐non‐gated delivery), conformity index, and dose heterogeneity values (increased with a factor 3 to 6 when motion was induced; comparable values between the theoretical, static and gated situations). The normal tissue complication probability was affected to a lesser degree. The breathing synchronization technique introduced an increased treatment time by a factor 3 to 4. Conclusion: The use of measured fluence fields, delivered in non‐gated and gated mode, in the TPS is an interesting QA‐tool to assess the clinical impact of dose blurring, as well as the potential of breathing synchronization to resolve this issue. Conflict of Interest: Part of this work has been supported by BrainLAB AG.
SU‐FF‐T‐106: Assessing the Impact of Radiotherapy Dose Uncertainties On Tumour Control Probability: Application of Equivalent Stochastic Dose (ESD)
Authors: Zavgorodni S , Cranmer‐Sargison G
Published: July 2006

Purpose: To demonstrate a method of TCP evaluation that intrinsically accounts for the dose uncertainties. Method and Materials: The dose uncertainty is taken into account through the concept of an equivalent stochastic dose (ESD) defined as the dose to a voxel that results in the mean expected survival fraction from a process randomly depositing a dose D. Applying ESD to a non‐uniform dose distribution yields the concept of equivalent uniform stochastic dose (EUSD). TCP was calculated to include dose uncertainty and dose inhomogeneity as TCP(EUSD). We show that Webb‐Nahum and Niemierko‐Goitein TCP models both converge to TCP(EUSD) when dose uncertainty is taken into account. Voxel control probabilities (VCPs) were modeled for a single voxel irradiated with a uniform prescribed dose of 60 and 70 Gy at 2 Gy per fraction. Effect of the dose fractionation on TCP in the presence of the dose uncertainty was also investigated using our TCP(EUSD) model. Tolerance uncertainty on the dose resulting in tolerance TCP loss (assumed as 5%) was calculated for a range of radiobiological parameters. Results: TCP degradation due to the treatment dose uncertainty in the whole tumour, as well as in its fraction was evaluated and shown that degradation of the TCP was controlled by the voxels where the dose is not known exactly. It is shown that cell radiosensitivity, the α/β ratio and the cell density each influence the TCP degradation due to the dose uncertainty. For a modeled tumour (α=0.3, α/β=10, N0=108) irradiated with 60 Gy, 10 % dose uncertainty reduced the TCP from 95% to 45%. Conclusion: Presented TCP(EUSD) model demonstrated capability to robustly evaluate the loss of TCP due to the dose uncertainties. It is shown that the tolerance uncertainty reduces with decreased number of fractions indicating that hypo‐fractionated treatments may require more accurate dose delivery.
SU‐FF‐T‐118: Can the Use of Respiratory Gating Reduce Radiation Dose to Heart in Whole Left Breast Irradiation Treatment? — A Preliminary Study
Authors: Kim H , Huq M , Heron D , Yue N
Published: July 2006

Purpose: To investigate the potential benefit of using respiratory gating technique to reduce radiation doses to patient's heart in whole left breast irradiation treatment. Method and Materials: Conventional helical CT scan and retrospective 4D CT scans were acquired for 5 left sided breast patients. The 4DCT scans were sorted into 10 phases per respiratory cycle. Treatment plans, which consisted of two wedged tangential fields, were designed based on the helical CT scan. The PTV coverage, heart and lung doses in this plan were used as references for comparison. 10 separate optimal plans were generated from the 10 different phases obtained from the 4DCT images. All beam parameters, except for beam energy and wedges, were adjusted so that the PTV coverage in these plans was similar to or better than that in the corresponding reference plan. The heart and lung doses were then computed from these plans and compared to the corresponding doses in the reference plan. Results: The present results show that one patient would benefit from gated treatment at any given phase in terms of heart dose reduction. For four other patients respiratory gating would reduce dose to the heart. Gating at a given phase would reduce mean dose to the heart by 24%, 17%, 8% for 3 patients; for another patient a 17% reduction in maximum dose to the heart was found while the impact to the mean dose was insignificant. Conclusion: The use of respiratory gating in the irradiation of whole left breast treatment has the potential to reduce dose to the heart.
SU‐FF‐T‐119: Cardiac Dose Heterogeneity and Relationship to Tumor Location in Patients Receiving Radiation for Lung Cancer
Authors: Zeng J , Zhang J , Koweek L , Blazing M , Zhou S , Marks L
Published: July 2006

Purpose: To assess dose heterogeneity to subregions of the heart (e.g. four chambers), and its relationship to tumor location in patients irradiated for lung cancer. Methods and Materials: 236 patients receiving 3D‐planned radiotherapy for lung cancer from 1991–2005 were retrospectively analyzed. Pre‐treatment CT planning images were segmented to define the whole heart, four cardiac chambers, and aortic root. For each patient, the mean dose to the heart and each cardiac subregion was calculated, reflecting tissue density inhomogeneities. Population average of those doses, their differences, and 95% confidence intervals were computed. Patients were divided into subgroups based on tumor location (left vs right, superior vs inferior, and central vs peripheral). Differences between subgroups were assessed using Student t‐tests. Results: Mean doses to the whole heart and each subregion are shown in Figure 1. Compared to the mean heart dose (the parameter typically considered in treatment planning), atrial doses are higher (11Gy for left and 3Gy for right), ventricular doses are lower (9Gy for left and 5Gy for right), and aortic root dose is 23Gy higher. Table 1 shows the impact of tumor location on cardiac subregional dose heterogeneity. Dividing patients into superior versus inferior lung tumors changes the dose to most subregions. Similarly, dividing patients into central versus peripheral tumors leads to dose changes in several subregions, but only for peripheral tumors. Dividing patients into left versus right lung tumors only changes the dose to one subregion. Conclusions: Doses to cardiac subregions are significantly different than mean dose to the whole heart. Treatment planning using DVH of whole heart does not adequately portray doses to cardiac subregions, which have distinct functions in cardiac physiology. DVHs inherently discard all spatial information and may be suboptimal to describe exposures to heterogeneous organs such as the heart. Supported in part by NIH‐R01‐Grant CA69579.
SU‐FF‐T‐121: Characterization and Real‐Time Measurements of Optical Density with GafChromic EBT Film
Authors: Rink A , Vitkin A , Jaffray D
Published: July 2006

Purpose: Over the last few decades, various groups have investigated optical fiber‐based dosimeters for in vivo measurements. Recently, two radiochromic films have also been considered for real‐time in vivo point‐based optical dosimetry, and both GafChromic MD‐55 and GafChromic EBT films faired well in several criteria. However, GafChromic MD‐55 was shown to have the change in optical density depend both on dose rate and on temperature, as measured immediately at the end of a given exposure. To continue with the search for a suitable medium for real‐time fiber‐based in vivo dosimetry, GafChromic EBT film is also being investigated for dose rate and temperature effects. Method and Materials: 1 cm × 1cm pieces of film were irradiated with a 6 MV beam within a 30 cm × 30 cm × 10 cm Solid Water phantom fitted with optical fibers for real‐time measurements, using a 10 cm × 10 cm field at SAD, 100 cm SAD, and 1.5 cm depth. Each exposure delivered a dose in 5 to 1000 cGy range at a dose rate in 14 to 520 cGy/min range. Changes in OD obtained immediately at the end of each exposure were compared. The percent uncertainty in ΔOD for a given dose was calculated by using all the values obtained, regardless of the dose‐rate used. Results: The percent uncertainty ranged between 1.8% and 3.6%, with an average of 2.8%. The expected variation in ΔOD within a single sheet of GafChromic EBT film for a given dose is 1.5% (two standard deviations). The average increase in uncertainty is 4%. Conclusion: An increase in uncertainty of real‐time ΔOD measurements is observed when variations in dose‐rate are introduced for the doses in 5 to 1000 cGy range, and dose‐rates between 14 and 520 cGy/min.
SU‐FF‐T‐137: Commissioning the Eclipse AAA Algorithm with Golden Beam Data
Authors: Rathee S , Murray B , Newcomb C , Breitman K , Connors S , Dunscombe P , Field C , MacKenzie M , Robinson D , Warkentin H , Fallone B
Published: July 2006

Purpose: Evaluation of the accuracy of Eclipse AAA TPS when commissioned with Golden Beam or measured data. Method and Materials: Two cancer centers independently performed preliminary evaluation of the Analytical Anisotropic Algorithm (AAA) implemented in Eclipse TPS. The AAA photon algorithm was commissioned with vendor supplied “Golden Beam Data”(GBD). We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beam; wedged square and half‐blocked beam; square fields at three different source to surface distances; open and wedged beam at oblique incidence; beams shaped using cerrobend blocks and MLC. The measurements were performed on the Varian 2100 EX linear accelerators installed at the two institutions. After the initial tests, Eclipse was recommissioned with measured data from one of the machines. Results: The evaluation of the profiles was performed in the buildup, penumbra, inner and outer beam regions as per AAPM TG53. With the GBD the agreement of the measured and calculated profiles at the two institutions was very good in all regions except for the inner beam region on one machine. The tests that had a significant number of failures in the inner portion of the beam were mainly those cases where the TG53 tolerances are very tight. In these cases a significant number of points were just beyond the tolerance, and some of the off axis scans had 100% fail. On one particular test case where 16 profiles were measured for a particular geometry, 59% of points passed in the inner section with GBD, while 91% of points in that region passed once the beam was commissioned. Conclusion: When Eclipse is commissioned with GBD it was quite accurate, however, commissioning with measured data can improve the overall match. Conflict of Interest: Funding provided by Varian.
SU‐FF‐T‐298: Investigation of Effects of Magnetron Replacement On a Helical Tomotherapy Unit
Authors: Wu C , Liu T , Yang C , Guo F , Purdy J
Published: July 2006

Purpose: To quantitatively evaluate the physical effects of magnetron replacement on a Helical TomoTherapy system (TomoTherapy Inc., Madison, WI, USA) Methods and Materials: A Helical TomoTherapy (HT) (TomoTherapy Inc., Madison, WI, USA) is in clinical use at our institutions. Due to various design, engineering and manufacture issues, the magnetron (e2v Technologies, Essex, UK) used on HT system needs to be replaced some times during unit maintenance or repairing. The magnetron provides microwave power to LINAC accelerator and thus its output has direct effect on the LINAC beam quality and subsequent dosimetry characters. In this work, the effect of replacing a magnetron on HT system is investigated. A Tomo water tank, 8 channel Tomo‐Electrometer and X1Sl ion chambers (Standard Imaging Inc., Middelton, WI, USA) are used to measure beam output, beam PDD curves and profiles before and after the magnetron replacement. Results: The central axis output of the machine is found to be about 2% lower compared to measurement before magnetron replacement. Both transverse and longitudinal beam profiles at various depths are found to be in a very good agreement before and after magnetron replacement, as was the central axis percentage depth (PDD) curves for various available jaw sizes (5 cm, 2.5 cm and 1 cm). Conclusions: Magnetron is a major part of a LINAC system and its replacement should be checked carefully. In this study, a series of measurements are performed to compare the machine beam properties before and after magnetron change. It is shown that the central axis output changes about 2%, and no significant change is found on beam profiles and PDDs before and after magnetron replacement.
SU‐FF‐T‐331: Monte Carlo Simulation of Backscatter in a Flattening Filter Free Clinac
Authors: Titt U , Poenisch F , Kry S , Zhu X , Mohan R , Vassiliev O
Published: July 2006

Purpose: Report the influence of backscatter to the chamber signal in a flattening filter free Varian 2100 Clinac. Method and Materials: Monte Carlo simulations of the chamber signal as a function of various field sizes in a flattening filter free treatment head were performed with MCNPX. Benchmarking simulations were carried out with a flattening filter in the beamline and compared with published results. Results: The measured and calculated backscatter contribution to the ionization chamber signal of a system with a flattening filter showed good agreement with published data. Simulations without the flattening filter showed no significant differences to the data with the flattening filter. Conclusions: Backscatter into the ionization chamber must be taken into account, when interpreting dose to a patient, because the monitor chamber signal will change as a function of field size. In the case of a flattening filter free beamline, the differences to a standard beamline are smaller than the statistical uncertainties of both measurements and simulations, and it can be treated as a conventional setup with a flattening filter. Experiments and simulations did not show evidence of any additional effects on backscatter, when the flattening filter is removed from the system. Conflict of Interest: Research is sponsored by Varian Medical Systems.
SU‐FF‐T‐344: On the Equivalency of the Population and Individual TCP Models
Authors: Schinkel C , Stavreva N , Carlone M , Stavrev P , Fallone B
Published: July 2006

Purpose: To demonstrate that the functional form of the population tumor control probability (TCP) model is well approximated by the individual TCP model. Method and Materials: Due to inter‐parameter relations, the parameters of the population based Poisson TCP model in the limit of large heterogeneity reduce to the geometric parameters D50 and γ50:1 . On the other hand the individual TCP model can also be written in the terms of the geometrical characteristics of the dose‐response relationship — D50 and γ50: . For typical clinical values2 of the parameters D50 and γ50, we evaluate and compare the individual and population‐based TCP models. Results: The two models are approximately equivalent for different values of D50 and γ50. When plotted, the individual and population‐based TCP models almost overlap over a wide range of doses for all D50 values and γ50 higher than 1. When γ50 is less than 1 both functions start to differ from one another. Conclusion: The equivalency between the individual and population TCP models is demonstrated. The individual TCP model has been observed to fit clinical datasets reasonably well, and this phenomenon may be attributed to the similarity of the models. When fitted to clinical data, the individual TCP model will produce parameter estimates completely emptied from their biological meaning and become purely phenomenological.
SU‐FF‐T‐346: On Using Two Parallel Opposed Fields for Grid Therapy
Authors: Ha J , Zhang B , Zhang G , Naqvi S , Regine W , Yu C
Published: July 2006

Purpose: To study the feasibility of creating well‐aligned opposed grid fields and to show their dosimetric advantages for grid therapy. Method and Materials: A Varian MLC (multileaf collimator) is used to create two opposed grid fields. The key in getting the two opposed beams aligned collinearly is to restrict the grid size to the region near the beam axis. A larger grid can then be assembled from a series of smaller ones, each with its own isocenter. To give a more quantitative assessment as to what the maximum size of these grid subunits can be, we choose the criterion of no crossings between neighboring rays. The dosimetry was studied by sandwiching radiographic films between phantom blocks. Results: The mathematical analysis shows that the size of each of the subunit grids is governed by the series {1/3, 1/5, 1/7,…} of the SAD (source‐axis distance), corresponding to the number of added columns; that is, two columns would reduce the “valid” grid coverage distance from the isocentric plane to 1/3 of the SAD, three columns to 1/5, and so forth. The percentage depth dose of a 1cm × 1cm opening along the beam axis is found to be 70% and 90% for the respective 6 and 18 MV photons traversing a water‐equivalent material of 30 cm thick. Conclusions: Well‐aligned opposed grid fields shaped by a MLC have been successfully demonstrated in this study. The method gives a much more dose uniformity in the areas of the grid openings as compared to a single grid field approach. This feature is important in treating a large size tumor, as such is the nature of grid application today.
SU‐FF‐T‐347: Optimization of Dose Distribution for HDR Brachytherapy of the Prostate Using Attraction‐Repulsion Model
Authors: Sumida I , Takahashi Y , Nose T , Ito A , Yamashita T
Published: July 2006

Purpose: To optimize dose distribution for high‐dose‐rate (HDR) brachytherapy for prostate cancer, we have developed a new algorithm named Attraction‐Repulsion Model (ARM). In this study, we compared the ARM with geometric optimization (GO). Method and Materials: ARM was used to optimize the dose distribution by finding the best dwell time combination, and this model uses a feedback mechanism. ARM requires grids inside the clinical target volume (CTV) and critical organs. These grids generate attraction or repulsion based on specific dose constraints. After calculations were performed repeatedly until the attraction and repulsion forces reached equilibrium, the optimal dwell time distribution was established. We compared the ARM with GO for ten patients using dose volume histograms and some volume indices. Results: The CTV ranged from 23–48 cc and the CTV V150 ranged from 52–79 %, and 23–44 % for GO and ARM respectively. This indicates that the dose homogeneity indices as well as the conformal indices were higher for ARM than for GO. The urethra V150 was 0–99% and 0–1 % for GO and ARM respectively. Conclusion: ARM proved to be superior to GO in minimizing the dose to normal structures and improving dose homogeneity for the target while reducing the dose to normal tissues.
SU‐FF‐T‐351: Optimum Parameters An a Model for Tumor Control Probability Including Interpatient Heterogeneity: Evaluation of the Log‐Normal Distribution
Authors: Keall P , Webb S
Published: July 2006

Purpose: The heterogeneity of human tumor radiation response to is well known. Researchers have used the normal distribution to describe interpatient tumor radiosensitivity. However, many natural phenomena show a lognormal distribution. Lognormal distributions are common when mean values are low, variances are large and values cannot be negative. These conditions apply to radiosensitivity. The aim of this work was to evaluate the lognormal distribution to predict clinical TCP data and compare the results with the homogeneous (single α value) and normal distributions. Method and Materials: The clinically‐derived TCP data for four tumor types: melanoma, breast, squamous cell carcinoma and nodes were used as the benchmark to fit the TCP models. Three forms of interpatient tumor radiosensitivity were considered: the lognormal, normal and δ‐function (homogeneous). The free parameters in the models were the radiosensitivity mean, standard deviation and clonogenic cell density. The evaluation metric was the minimum square difference between the predicted and clinical data. Results: The normal and lognormal distributions match the clinical data significantly better than the δ‐function distribution. The means and standard deviations of the normal and lognormal distributions are similar. Though the clinical data match was slightly better for the lognormal data, the difference was not statistically significant (p=0.13). The clonogenic cell density value yielding the best clinical data match (101–105) is much less than the expected tumor cell density (107–109). Three explanations are intratumor radiosensitivity heterogeneity, tumor ‘stem’ cells and multiple cells required for tumor growth. Conclusion: The lognormal and normal distribution of interpatient tumor radiosensitivity heterogeneity more closely describe clinical TCP data than a single radiosensitivity value. The lognormal distribution trends towards significance in its ability to match clinical TCP data compared to the normal distribution. The lognormal also has some theoretical and practical advantages over the normal distribution.
SU‐FF‐T‐352: Output Verification and Clinical Implementation of Leipzig Applicators for Treating Small Skin Lesions by HDR‐Brachytherapy Using Multiple Dosimetry Systems
Authors: Kazi A , Holmes T , Dziuba S , Hudes R
Published: July 2006

Purpose: The purpose of this paper is to provide a guide to commissioning the Nucletron Lepzig Applicators in a clinical setup for treating small surface lesions using HDR brachytherapy. Method and Materials: Recently we have acquired a set of six Leipzig Applicators (3 Horizontal applicators with inner diameters 1 cm, 2 cm and 3 cm and 3 Vertical with same diameter sizes) for our microselectron‐HDR V2 afterloader. Initially we have measured dose rates for 3 Horizontal applicators using Standard Imaging Exradin A10 parallel plate ion chamber and films (gafchromic and EDR2). Since the sensitive area of this ion chamber is comparable to the effective field area for the smaller 1 cm diameter applicators we felt the necessity to verify the dose rates using gafchromic films as well as EDR2 film. We have also used MOSFET and a micro parallel plate chamber (Exradin A14P, provided by Standard Imaging) and small cylindrical chambers (Exradin A1SL,and A14SL) for independent verification of our measurements. Results: To implement these applicators for clinical use we measured surface dose rates and depth dose rates for each applicators using variety of dosimetry. We subsequently developed an Excel Spreadsheet program to compute dwell time for a given prescription and to print a plan report documentation for the treatment. Conclusion: In this presentation we compare our data with published data and describe the details of clinical implementation of these applicators.
SU‐FF‐T‐358: PlanCheck: A System for Routine Clinical Comparison of IMRT Treatment Plans with Monte Carlo Recalculations
Authors: Zakaryan K , Cui J , Alaly J , El Naqa I , Simon J , Simon W , Low D , Deasy J
Published: July 2006

Purpose: Current IMRT QA methods are cumbersome and are not comprehensive. The purpose of the PlanCheck dose recalculation system is to provide independent verification that MLC leaf‐sequences generated by commercial treatment planning system will result in an acceptable dose. Method and Materials: The PlanCheck Beam Commissioning process was developed for medical linear accelerators and includes modeling of the photon energy spectra, off‐axis softening, electron contamination, flattening filter and penumbra blurring. The Monte Carlo beam parameters are derived by fitting treatment planning dose in water and to the measured dose. The system will regenerate the dose for each treatment and for the whole planned dose utilizing the Monte Carlo engine based on beam sequence DICOM/RTOG information imported into PlanCheck. The comparison metrics, including dose‐volume histogram comparisons, report the validation quality and dose agreement. Results: Monte Carlo commissioning was tested for Varian Linear Accelerator (Clinac 2100) for 2×2 cm2, 5×5 cm2, 10×10 cm2 and 20×20 cm2 open fields in water for 6MV, 10MV and 18MV photon beam. The profiles and comparison results show good agreement for Eclipse (Varian) open field dose in water. The IMRT treatment plans from systems such as XiO (CMS), Eclipse (Varian) and Pinnacle (Phillips) were tested with Plancheck for dose agreement with Monte Carlo Dose and found to show adequate agreement. Conclusion: PlanCheck Monte Carlo calculations shows good overall agreement with treatment planning results except for regions with complex heterogeneities. Sun Nuclear Corporation is currently developing this product for intensive commercial use. The system dose engine is currently in process of integration with a 64‐bit/16‐node calculation cluster, which we expect will make the typical IMRT plan calculation time 30 minutes or less for the total planned 3D dose.

This research was partially supported by NIH grant R01 CA90445 and a grant from Sun Nuclear, Corp.
SU‐FF‐T‐373: Quality Assurance of Individual Segments of Step‐And‐Shoot IMRT
Authors: Louwe R , van Bolderen L , Lans M , Philippens M
Published: July 2006

Purpose: Absolute point dose measurements as part of the quality assurance of step‐and‐shoot intensity modulated radiation therapy (IMRT) often reveales unexpected deviations. This study investigates the feasibility of IMRT‐QA of individual segments to explain these deviations, and the efficacy of different evaluation methods. Method and Materials: The dose in a small volume around the point of interest (POI) was calculated for the individual segments of an IMRT plan for which a software script was added to our treatment planning system (Pinnacle3 7.6c, Philips). Furthermore, two input parameters for analysis of the measurements, i.e. the dose per monitor unit (D/MU) and the maximum dose gradient (δD/δr), were calculated for each segment. A software application was developed for automated data acquistion and analysis. The analysis was carried out either disregarding measurement data with a low confidence level (δD/δr or D/MU outside a critical range), or using γ‐evaluation. Results: IMRT‐QA was performed for 10 prostate and 5 head and neck patients. We found significantly higher deviations for segments at which the detector turned out to be located within the penumbra, which was consistent with 0.15 < D/MU < 0.55 cGy/MU, or δD/δr > 0.25 cGy/mm. When these data were filtered out, the total dose deviation ranged from −2% to 3% (not filtered: ‐ 5% to 5%). However, only 78% of the fraction dose was verified on average due to filtering. When γ‐evaluation was used, 93% of the fraction dose was within acceptance criteria (2.5% Dsegm; 2.5mm) on average. Datapoints outside these acceptance criteria were predominantly related to measurements behind jaws or leafs (D/MU < 0.15). Conclusion: IMRT‐QA of individual segments revealed that penumbral ionometric uncertainty is the main cause of deviations in overall IMRT‐QA. This method is feasible in daily clinical routine and provides more insight in deviations between measurements and calculations.
SU‐FF‐T‐381: Real‐Time Treatment Planning System for Prostate Photodynamic Therapy
Authors: Li J , Zhu T
Published: July 2006

Purpose: To develop a real‐time treatment planning system for prostate photodynamic therapy (PDT). Method and Materials: A real‐time treatment planning system has been developed for prostate PDT, which is composed of two sub‐systems, i.e., image acquisition and light fluence rate calculation engine. The whole system is built in a personal computer. The image acquisition is implemented using an image frame grabber (DT‐3120, Data Translation, Inc., MA) to acquire ultrasound images from a transrectal ultrasound unit in real time during a treatment. The software converts information about the organ geometries and the actual locations of the light sources on a template with 0.5‐cm grids. These data are then input into the calculation system for prediction of light fluence rate distribution. The three‐dimensional (3D) geometry of the prostate and the actual source arrangements are reconstructed and are used in the calculation, which takes into account the effect of optical heterogeneity. Treatment planning is accomplished with the prediction of 3D light fluence rate distribution. Results: The system has been tested in experiments. In a phantom experiment, transrectal ultrasound images of the prostate phantom during treatment were acquired and the actual locations of the linear sources were recorded. The geometry of the phantom was reconstructed and light fluence rate was calculated in the geometry with the accurate source locations. The capability of calculating light fluence rate distribution in a heterogeneous phenomenon was tested in a patient prostate. The reconstructed prostate geometry, actual source strengths, and measured heterogeneous optical properties, were used in the calculation. The predicted light fluence rate distribution showed the effect of heterogeneous distribution of optical properties. Conclusion: The real‐time treatment planning system provides an efficient tool for prostate PDT, which improves accuracy in treatment planning.
SU‐FF‐T‐388: Secondary Radiation Doses From CyberKnife SRS/RT
Authors: Yu C
Published: July 2006

Purpose: Because of extensive use of conventional x‐rays in CyberKnife SRS/RT for treatment tracking and large number of monitor units (MU) in beam delivery, it is of great concerns of potential secondary radiation risks for patients with benign lesions. This study intend to determine the total secondary radiation dose and its components from both Linac and conventional x‐rays, to estimate the risks, and to assist to better manage frequency of x‐rays acquisition for treatment tracking and better guide through the treatment planning. Comparison with other modalities will also be discussed.

Method and Materials: A human phantom was constructed from the Accuray head phantom adjacent to a thoracic and pelvic body section from a commercial humanoid phantom. Measurements were taken at the surface and depth for both high‐energy photons and conventional x‐rays, respectively. Thermoluminescent dosimeter (TLD) chips were placed at 0, 10, 20 and 30cm from the pseudo isocenter. At depth, TLD chips were covered with bolus material for backscatter and/or dose buildup.

Results: The secondary radiation dose from conventional x‐rays was about 0.1cGy per exposure inside the radiation field and fell off rapidly outside. The secondary dose from the high‐energy photons was approximately 0.6cGy per 100MU at the distance 10cm from the pseudo‐isocenter and fell off gradually. On average, the total secondary dose to a patient under CK SRS/RT would be 84, 42 and 40cGy at 10, 20 and 30cm away from the pseudo‐isocenter, respectively. Conclusion: The secondary radiation dose to a patient under CK SRS/RT could be significant and high as compared to other similar modalities. During patient treatment, the frequency of x‐rays acquisition should be carefully evaluated between the clinical necessity and potential radiation risks. This is also true in the process of treatment plan development during which the number of fractions and MUs would be determined.
SU‐FF‐T‐429: The Use of a New Dynamic Motion Phantom for Patient Specific QA in Tracking Therapy
Authors: Chuang C , Verhey L , MA L
Published: July 2006

Purpose: Respiratory gated/tracked radiation delivery with high tumor dose conformity is now a reality. This study aims to develop a patient‐specific quality assurance procedure for 4D tracking delivery using patient‐specific motion probability distribution functions and a dynamic motion phantom. Method and Materials: A commercial 4D motion phantom (CIRS, Norfolk, VA) was used in the study. A CT scan of the phantom was taken at a specific phase of motion. Patient motion trajectories during treatment were extracted using chest‐wall LED markers and fiducials implanted inside target. To produce the patient‐specific motion, a probability distribution function was generated from the extracted target motion trajectory. Sampled points from the probability function were input into the phantom motor controller, and patient treatment plan was delivered to the moving phantom with and without motion tracking. Dose distributions can then be compared with the original static plan calculations. Results: The tumor motion probability function for a 36‐minute delivery showed significant differences from a 5‐minute snapshot of tumor motion, with a median of 0.64 mm vs. 1.98 mm in the S/I direction, 0.56 mm vs. 1.22 mm, and 0.28 mm vs. 0.83 mm for A/P and R/L, respectively. The variances for the S/I direction were 5.58 mm vs. 7.24 mm, 0.98 mm vs. 0.84 mm, and 4.82 mm vs. 5.66 mm for A/P and R/L, respectively. A study of target motion compared with center of mass motion also showed significant differences, particularly in the S/I and R/L directions. Results of dose comparison with/without 4D tracking will be presented. Conclusion: A tumor probability distribution function is a more accurate representation of tumor motion than the center of mass method. To conduct patient‐specific QA for 4D tracked dose delivery, it is more accurate to use the probability distribution and a programmable motion phantom is an excellent tool for this.