bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐143: Evaluation of the Lung Dose Calculation Accuracy for An IMRT Planning System
Authors: Aljarrah K , Flampouri S , Pope C , Jiang S
Published: June 2005

Purpose: Image guided treatment of lung provides the ability to deliver dose precisely to the target, however, lung dose calculation is often difficult and might be the weakest link now in the chain of lung IGRT. The purpose in this work is to find out the accuracy of a common TPS system in lung dose calculation. Method and Materials: So far lung dose measurements have been done only in slab geometry and for a single beam. In this work, we used a realistic lung phantom and delivered all fields of realistic IMRT plans. The phantom is supplied with cylindrical inserts, made of lung, bone, and tissue, which were used to load the dosimetry equipments. The dose to the phantom was calculated, for 10 IMRT treatment plans, with the Corvus system. Validation of the calculated dose was performed with LiF thermoluminescent dosimeters (TLDs) measurement, Ionization chamber measurement, and Monte Carlo simulation. For each treatment plan, the dose was verified at points located in lung, bone, and tissue. Results: The comparison of the collected data shows that the dose to the lung calculated with Corvus was overestimated by 10% relative to the Monte Carlo results, and by 7% relative to the chamber measurements. The TLDs dose results show better agreement to the Monte Carlo results than to the Corvus results. In bone the Monte Carlo agreed well with both TLDs and chamber measurements (within 3.5%) while Corvus dose was 6.5% different. The dose to the tissue shows good agreement (within 2%) between all the dosimetry tools. Conclusion: The dose calculation accuracy in lung has been estimated for IMRT planning system. It indicates some better dose algorithms might be needed in order to have an accuracy of a few percent.
SU‐FF‐T‐147: Improved Calibration Method of EDR Films for IMRT‐QA
Authors: Mota H , Sibata C , Sasidharan S , White K , Wolfe M , Jenkins T , Patel R , Allison R
Published: June 2005

Purpose: Due to film processing variation and the optical density not being linear with the dose, a full calibration has to be obtained for every IMRT‐QA with film. We are proposing the application of a linearization method and the use of a single dose calibration EDR film for relative and absolute dose measurement in IMRT‐QA. Method and Materials: The linearization method was studied for the EDR films using 6, 10 and 15 MV photon beams. The films were developed in a Konica film processor, then scanned with a Vidar VXR‐16 scanner and analyzed using RIT‐114 version 4.1. A standard 30‐point calibration curve was obtained with the 6 MV beam. Using the method developed in a previous paper, a curve fitting was obtained for a sigmoid expression modulated by a 3rd degree polynomial: ; where x is the net optical density, m is the net saturation density, b, a1, a2 and a3 are parameters of the model. Results: A value of 1870.1945 was obtained for b that is a parameter related to the dose unit. The net saturation density was 3.5587 for our system. The parameters a1, a2 and a3 are, respectively, −0.4551, 0.1167, −0.0134. For every IMRT‐QA a spreadsheet is used to obtain a 70 point calibration curve for RIT, using only one exposed film developed together with the composed and enface films, obtaining <5% uncertainty. Conclusion: Some softwares are available to make film dosimetry in IMRT QA less cumbersome, however daily calibration still remains a time consuming procedure. Absolute dosimetry requires a full calibration every time film is used. The linearization method presented here is being used in our department for isodose comparison and for absolute measurement at calculation point. The overall time is reduced while obtaining uncertainty comparable to the existing methods.
SU‐FF‐T‐148: IMRT Head and Neck Phantom Irradiations: Correlation of Results with Institution Size
Authors: Molineu A , Hernandez N , Alvarez P , Followill D , Ibbott G
Published: June 2005

Purpose: To analyze the results from 136 IMRT H&N phantom irradiations. Method and Materials: A mailable anthropomorphic IMRT head and neck phantom was irradiated 136 times by 104 institutions. Some institutions irradiated multiple times. Institutions imaged the phantom, planned an IMRT treatment, performed their routine IMRT QA checks, and irradiated the phantom according to their plan. The phantom contained imageable structures representing a planning target volume (PTV) close to an organ at risk (OAR), simulating an oropharyngeal tumor and the spinal cord. The phantom also contained a secondary PTV that simulated peripheral nodes. TLDs were placed in each structure and a set of orthogonal radiochromic films intersected in the primary PTV. The following criteria were used to evaluate the measurements: TLD/institution dose − ± 7%; distance‐to‐agreement in the high dose gradient region near the OAR − ⩽4 mm. The failure rate of institutions that housed 3 or fewer megavoltage therapy machines was compared to that of larger institutions. Results: 41 irradiations failed to meet one or more of the criteria. 24 of the failures were dose discrepancies measured with TLD, 5 were dose distribution discrepancies measured with radiochromic film and 12 were disagreements in both TLD and film measurements. There was a 38% discrepancy rate in first‐time irradiations at the institutions with 3 or fewer machines and a 26% rate at the larger institutions. All of the institutions that failed multiple times were smaller institutions. Conclusion: Institutions of all sizes are capable of making mistakes in IMRT treatments. Sufficient physics coverage is an important aspect of IMRT quality assurance. Conflict of Interest: The investigation was supported by PHS grants CA10953 and CA81647 awarded by the NCI, DHHS.
SU‐FF‐T‐149: IMRT Pre‐Treatment Verification with Ionization Chamber, Film and EPID: Quality Vs. Time Consumption
Authors: Starmans M , Dekker A , Nijsten S , Minken A , Lambin P
Published: June 2005

Purpose: The objective of this study was to obtain a standard IMRT pre‐treatment verification protocol based on both quality and time‐consumption parameters. A further objective was to define pass/fail criteria for the protocol. Method and Materials: Three existing pre‐treatment verification methods were compared. 1) Total plan dose verification in a PVC slab phantom with an ionization chamber. 2) Total plan dose verification in PVC slab phantom with Kodak EDR2 film. 3) Single beam dose verification with the Siemens Optivue 500/1000 EPID. A prostate and head and neck plan were made with the XiO 4.2 IMRT TPS to evaluate quality (reproducibility and potential to detect significant errors) and time‐consumption for each method. To evaluate reproducibility the plans were measured 10 times. The potential to detect errors was measured by introducing errors in the delivery by changing the number of monitor units, deletion of segments and MLC misalignment. For the time‐consumption, measurement and analysis time were registered. A MATLAB tool was written to perform the analysis. Use of the gamma‐index with different values for the dose difference percentage and distance to agreement was evaluated for methods 2 and 3. For the single beam dose verification method an additional criterion was defined and evaluated. This criterion was a combination of the mean percentage dose difference in a beam and the beam weight dependency. Results: The MATLAB tool provided a fast and accurate way to analyse the measurements. We were able to show clear differences between the three methods. As expected film verification was the most time consuming method and EPID verification could be performed the quickest, but still with high quality, i.e. reproducible and correct error detection. Conclusion: A comparison between the three methods resulted in the definition of a standard IMRT pre‐treatment verification protocol with a favorable quality/time‐consumption ratio and suitable criteria.
SU‐FF‐T‐150: In Vivo Diode Dosimetry for Helical Tomotherapy Dose Verification
Authors: Hui S , Alaei P , Higgins P , Gerbi B
Published: June 2005

Purpose: We propose and assess the feasibility of in vivo diode dosimetry for helical tomotherapy treatments. We aim to evaluate the influence of treatment parameters (grid size, field width, pitch and modulation factor) on diode measurements. Method and Materials: Isorad‐p type diodes were calibrated and diode characteristics (angular dependence, etc) were studied. Treatment plans with different delivery parameters were performed on a s30 cm diameter solid water cylindrical phantom to deliver: 1) homogeneous dose to the whole phantom target; and 2) dose to a small target located at the center of the phantom. Diodes were placed in various positions at the surface of the phantom. The megavoltage CT (MVCT) was utilized for accurate phantom and diode localization. Diode measurements on patients have been performed and will also be analyzed. Results: Diode directional dependence in the transverse direction was less than ± 1% over full 360° rotation. On the whole phantom target, in general measured data were within ± 5% of the planned dose. Use of a fine dose grid (2mm) in planning generally showed better agreement with measurements. Field width has no effect on diode response. Higher pitch resulted in poor agreement. Diode measurements were within 10% of expected dose for a centrally located small target. Dose measurements for patients have been within 10% of expected dose. Conclusion: Isorad‐p type diodes agree to ± 5% for some situations and better than ± 10% for in vivo diode dosimetry for helical tomotherapy treatments. A basic understanding of the effects of physical factors (field width, pitch and modulation factor, grid size) on variations in diode response is essential to select the accepted limit. In vivo dosimetry can be very useful as a secondary check for complicated helical tomotherapy radiation delivery.
SU‐FF‐T‐154: Long‐Term Reproducibility of IMRT Planning and Dosimetry Using Clinical Fields
Authors: Lim S , LoSasso T , Hunt M , Mechalakos J , Chui C
Published: June 2005

Purpose: To augment our patient‐specific IMRT QA program by checking the individual and overall reproducibility of the DMLC planning and delivery systems periodically using standardized patient data sets and a commercial diode array. Method and Materials: Clinical fields were chosen from IMRT treatment plans using 6MV and 15MV, respectively. It is our intent to completely re‐plan the same patients, generate leaf sequence files, and use the diode array to measure the absolute dose distributions for at least one different linac/MLC combination each month, thereby performing the evaluation for all MLC‐equipped linacs every six months. The dose distributions are recalculated in a flat homogeneous phantom using the clinical treatment planning system. Data are analyzed using dose‐difference and distance to agreement (DTA) methods. The supplied analysis program also provided a score for each field, which is defined as the percentage of the number of detectors satisfying the user defined dose‐difference and dose threshold constraints and DTA constraint. Results: The scores for the fields used in this study range from 75–100% of an average ∼200 points for each field for the 3% dose, 10% dose threshold, and 3 mm constraints that we specified. The diode array is able to identify the dose discrepancies relative to treatment planning calculations and earlier measurements and provides a quantitative score to grade each comparison, which along with the relative distribution of the failed points will serve as means for comparisons of measurements and/or calculations over time. Conclusion: The protocol that we present here enables us to provide a quick, accurate and reproducible evaluation of the overall DMLC planning and delivery process. With periodical evaluation using the same patients' data sets, we should be able to detect differences in performance among similar MLCs and linacs, and very small variations introduced over time for the same components.
SU‐FF‐T‐157: Optimal Matching of 3D Film Dosimetry with Calculated Dose for IMRT Quality Assurance
Authors: Park S , Shin D , Lee S , Kim J , Kim D , Kim T , Cho K
Published: June 2005

Purpose: To develop an optimal matching of the three dimensional film dosimetry with calculated dose for Intensity Modulated Radiation Therapy (IMRT) quality assurance Method and Materials: We have fabricated the 3D IMRT phantom which consists of the base frame (30×30×5 cm3) and 12 acrylic plates (30×30×1 cm3). The EDR2 films were embedded in each plate for 3D film dosimetry. Both absolute point dosimetry and 3D film dosimetry were performed throughout the IMRT using Clinac 21EX's millennium MLC. With custom‐written software modules, the measured and calculated dose distributions for axial, coronal and sagittal planes were superimposed by coincidence of their origins, followed by comparison of the point doses at all matched positions. Then, with the optimization algorithm the setup errors were recovered. Results: We developed custom‐written software modules for managing dose‐distribution files, optimizing the position, calculating dose differences, and printing out the report. The dose verifications for axial, coronal and sagittal planes were obtained and graphically shown. Differences between the calculated and measured doses over the 3% criterion could be reduced by 15% after applying the optimization algorithm. Conclusion: We have presented a 3D dose‐verification scheme with an optimization algorithm for IMRT that determines the setup errors in the measuring device by minimizing the average dose difference between the calculated and measured doses. Optimization dramatically reduced the difference between measured and calculated dose distributions in all cases investigated. Conflict of Interest: This investigation was supported by a research grant from the National Cancer Center, Korea (no. 0410310).
SU‐FF‐T‐163: Quantitative Verification of IMRT Intensity Maps Using An Amorphous Silicon Electronic Portal Imager
Authors: Yao B , Peng W , Santanam L , Van den Heuvel F , Burmeister J
Published: June 2005

Purpose: Common quality assurance procedures for intensity modulated radiotherapy include ionization chamber and/or film measurements in phantom for comparison with point doses and dose distributions predicted by the treatment planning system. We have developed a method to quantitatively compare individual intended fluence maps with measured intensity distributions using an aS500 amorphous silicon electronic portal imager (EPI). Method and Materials: A software program recognizes the intensity map and extracts the intensity in each bixel relative to the maximum bixel value. A simple single‐bixel scatter model was applied to account for scatter in the imager. The software quantitatively compares normalized bixel values in the EPI intensity map with the fluence map exported from the Corvus treatment planning system after correction for scatter. Results: Applying the scatter correction reduces the deviation between EPI maps and planned fluence maps, decreasing the correlation coefficient from 0.59 to 0.24. For 14 gantry angles and a total of 1663 bixels, we compared EPI intensity maps with scatter‐corrected planned fluence maps. The mean error was 1.0% ± 4.1% (1σ) with scatter correction, and 2.4% ± 4.9% (1σ) without scatter correction. Our clinical intensity maps utilize ten intensity levels. When rounded to the nearest intensity level, 69% of all bixels agree with the predicted intensity level, while 99% of bixels are within 10% of the predicted intensity. The majority of bixels in disagreement have low intensity. The mean error for bixels with a planned relative intensity of 10% was 6.3% ± 5.2%. Conclusion: Future refinement of the normalization and scatter correction methods will improve agreement in the low intensity bixels. This would improve the sensitivity of this tool and potentially allow it to replace the more time consuming film and ionization chamber techniques commonly used for IMRT quality assurance.
SU‐FF‐T‐164: Radiochromic EBT Film Characterization and Applicability for IMRT QA Verification
Authors: Ding L , Urie M , Lo Y
Published: June 2005

Purpose: Characterization of ISP's new EBT radiochromic film for clinical dosimetry, with emphasis on the usefulness for routine QA of IMRT treatments. Method and Materials: EBT is a new formulation of radiochromic film by ISP. Sheets were exposed to 6 MV and 18 MV photons from a Varian 2300C/D accelerator over clinically useful dose ranges. Density was measured by scanning the film in a Vidar VXR‐16 Dosimetry Pro scanner and the data analyzed using RIT113 v4 software. The reproducibility, stability, and temperature sensitivity were investigated. Dose distributions for IMRT treatments delivered with a Millinieum 120 leaf MLC were measured with the EBT film and compared to those of Kodak EDR2 film and to the predictions of the Eclipse treatment planning system. Results: EBT films from the same batch have a consistent response to doses ranging from 50 to 500 cGy for 6 MV and 18 MV photons. The density readings are stable from 1 to 75 hours post exposure when stored in the dark at room temperature. Beyond 75 hours, the density slowly increases. The film is insensitive to cold, but shows significant degradation when exposed to 60 C for as little as two hours. For IMRT dose distributions where the dose is below 200 cGy, the EBT film has similar responses as Kodak's EDR2 film and very close agreement to the Eclipse predictions. In regions of IMRT doses greater than 200 cGy, the EBT film loses sensitivity as compared to EDR2 film and to the Eclipse calculations. Conclusion: ISP's EBT radiochromic film is reproducible and stable under normal clinical conditions. For IMRT dose verification, the EBT film is in close agreement with calculations and EDR2 film for doses less than 200 cGy. Further research is needed to understand the reduced response of EBT to IMRT doses above 200 cGy.
SU‐FF‐T‐166: Reproducibility Of A Method For The Quanitative Assessment Of The Agreement Between Planned And Measured Dose Distributions
Authors: Witten M , Wuu C
Published: June 2005

Purpose: To introduce a new quality assurance parameter, the index of agreement (IOA), with which to quantify the agreement between planned and measured radiation dose distributions, and to assess the reproducibility of the parameter. Method and Materials: One IMRT radiation portal, consisting of three segments, was used to irradiate twenty separate pieces of calibrated EDR film. The field was delivered with a Siemens PRIMUS linear accelerator with 10 MV photons. A planar dose file was generated with the ADAC Pinnacle RTP system at a resolution of 2 mm by 2 mm to represent the dose distribution of the field. Both the planar dose file and the measured dose distributions derived from the film were imported into MATLAB for processing and analysis. At each pixel of the planned dose distribution, the pixel of the measured dose distribution, within a closed neighborhood of radius c (chosen to be 3 mm) that exhibits the smallest absolute fractional dose deviation was found. If the absolute deviation was less than p% (chosen to be 3%), the pixel was assigned a score of zero, while if the absolute fractional deviation exceeded the tolerance value, the pixel was assigned a score of zero. The index of agreement (IOA) was then calculated as the ratio of the sum of the score values of all clinically relevant pixels divided by the total number of clinically relevant pixels. Results: The IOA values were approximately normally distributed, with a mean value of 0.9330 and a standard deviation of 1.1%. Areas of poor agreement were reproduced during each irradiation. Conclusion: The IOA is a promising new quantitative quality assurance tool, and its value is highly reproducible.
SU‐FF‐T‐171: TomoTherapy HI‐ART Heterogeneity Calculations
Authors: McGary J
Published: June 2005

Purpose: To examine the accuracy of the TomoTherapy HI‐ART® treatment planning system's dose heterogeneity algorithm. Method and Materials: The TomoTherapy HI‐ART® treatment planning system uses the superposition/convolution dose algorithm for calculating heterogeneous dose distributions. To evaluate the combined delivery and dose calculation of this system, a series of treatment plans and corresponding measurements were made with: (1) calibrated CT density plugs and embedded TLD (2) film phantom and calibrated CT density plugs, (3) anthropomorphic phantoms containing TLD and film, and (4) different phantom materials with chamber inserts. Treatment plans were delineated with one cylindrical target calculated with homogeneous dose distributions at 60 Gy (1 Gy/fx, S.D. < 0.7). Results: The film studies using the film phantom show that the predictions are within 5–10 % along the interface and within the heterogeneity materials. TLD values using embedded plugs in various phantoms indicate comparable agreement. Phantoms with slab materials, composed of electron densities within 10% of water, are within 5% between calculations and ion chamber measurements. The lung phantom indicates that the readings agree with predictions to within 4%. Conclusion: The heterogeneity calculations agree with measurements to within 10%. Film measurements demonstrate more variability and discrepancy than the other measurements. The interface measurements demonstrate the largest disagreement where the dose gradient is approximately 10% for a uniform dose calculation between two different electron density materials.
SU‐FF‐T‐179: Accuracy of CT Numbers and Its Effect On Dose Calculations
Authors: Wu R , Dong L , Tung S , Gillin M
Published: June 2005

Purpose: The CT‐to‐density conversion table can vary for various CT scanners or when using different scanning parameters (such as KVp, field‐of‐view etc.). These CT number variations could be up to −/+100 for the same material. However, the dosimetric impact of these CT number variations was not well studied in the past. This study is to report the variations of CT numbers and their clinical impact on dose calculations. Method and Materials: The CT‐to‐density conversion table was measured on six different scanners. Two special tables were created with +/− 100 CT‐number shifts from the default conversion table in our clinic. Data showed that these two tables represented two bounding situations to accommodate the variations in different CT scanners or scanning parameters. Treatment plans using lung, head and neck, prostate, and breast cases were recalculated using these new CT conversion tables for both photon and electron beams. Results: There were noticeable differences across different density ranges for different CT scanners. Using the two extreme CT‐to‐density conversion tables, it was shown that the differences in dose calculations were −1.6% to 0.3% for head & neck (6MV), −2.5% to 0.9% for lung (6MV), −4.0% to 3.0% for prostate (6MV), and −2.3% to 1.7% for prostate (18MV), respectively. It showed that the impact was more notable for treatment targets at deeper depths or at lower beam energy. For electron beams, the distal 90% fall‐off edge was changed approximately 2 mm for commonly used electron energies. Conclusion: Dose calculations seem to be not sensitive to different CT‐density conversion tables from different CT scanners or using different CT scanning parameters. Considering most scanners were calibrated to water, our results could be the worst case scenario. A single CT‐to‐density conversion table could be used for all CT scanners.
SU‐FF‐T‐181: An Investigation of Surface Dose Changes for Therapeutic Kilovoltage X‐Ray Beams with Underlying Lead Shielding
Authors: Hill R , Healy B , Holloway L , Baldock C
Published: June 2005

Purpose: The effect on surface dose from underlying lead shielding in water was investigated for therapeutic kilovoltage x‐ray beams by experimental and Monte Carlo methods. Method and Materials: A Farmer type ionisation chamber was used to measure the surface dose in a water phantom for x‐ray beams with energies from 75 to 135 kVp. A 5 mm thick lead sheet was positioned at various depths below the surface. The surface dose ratio was calculated by comparison with the surface dose with no lead sheet present. A Monte Carlo model of the x‐ray beam and the phantom was generated using the EGSnrcMP code (V4.2). The initial energy spectrum was determined using an empirical method and verified by calculation of depth dose data. The dose was scored in a 1 mm thick slab at the phantom surface. The change in surface dose was calculated as a function of depth to the lead and compared to measured data. Results: The reduction in surface dose was a function of x‐ray beam energy, beam area and the depth of water to the lead. As the depth of water to the lead sheet decreased, there was a reduction in the surface dose. With the 8 cm diameter applicator and 1 cm depth of water to the lead, the surface dose ratio was 0.918 for the 75 kVp x‐ray beam and 0.890 for the 100 kVp x‐ray beam. For the smaller applicators, there was less reduction in the surface dose ratios. Surface dose ratios calculated by the EGSnrcMP code were in good agreement with measured data, with a maximum deviation of 1.2%. Conclusion: The surface dose for kilovoltage x‐ray beams is reduced when lead is underlying in the phantom. The Monte Carlo results indicate the model is sufficiently accurate to predict changes in the surface dose.
SU‐FF‐T‐184: Automated Dose Determination On the Midsagittal Plane Based On Skin Measurements During TBI
Authors: Zaini M , Loshek D , Olds W
Published: June 2005

Purpose: To determine the patient dose in the midsagittal plane based on a series of MOSFET measurements on the patient's skin during total body irradiation (TBI). Method and Materials: The patient is in the sitting position with her midsagittal plane perpendicular to the photon beam and covered with water‐equivalent rice bags on the beam side during TBI. The rice bags simulate a box of water surrounding the patient. A plastic plate rests against the patient and the rice bags on the beam side (irradiated from both left and right). Eight to ten MOSFET dosimeters are placed under bolus pieces on the patient's skin on both sides of her body during each beam irradiation. Conversion of the MOSFET readings to midsagittal dose is necessary for the MU adjustments, if necessary. The reasons this is non‐trivial and needs to be done quickly are, (a) the imitated box is not complete on the exit side of the beam, (b) the summation of both the entrance and exit beams need to be considered for dosimetry, (c) some irradiation sites such as the neck and legs have different thicknesses than that of the torso, and (d) MU adjustments need to be determined promptly, since a TBI patient is under tremendous stress. Results: The midsagittal dose can differ from the skin dose by as much as −25% to 40% depending on the irradiation, measurement, and anatomical geometry. The automated code uses the pre‐measured PDD for the TBI setup. The variables are the thickness of the torso, the thickness of the anatomical site (e.g. pelvis or neck), and the distance between the dosimeter and the plastic plate. Conclusion: The automated code designed for TBI dosimetry is robust, accurate, fast, and displays the parallel‐opposed beam geometry that is different for every patient and dosimeter placement.
SU‐FF‐T‐187: Comprehensive Empirical Formulation for Treatment Planning of Small Clinical Electron Fields
Authors: Khan I , Malhotra H , Bobeica M , Avadhani J , Podgorsak M
Published: June 2005

Purpose: To develop a comprehensive empirical formulation to predict the dosimetric properties of small electron fields at nominal and extended SSDs. Method and Materials: A set of circular electron cutouts with physical diameters from 2 to 9 cm was constructed in the usual clinical manner. A full dosimetric analysis of the fields defined by each cutout was done for nominal electron energies between 6 and 20 MeV using a 3D water phantom and a pin‐point ion chamber. Properties studied included depth dose, in‐air inverse‐square fall‐off, and beam profiles. After appropriate benchmarking, these properties were further studied with Monte Carlo simulations using the BeamNRCMP code. From the physical data and simulations the following parameters were determined for each cutout/energy combination: R50, R90, widths of 90% and 95% isodose surfaces, effective SSD, and dose output factor. The parameters were then correlated with collimator size, giving a suite of equations accurately defining the dosimetric properties of these fields. Results: Rules of thumb regarding properties of electron fields are typically used in deciding the shape and nominal energy of electron fields. The cadre of formulas developed in this work more accurately predicts the dosimetric behavior of small electron fields where significant deviation from expected dosimetry can occur. We found a large difference in the 95% and 90% isodose widths as a function of field size and nominal beam energy. Moreover, the effective SSD was found to be a strong function of field size. Conclusion: The formulation developed in this work accurately describes the clinical dosimetric properties of small electron fields. It can be used to select the size and shape of an electron cutout needed to provide the desired coverage at a given depth. Furthermore, the output of these small fields can also be calculated, obviating the need for a measurement.
SU‐FF‐T‐189: Custom‐Made Ion Chamber for Daily QA in RT
Authors: Park Y , Huh S , Ha S , Cho W
Published: June 2005

Purpose: A computerized dosimetric system with air‐tight ion chambers and on‐board electrometer has been devised to measure daily output checks of 4/6/10/15 MV X‐ray of linear accelerators. It can be inserted into the blocking tray or can be setup on the treatment table of the linear accelerators. Method and Materials: The dosimetric systems consists of three parts : (1) air‐tight plane‐parallel ionization chamber, (2) on‐board electrometer, (3) computer interface. The ionization chambers (10mm diameter * 2mm width) are implemented on printed circuit boards (PCB) with acrylic plates. Three electrodes are fabricated using approximately 100um thick gold/lead/cupper coating on the PCB board. The buildup plates ranging from 1cm to 3cm depending upon x‐ray or electrons. The electrometer is designed using a typical current‐to‐voltage converter (ACF2101, Burbrown, USA) with 20‐bit ADC circuits (ADC1210, Burbrown, USA), (3) The ADC output is transmitted via RS‐232 interface to a personal computer. The power from rechargeable battery is supplied all the time to the electrometer, thus eliminating warm‐up time. Results: The short and long term stabilities of the ion chamber are found to be less than 0.2% (one standard deviation) with reference of standard dosimetric system. Also, it is extended to be a 9‐channel dosimetric system as a QA device of implementing transit dosimetry (±/−3% agreements between expected and measured values) to determine the overall performances of radiation therapy for the head/neck and breast treatments. Conclusion: The daily QA devices of measuring the output of linear accelerators (6EX and 21EX of Varian linear accelerators) are proven to be very effective. It will be modified to measure the output and the PDD (in a step of 1cm) of the electron beams as a weekly QA.
SU‐FF‐T‐191: Design and Construction of a Realistic Pelvic Phantom for a Level III Dosimetry Study
Authors: Ebert M , Harrison K , Cornes D , Denham J , Howlett S
Published: June 2005

Purpose: Verification of dose delivery is a basic necessity of quality assurance in radiotherapy treatment, and is extremely valuable in determining uniformity of protocol adherence in the context of a multi‐centre trial. To verify the delivery of radiotherapy doses and dose distributions it is necessary to replicate the entire treatment process as accurately as possible in a way that allows the direct measurement of doses. A pelvic phantom was constructed to investigate the 3D delivery of radiotherapy (Level III dosimetry) for rectal and prostate cancer treatment in collaboration with the Trans‐Tasman Radiation Oncology Group. Method and Materials: A CT dataset of a male pelvis was used to define the geometric boundaries of principle tissues — bone, prostate, seminal vesicles, bladder, rectum and muscle. Points of interest were located throughout the dataset to identify appropriate placement of TLD materials. The centre of the prostate was identified as the location for point‐measurement with a small‐volume ionization chamber. Polyurethane materials for phantom construction were evaluated in terms of their durability, ability to be milled and radiographic qualities. Results: The pelvic phantom has been manufactured and has undergone commissioning. CT numbers for different density materials match well with typical tissues. TLD measurements in the phantom show a reproducibility of 2 %. The proposal for the inter‐centre study has led to a high (> 85 %) accrual of radiotherapy centres in Australasia. The phantom has now been used at a large number of participating centres. Conclusion: This study will provide estimates of variation in critical structure outlining, expansion algorithms and dose volume histograms across different planning systems. In addition local irradiation techniques, geometric and dosimetric accuracy dose distributions in realistic materials, and the feasibility of multi‐centre dosimetry studies is assessed.
SU‐FF‐T‐205: Ionization to Dose for Electron Beams: A Comparison of Three Approaches
Authors: Antolak J
Published: June 2005

Purpose: To compare methods of converting depth‐ionization to depth‐dose for electron beams. The current TG‐25 standard is based on TG‐21 calibration protocols and uses stopping power ratios for monoenergetic beams. The current IPEMB code of practice uses stopping power ratios for realistic electron beams, which are also used for the current TG‐51 and other electron dose calibration protocols. Method and Materials: Depth‐ionization curves measured with a cylindrical ion chamber were converted to percentage depth dose (pdd) using the current TG‐25 approach, the current IPEMB code of practice, and a hybrid method that uses the stopping power ratios of the latter and the replacement correction of the former. For depths greater than the practical range, the stopping power ratio and the replacement correction were set to the values at the end of the practical range. Results: For energies of 12 MeV or lower, and depths greater than dmax, all method give results that are almost identical. In the buildup region, the hybrid pdd was up to 0.8% higher than the TG‐25 approach and up to 1.0% less than the IPEMB code of practice. For 16 MeV in the buildup region, the hybrid pdd was almost identical to the TG‐25 pdd but 0.7% lower than the IPEMB pdd. For 16 MeV at R90, the hybrid method was about 0.6% less than TG‐25 and 0.7% greater than IPEMB. For the 20‐MeV beam at R90, the hybrid and TG‐25 results were about 1% higher than the IPEMB results. Conclusion: The hybrid method proposed here gives very similar results to the previous TG‐25 method and the IPEMB code of practice. The IPEMB method has replacement corrections for only one size of ion chamber, so the hybrid method is more widely applicable, and should be considered as an alternative or replacement for the TG‐25 method.
SU‐FF‐T‐206: Laser Rangefinder for Automatic SSD Measurement and Inter‐Fraction Patient Movement Monitoring
Authors: Bonnerup C , Sibata C , Allison R
Published: June 2005

Purpose: Source to Skin Surface Distance (SSD) is one of the most frequently used reference parameters for patient setup to verify the depth of treatment for external beam treatments in clinical radiotherapy. Modern machines use an Optical Distance Indicator (ODI) mounted inside the linear accelerator or a Front Pointer (FP) where the use of the ODI is precluded. In IMRT and IGRT, more precise tools are needed to monitor and record SSD, and provide information on patient movements during therapy. Additionally, automatic capture and recording of SSD data is preferential in busy clinical settings. We describe the use of a commercially available laser rangefinder to report SSD and inter‐fraction patient movement in real‐time. Method and Materials: The Acuity Research Model AR4000‐LV was evaluated as a tool to measure SSD and inter‐fraction movement. The device emits a visible laser spot (670 nm), and a time‐of‐flight measurement is obtained from the reflected beam from diffuse reflectance surfaces. Output data is provided either as a voltage or as a digital stream. The device was tested both on an optical bench (for absolute measurements) and on a phantom on the treatment couch. Tests were performed statically and during measured oscillations of the phantom. Results: Un‐calibrated accuracy was as described by the manufacturer; ±3.8 mm. However, precision was much better at ± 0.254 mm. Calibrated measurements improved accuracy to ± 1mm. Measurements on non‐planar and tilted surfaces were also tested, and were unchanged at angles up to 45°. Conclusion: We show the ability of off‐the‐shelf technology to improve the efficiency and repeatability of SSD measurements. Also, minor patient movement can be monitored in real‐time. We find the laser rangefinder to be precise, time efficient and very accurate for determining and verifying SSD. We will evaluate its use in clinical studies.
SU‐FF‐T‐208: Modeling the Headscatter Off‐Axis in Megavoltage Photon Beams
Authors: Finlay J , Li J , Zhu T
Published: June 2005

Purpose: To accurately model off‐axis headscatter from a linear accelerator at off‐axis positions using a parameterized, two‐component model. An improved model is now proposed to model headscatter outside beam collimation. Method and Materials: We define the head‐scatter off‐axis ratio, HOA, as the ratio of the kerma due to head‐scatter photons at the off‐axis position x to the kerma from direct primary photons on the central axis. “Direct primary” are those photons that come from the source without interactions in the intervening structures. We measured HOA along the X jaw direction from the central axis to a distance of 25 cm from the field edge for various collimator settings. We fit the measured HOA using an improved two‐component model. The first component is a Gaussian source projected on a plane below the x‐ray source, representing the headscatter from the flattening filter and the primary collimator. Compared to an earlier model [1], this model models the second component for photons scattered in the secondary (variable) collimators using a linear edge kernel integrated over the four collimator jaws. Headscatter off‐axis outside beam collimation is examined more extensively with this study. Results: The two‐component model can be fit to data obtained from a variety of fields with a mean (maximum) local relative error of 10% (35)%. The region outside the penumbra region is modeled primarily by the collimator component. Conclusion: A two‐component model in which the headscatter from the flattening filter and the collimators are modeled separately can accurately predict HOA values for arbitrary collimator settings.
SU‐FF‐T‐385: Tolerance Levels for Dosimetric IMRT QA Based On Radiobiological Parameters
Authors: Miften M , Werts E , Trombetta M , Fuhrer R , Parda D
Published: June 2005

Purpose: Tolerance levels for dosimetric verification of IMRT plans are usually set at constant values. This QA process is not patient specific. It ignores the potential role of fraction size and treatment site in determining the clinical consequences of an error in dose calculation and/or delivery. The objective of this work is to develop dose tolerance levels for IMRT based on radiobiological parameters. Method and Materials: The linear quadratic cell survival model which accounts for the effects of both total dose and fraction size was used. A technique was developed to convert dosimetric tolerance levels between treatment schedules while maintaining a constant uncertainty in a radiobiological parameter. Specifically, a constant tolerance level in a radiobiological parameter is set, rather than in calculated/delivered dose. Calculations were performed for different treatment fractionations and α/β ratios mimicking a wide range of clinical situations. Results: Tumors with low α/β ratio (melanoma, prostate) are more sensitive to errors in total dose. Dosimetric tolerance for such tumors should be approximately 65% of those of other tumors. For such malignances, if a 3% error in calculated/delivered dose is acceptable for treatment of most tumors, this should be reduced to 2% for treatment of these more radiosensitive tumors. In addition, results show that in field late and early responding normal tissues are more sensitive to errors in dose than out of field tissues. Conclusion: Invariant tolerance levels are appropriate for a wide range of clinical IMRT schemes, especially given current uncertainties in clinical α/β values. Assuming that current dosimetric levels are appropriate for some clinical applications, the appropriate values are for all other applications are between 0.5–2 times the currently used values for IMRT QA. In conclusion, tolerance levels for dosimetric QA based on radiobiological parameters provide useful information and may be more appropriate for verifying calculated/delivered dose.
SU‐FF‐T‐386: Treatment Optimization for Prostate IMRT Incorporating Utility Analysis and Patient Decisions
Authors: Ma C , Li J , Price R , Chen L , Konski A , Watkins‐Bruner D , Pollack A
Published: June 2005

Purpose: Radiation target dose, target margin, and the doses to the surrounding normal tissues are traditionally applied in a standard way (e.g., 76–78Gy to patients with intermediate to high risk prostate cancer). The hypothesis here is that quality of life will be improved on an individual basis by allowing these aspects to vary, within limits, based on the preferences of a given patient in terms of local control and normal tissue toxicities. The goal is to develop planning methods for incorporating utilities. Method and Materials: Patient utilities are individual valuations of various health states. Through the incorporation of utilities in the IMRT planning process, the treatment may be tailored to the individual patient desires. The assessment of utilities is ongoing. In this study, DVHs for the target and critical organs are used to estimate tumor control and normal tissue complications and then converted to utility scores. We included utility assessment for baseline general health, and urinary, sexual, and bowel functions. Preliminary results are based on multiple IMRT plans using a commercial TPS. An automated planning process is being developed using a home‐grown optimization system. Results: Outcome data from the literature and our own institution are used to establish a consistent DVH‐utility conversion system. Although such a scoring system is based on the mean values of a patient population the changes in the utility scores for a particular patient are relative and relevant based on the dosimetric changes in the treatment plans. Emphasis is given to dose in the target, rectum, bladder and erectile tissue to maintain the balance between local control, rectal/bladder bleeding, urinary functions and impotence. Patients are given the opportunity to make treatment decisions by selecting personalized utility criteria. Conclusion: Treatment optimization based on utility analysis allows both the oncologist and patient to make personalized treatment decisions.
SU‐FF‐T‐392: A Technique for Non‐Coplanar Helical Tomotherapy Cranial Radiosurgery Treatment
Authors: Akpati H , Wu H , Rosen L , Katz S
Published: June 2005

Purpose: To demonstrate a new technique for non‐coplanar helical Tomotherapy cranial radiosurgery treatment. Evaluation of the improvement, in terms of minimizing the dose to healthy tissue, of the non‐coplanar technique over the conventional coplanar treatment. Methods & Materials: The proposed technique for non‐coplanar helical Tomotherapy cranial radiosurgery is demonstrated using an anthropomorphic phantom (Rando phantom). Obtaining a composite dose distribution of the optimized non‐coplanar plans is not possible with the current helical Tomotherapy planning system. For comparison of the dose coverage, conformality and dose volumes between the non‐coplanar and the conventional coplanar treatment techniques, film dosimetry is used to obtain isodose distributions. A calibrated Welhofer scanner was utilized for the film dosimetry. Results: The results show significant reduction in the volumes receiving 60% or less of the prescribed dose. There is no appreciable change in volumes receiving 70% or more of the prescribed dose. Dose volume ratios, DV20/80 and DV30/70 (defined as the ratio of the volumes receiving 20% and 80%, and the ratio of volumes receiving 30% and 70% of the prescribed dose, respectively) are used to quantify the improvement of a non‐coplanar helical Tomotherapy cranial radiosurgery technique. The DV20/80 and DV30/70 dose volume ratios obtained for the non‐coplanar technique are 9.33 and 4.57, respectively, whereas ratios obtained for the standard coplanar technique are 11.33 and 5.01, respectively. Conclusion: The results of the study clearly demonstrate that the dose to healthy tissue can be minimized by using three non‐coplanar angular orientations for helical Tomotherapy cranial radiosurgery. More marked improvements can be obtained by utilizing more than three non‐coplanar angles, but at a high cost in terms of planning and treatment times. Balancing the improvements in DV20/80 and DV30/70 ratios and the required treatment and planning times entails making proper clinical judgment for each case.
SU‐FF‐T‐393: An Anthropomorphic Phantom for Respiratory Motion Research in Dynamic SRS
Authors: Dieterich S , Stoianovici D , Wong K , Cleary K
Published: June 2005

Purpose: The purpose of our experiment is to study the use of a respiratory anthropomorphic phantom evaluating the respiratory motion tracking capabilities in dynamic stereotactic radiosurgery (SRS). Method and Materials: An anthropomorphic phantom was scanned with 1.25 mm slice spacing on a 4‐slice GE Lightspeed CT. A Harvard lung pump was attached to the lung bellows of the phantom and set to 15 breath/min, 40% expiration. The abdominal cavity was filled with humid sponges to resemble lung tissue density. A mini‐ballcube containing a pair of orthogonal GaF‐chromic films was attached to the diaphragm. The spherical target was treated to 3000cGy at 100% isodose line using both a static SRS and dynamic SRS (Synchrony) treatment. The films were analyzed using a Vidar VXR‐16 scanner and the Accuray film analysis tool. Results: CT numbers measured in the phantom range from 42–162 for the sponges, 1111–1193 for the silicone tissue, and 1412 – 1512 for the embedded plastic skeleton. The total targeting error for the diaphragm treatment was 0.77 mm in the static and 0.91 in the dynamic case. The eccentricity for the spherical target was 1.27 mm in the axial plane for both treatments, and 1.34 mm for the static and 1.41 mm for the dynamic treatment in the coronal plane. The respiratory pattern was very similar to patterns observed in Synchrony patients. The extent of diaphragmatic motion was 8–10 mm in the superior/inferior direction. Conclusion: The respiratory anthropomorphic phantom is a promising tool for evaluating the respiratory motion tracking capabilities of dynamic SRS in a patient‐like environment. The accessible abdominal cavity allows for different target positions. It can easily be filled with different materials (sponges, saline solution, organic material, gels) to simulate different environments in the body. The electron densities (CT numbers) of the phantom material are very close to human tissue.