bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐FF‐T‐108: Evaluation of IMRT Plans Between Linac and Helical Tomotherapy Based On Dose Volume Indices and Normal Tissue Sparing in Prostate Cancer Patients
Authors: Sharma P , Murthy V , Master Z , Ph Reena , Patkar S , Chaudhari S , Kinhikar R , Mahantshetty U , Engineer R , Deshpande D , Shrivastava S
Published: April 2010

Purpose: The objective of this study was to evaluate and quantify the dosimetric gains of linac‐based sliding window IMRT technique versus Helical Tomotherapy (HT) in terms of Conformity Index (CI), Homogeneity Index (HI), and normal‐tissue sparing for prostate cancer patients Methods and Material: Eight locally advanced, non metastatic, prostate cancer patients were selected for this study. Volume delineation was done on contrast enhances CT scans with 5mm slice thickness Target volumes included PTV74 (prostate and seminal vesicle) and PTV55 (pelvic lymph nodal regions) and Organs at Risk (OARs) were contoured. The PTV74 volume ranged from 79.8 cc to 177.85 cc (mean 121.02 ± 28.9 cc), while PTV55 ranged from 431.08 cc to 941.52 cc (mean 677.78 ± 185.06 cc). The prescription doses were 74 Gy/37♯ and 55 Gy/37♯ to 95% of PTV74 and PTV55 respectively. Results: Mean CI ± SD for PTV74 were 0.69 ± 0.06 and 0.77 ± 0.06 with IMRT and HT respectively (statistically significant, p=0.025). The mean CI for PTV55 were 0.43 ± 0.04 and 0.57 ± 0.07 with IMRT and HT respectively (p=0.21). Mean HI ± SD for PTV74 were 0.07 ± 0.02 and 0.04 ± 0.01 for IMRT and HT plans respectively (p=0.012) while for PTV55 mean HI were 0.2 ± 0.09 and 0.06 ± 0.02 for IMRT and HT respectively (p=0.012). HI was statistically superior with HT than with IMRT for both PTVs. OARs sparing in HT was statistically superior in rectum (p=0.025), Femoral Heads (pr=0.012, Pl=0.017) and statistically non significant difference in mean dose of bladder (p=0.18), small bowel (p=0.33) compare to conventional IMRT. Conclusion: Helical Tomotherapy plans revealed better Conformity Indices and Homogeneity Indices compared to sliding window IMRT technique for PTV74. Overall, the HT plans also spared the OARs significantly more than the IMRT plans except the small bowel.
SU‐FF‐T‐263: Evaluation of a PAGAT Gel for Use in An Anthropomorphic Phantom
Authors: Heard M , Ibbott G , Followill D , Jackson E , Salehpour M , White A
Published: April 2010

Purpose: To evaluate the use of PAGAT, a normoxic polymer gel formulation, in the Radiological Physics Center's (RPC) Head and Neck Phantom. Methods and Materials: Conventional and IMRT treatment plans were developed for the RPC's head and neck phantom, which contained simulated planning target volumes and an organ at risk. A PAGAT dosimeter was placed in the phantom and the conventional treatment plan was delivered. A different PAGAT dosimeter was used to measure the IMRT treatment. An additional group of PAGAT dosimeters were irradiated to doses between 1 and 5 Gy through parallel‐opposed beams to develop a calibration curve. The dosimeters were imaged using an OCT‐OPUS™ laser CT scanner 24 hours after irradiation. Profile, isodose, and gamma index comparisons were performed between the treatment plan and the distribution measured with PAGAT. Results: Good agreement was observed between the conventional treatment plan and the gel distribution with the exception of one target region where the dose was most fractionated. The gel overestimated the dose in this region. The gel used to measure the IMRT treatment overestimated the dose by 35%. Relative comparisons of the IMRT treatment showed agreement consistent with that seen in the conventional treatment plan. Conclusions: The over‐response of the gel is attributed to the fractionation dependence of the gel, which was investigated. Calibration gels should be irradiated with a fractionation scheme similar to the experimental gels, particularly for IMRT treatments, so that absolute dose comparisons can be performed.

The investigation was supported in part by PHS grant CA 10953 awarded by the NCI, DHHS.
SU‐FF‐T‐268: Study of Field Size Factors for Passively Scattered Proton Therapy Beams
Authors: Wang X , Sahoo N , Zhu X , Ding X , Arjomandy B , Ciangaru G , Gillin M
Published: April 2010

Purpose: To measure the field size factors (FSF) for passively scattered proton therapy beams and study its dependence on aperture size, proton energy, air gap or snout position, and spread‐out Bragg peak (SOBP) width. Method and Materials: A pinpoint ion chamber in plastic phantoms was used to measure the FSF for square apertures with sides of 2, 3, 5, 18 cm and circular apertures with 2 to 6 cm diameter relative to the 10 × 10 cm2 apertures. The chamber location was kept fixed at the isocenter. The FSF were measured at the center of SOBPs for 120 to 250 MeV proton beams as a function of air gap between the aperture and phantom surface and SOBP widths. Results: The FSF dropped significantly when the aperture size became smaller than 3 × 3 cm2. For a 6 cm SOBP and largest possible air gap, the FSF for a 2 × 2 cm2 field was 0.95 for a 120 MeV beam and decreased to 0.84 as the energy increased to 250 MeV. Increase in SOBP width, decrease in the air gap, and the increase of aperture size beyond 3 × 3 cm2 caused relatively smaller change in the FSF. Conclusion: The change in FSF for small fields may be attributed to two competing processes, the reduction of the primary fluence due to the decrease in aperture size and increase in aperture edge scattering. For low energy beams and small air gaps, the FSF is close to 1 with the increased edge scattering compensating the loss of fluence. With increasing energy and air gap, primary fluence reduction overtakes the aperture edge scattering and causes larger reductions in FSF. A possible model for predicting the FSF for small irregular fields based on this study is being explored and will be discussed.
SU‐FF‐T‐275: Dose Delivery Effects in Electron Beams From Surgical Breast Clips
Authors: Gossman M , Zhao L , Cao M , Lopez J , Das I
Published: April 2010

Titanium clips are commonly used to delineate the location of the tumor bed during breast cancer surgery. Following a course of radiation from x‐ray beams, electrons are normally prescribed as a boost for cancer treatment. This research employs a technique to determine the effect on such boost treatments as a result of metallic clip interference potentially exhibited though attenuation and scatter processes. Both Generalized Gaussian Pencil Beam and Electron Monte Carlo algorithms were used for dose delivery simulation at therapeutic energies of 6 MeV and 9 MeV. The potential effect of clip interference with an incident electron beam is concluded as unidentifiable on computer simulation in planes 3 mm from clips for both algorithms. Film dosimetry with Gafchromic EBT media did detect dose perturbations caused by the titanium clips in a more proximal plane, 0.23 mm away, resulting in a finding of 3.0 % backscatter and 2.4 % attenuation at 6 MeV and 2.0 % backscatter and 6.7 % attenuation at 9 MeV. At 6 MeV the dose to the film reduced by only 2.4 % whereas the dose reduction from the 9 MeV electron beam was greater at 6.7 %. Between 75–300 cGy, the Vidar scanner has a dose resolution of 0.5 %.5 The noise contribution of the scanner is further estimated to be about 0.4 %.5 As a result of having to couple these uncertainties with the uncertainty of 2 % for film calibrations, we conclude the magnitude of difference between measurement and accuracy is significant. With an end error analysis totaling 3 %, the uncertainty in measurement and the magnitude of the experimental findings is a strong indicator that the results are not significant. Results here are consistent with computer modeled simulations conducted initially in this investigation and deemed negligible for clinical findings.
SU‐FF‐T‐279: The Influence of Readout Parameters On the TL Dose Response of TLD‐100
Authors: Liang Q , DeWerd L
Published: April 2010

Purpose: To investigate the influence of TLD‐100 readout parameters including maximum readout temperature and heating rate for different doses. Method and Materials: The influence of readout parameters on the response of LiF:Mg,Ti (TLD‐100) chips was investigated by using different maximum readout temperatures and heating rates. TLDs were irradiated to doses to water ranging from 10cGy to 1000cCy with 60Co. The maximum readout temperatures ranged from 240°C to 380°C with the heating rate set at 15°C/s. For the 350°C maximum temperature, heating rates of 2°C/s to 30°C/s were investigated. Results: When the maximum temperature is set to 240°C, an incomplete glow curve is seen with more residual signal in the TLDs and relatively low TL dose response. When the maximum readout temperature is as high as 380°C, the higher temperature peaks (peak 6, 7) can affect the integral TL emission. However, if only the height of peak 5 is used, the response‐dose curves are quite similar for maximum readout temperatures between 260°C and 380°C. Different heating rates from 2°C/s to 30°C/s were used to heat the TLDs to 350°C, with the same holding time after reaching the maximum readout temperature. The results show that different heating rates do not appreciably change the TL dose response when the heating rate is higher than 5°C/s. However, the signal to noise ratio decreases noticeably when the heating rate is lower than 5°C/s especially at low doses. The glow peak temperature shifts upward with increasing heating rate, which means a higher maximum readout temperature is required to obtain the full glow curve. Conclusions: Integral TL response is strongly dependent on maximum readout temperature, so care must be used in choosing and documenting this temperature. Heating rate may also affect the readout uncertainly, especially at low doses.
SU‐FF‐T‐286: Dosimetric Differences in Dynamic MLC Performance as a Result of Alignment and Software Configuration
Authors: Curran B , Roberts D , Cardarelli G , Sternick E
Published: April 2010

Purpose: During commissioning of a new linear accelerator (Varian Clinac 21iX), differences were found in dynamic MLC (sliding window) delivery of 2.5–5.5% when compared to an existing matched linear accelerator (Varian Trilogy). An investigation was done to determine the causes of these differences. Method and Materials: Ion chamber and film measurements were done on several accelerators while varying MLC alignment parameters. Test deliveries, including open fields, MLC fields, “sweeping gap”, and dynamic MLC fields were made using a central ion chamber and film. A difference in the software configuration of the MLC (“SureTouch”) resulted in a significant dosimetric effect and measurements were performed to ascertain its impact. Results: Except for dynamic MLC delivery, the accelerators were well matched, with all differences less than 0.5%. Differences in delivery of dynamic IMRT treatments were found in three areas: 1) Calibration of the MLC alignment can introduce up to 2% variation in dMLC performance; 2) A variation of 1.5% appears to be inherent in the mechanical performance of MLCs; 3) A 1% difference appears to be a result of the software configuration of the MLC by service engineers. This configuration can be optionally introduced in cases where poor performance of the MLC is noted during patient deliveries. Conclusion: With proper configuration and calibration of an MLC, machine‐to‐machine performance should be within 2%. As noted in several previous publications, careful calibration and testing of MLC leaf position and alignment are critical to proper delivery of dynamic MLC treatments. Software configuration of the MLC by service engineers can affect dosimetric performance for IMRT deliveries. Medical physicists should monitor MLC configuration changes installed by service personnel carefully, as these changes may require alterations in treatment planning system models.
SU‐FF‐T‐289: Gamma Is a Necessary, But Not Sufficient Criteria for Comparing Dose Distributions
Authors: Siebers J , Wang S , Gardner J
Published: April 2010

Purpose: To demonstrate via simple test cases that although percent pass gamma (γ<1) may be a necessary condition for acceptance in dosimetric analysis, it is not a sufficient condition to ensure dose integrity and therefore should not be used as a sole of criterion to judge dosimetric acceptability. Method and Materials: Simple modifications to the delivery of treatment fields are created such that, although > 98% of points have γ<1 with respect to the reference field when 2D planar fluence/dose verification images are acquired, the resultant dose distributions are clinically unacceptable. Gamma criterion of 3%, 3 mm and 5%, 5mm are used. Reference test fields include a 10×10 cm2 open field with 5mm gap sweeping and two modified 10×10 cm2 fields. Deviations include shrinking the dose distribution by 3 mm, and introducing a non‐moving 5 mm wide MLC leaf in the interior of a field. Results: The two‐dimensional field that is narrower than the reference field can have 100% of the points with γ<1. A non‐moving 5 mm MLC leaf in a 10 ×10 cm2 sliding window can have 94.5% of the points passed 3%, 3mm gamma test and 97.8% of the points passed 5%, 5mm gamma test. Similarly, fields with dose peaks or valleys in them with dimensions up to two‐times the DTA criteria can have 100% of points with γ<1. Exchanging to roles of the reference and test images in the gamma evaluation can be distinguish some, but not all, of the clinically relevant errors. Conclusion: Evaluation of the fraction of points with γ<1 may be a necessary condition, but it is not sufficient to state a plan is dosimetically acceptable. Evaluation of additional dose metrics is required to show clinical acceptability. Conflict of Interest: Research supported in part by Varian Medical Systems and NIH P01 CA116602.
SU‐FF‐T‐296: On the Comparison of Epson V700 and 10000XL Scanners for GafChromic EBT Film Dosimetry
Authors: Xu L , McEwen M
Published: April 2010

Purpose: This project investigated the differences between the Epson 10000XL and V700 flat bed scanners when used in EBT film dosimetry, to determine whether the cheaper model (V700) is suitable for clinical use. Method and Materials: Based on existing literature, we investigated three basic characteristics of both scanners ‐ the dose sensitivity curve of EBT film, the scanner uniformity correction, and the darkening of EBT film by the scanner lamp. Doses from 0.25 Gy to 8 Gy were delivered to 8 pieces of EBT film, and the same set of film was read by both scanners. Calibration curves obtained from both scanners were compared. Scanner lamp scattering effect (uniformity) was investigated in directions parallel and perpendicular to the scanner lamp. The geometric accuracy of both scanners were also verified using a test pattern consisting of a set of concentric rings. Scanner darkening effect was investigated by repeatedly scanning two pieces of blank film on both scanners. Results: We observed a strong darkening effect on EBT films by the V700 scanner, while the 10000XL showed no permanent darkening. The two scanners show similar calibration curves (both in terms of sensitivity and signal‐to‐noise) and geometric accuracy. The V700 shows a significantly higher lateral uniformity variation compared to the 10000XL, and the correction can not be adequately described by a simple equation. In contrast, the 10000XL exhibits a smooth uniformity variation that can be accurately corrected for. Conclusion: In our view, the darkening effect and irregular lateral uniformity correction are two significant drawbacks of V700 scanner that make it unsuitable for accurate EBT film dosimetry. These results are also applicable to other Epson consumer‐level scanners, as they all use the same type of lamp as the V700. The 10000XL remains the scanner of choice from the current Epson lineup for EBT film dosimetry.
SU‐FF‐T‐308: Output Factor Measurements and Aperture Size Dependence of Dose Distributions for Scanned Proton Beams at MPRI
Authors: Wolanski M , Pack D , Jesseph R
Published: April 2010

Purpose: Prior to a patient's proton beam treatment at MPRI, the Quality Assurance program entails measurement of the specific output factor for each field with its associated hardware in place. As a basis for comparison to patient specific measurements, and to characterize our dose delivery and monitoring system, we have measured a standard set of output factors (10 cm diameter aperture without compensator) over the energy range of clinical interest for all spread‐out Bragg peaks (SOBPs) commissioned for delivery. We have also made detailed measurements of on‐axis depth‐dose distributions as a function of aperture diameter from 10 cm to 2 cm for representative scanned‐beam fields to further characterize our system. Method and Materials: Single point output factor measurements were made in a water phantom using a Markus ionization chamber. Additionally, depth‐dose distributions were acquired with a multi‐layer ionization chamber (MLIC) array, which provides 122 depth channels spaced approximately every 0.18 cm. Lastly, beam profiles were measured with a commercial array of ionization chambers (MatriXX, IBA). Results: The results show the systematic decrease in dose along the SOBP plateau, and the increase of the SOBP entrance dose relative to the plateau dose, as a function of aperture size and treatment range. The need to carefully plan and deliver fields less than 5 cm diameter is obvious. Additionally, the application of the standard output factor measurements to predicting patient specific output factors for prostate patients has been verified with ±0.8% accuracy. Conclusions: Given the similarity of prostate fields and our ability to model the output factors within ±0.8%, we no longer measure patient specific output factors for prostate patients treated with opposed laterals. But, we do make these output factor measurements for other treatment fields while utilizing the MLIC to characterize depth‐dose distributions for fields smaller than 5 cm diameter.
SU‐FF‐T‐314: Patient‐Specific Quality Assurance Techniques for RapidArc Radiotherapy
Authors: O'Daniel J , McMahon R , Chang Z , Yan H , Sakhalkar H , Das S , Wu Q , Yin F
Published: April 2010

Purpose: To develop a patient‐specific quality assurance protocol for RapidArc™ radiotherapy. Method and Materials: Following the commissioning of RapidArc™ treatment delivery, we tested several methods of patient‐specific quality assurance (QA). Ion chamber and film measurements were used as the “gold standard”. One ion chamber measurement was taken in a region of low‐dose gradient. Four film measurements were taken in three planes: axial, sagittal, coronal, and a coronal180° (testing setup uncertainty). Measurements were also taken using the Matrixx® device (IBA Dosimetry, Bartlett, TN) in two orientations, sagittal and coronal. The Matrixx® consists of a 24×24cm2 grid of ion chambers, providing a 2D absolute dose comparison. All plans were designed and calculated using Eclipse (Varian Medical Systems, Palo Alto, CA). Eleven patient plans were evaluated, consisting of 4 brain, 1 head&neck boost, and 6 prostate radiotherapy treatments. Results: Ion chamber measurements were typically ∼2% greater than those predicted by Eclipse. Using a gamma index with 3%, 3mm, 5% threshold criteria, 88% of films had gamma pass rates of >90%. Three of 7 axial films failed, but passed with thresholds of 30–40%. Similarly, Matrixx® measurements had gamma pass rates of >90% for 100% of measurements. The average reading of 4 Matrixx ion chambers was significantly correlated with the single ion chamber results (Pearson correlation coefficient = 0.839, p = 0.002), and the 2D Matrixx results agreed well with film measurements (gamma pass rate >90% for 10 of 11 comparisons). Conclusion: These QA measurements produced good agreement between planned and measured doses. Through utilizing ion chamber and film measurements, we have gained confidence in the Matrixx® for RapidArc QA. The chosen passing criteria were +/−3% for ion chamber readings, and gamma passing rates of >90% for film and Matrixx® with 3%, 3mm, and 5% threshold.
SU‐FF‐T‐325: Dose Perturbation Study in a Multichannel Breast Brachytherapy Device
Authors: Slessinger E , Das I
Published: April 2010

Purpose: To determine the dosimetric effects due to air pockets and high Z contrast within a new multichannel breast brachytherapy device Method and Materials: The 5‐6cm diameter Contura brachytherapy device was inflated using 37cm3 of saline to a 40 mm diameter. Baseline dose falloff from an HDR Iridium‐192 source was measured in water using a Markus parallel plate chamber supported on a motorized vertical drive. Ionization measurements with the Iridium source centered in the central Contura channel and an anterior off‐center channel were performed. Data were collected at distances from 1 to 50 mm. Comparison studies were conducted with identical inflated volume containing varying air pocket volumes (1–4 cm3) and varying concentrations of Isovue contrast solution (3, 6, and 9% by volume). Dose perturbation factors (DPF) were computed and evaluated. Results: Dose perturbations due to air pockets and contrast solutions were observed. As the volume of air increased the DPF caused increased dose at a rate of approximately 2.25%/cm3. The effect was relatively consistent for both channels throughout the distance range. The effects due to contrast were more complex. Increased dose was observed from the 3% solution, by 2% from the source in the central channel (♯5) and by 4% with the source in the off‐center channel (♯1). A dose reduction of 1.0% from channel ♯5 and 1.5% dose increase from channel ♯1 occurred from the 6% contrast solution. The 9% contrast solution caused dose reductions by 3.5% (channel ♯5) and 2.5% (channel ♯1). The DPF from all contrast solutions moderated with increasing distance. Conclusion: Dose perturbations due to air pockets and high Z contrast solution can be significant. Treatment planning systems do not account for these. It is important to minimize internal air pockets and limit contrast solution concentration to avoid dose errors to the target and normal tissue.
SU‐FF‐T‐330: A Practical Method to Measure and Evaluate Different Dose Calculation Algorithms in Varian Eclipse Treatment Planning System
Authors: Lu L , Yembi‐Goma G , Wang J , Gupta N , Li K , Zhang H , Weldon M , Huang Z , Lo S , Martin D , Mayr N
Published: April 2010

Purpose: Electron Density Phantom (EDP) was used to evaluate dose calculation algorithms in Varian Eclipse Treatment Planning System (TPS) and to commission the Anisotropic Analytical Algorithm (Version 8.1.17, AAA_8117) with inhomogeneity correction for treatment planning. Methods and Materials: The EDP with various tissue equivalent plugs used to calibrate the CT simulator for dose calculation in Varian Eclipse TPS (Version 8.1.1.17) was adopted to evaluate different dose calculation algorithms in the TPS by measuring the actual dose delivered to the EDP prescribed by treatment plans. The treatment plans with field sizes of 20×20, 10×10, and 4×4 were created in the TPS using CT images of the EDP. The 200 cGy per fraction was prescribed to a point that is 4 cm behind the 8.2 cm long exhale‐lung tissue equivalent plug. Dose calculation for each plan was performed with calculation models of Pencil Beam Convolution (PBC_8117), the AAA_8117 without inhomogeneity correction, and the AAA_8117 with inhomogeneity correction, respectively. Then the individual plans were delivered to “treat” the EDP for measuring the actual dose. The 2‐D dose distributions were measured and analyzed with two independent methods (1) MapCheck, and (2) EDR‐2 film with RIT VXR‐16 Dosimetry Vidar System. The prescribed point dose was measured using an ion chamber. All the measured doses were compared with the calculated doses from the associated treatment plans. Results: The AAA_8117 with inhomogeneity correction is superior to the other algorithms in our study. The dose discrepancy is within 1% at the prescribed area. Conclusion: A practical method to evaluate different dose calculation algorithms in a TPS was proposed and performed in our study. Results suggest that the AAA_8117 with inhomogeneity correction should be used for treatment planning, especially when lung tissue is involved in the radiation field.
SU‐FF‐T‐341: Small Field Dosimetry for Electron Beams
Authors: Dimofte A , Zhu T , Novak J
Published: April 2010

Purpose: This study examines the accuracy of dose calculation for electron beam in small circular fields for Varian accelerators using an empirical method based on dose per incident fluence. Method and Materials: Fractional Depth doses (FDD) at SSD = 100 cm for open beam and small circular cutouts (r = 0.5, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0 and 5.6cm) are renormalized to surface. The resulting depth dose is then normalized to the broad beam FDD to determine a fractional FDD, FDDf for.all electron energies (4, 6, 9, 12, 16, and 20MeV). The head scatter factor (H), defined as the ratio of electron energy fluence at point of interest to open cone at otherwise the same geometry, is measured using a diode in air without buildup cap as a function of cutout radius for all electron energies and cone sizes (6, 10, 15, 20 and 25cm2). The phantom scatter factor, PSF=BF/H, defined as the ratio of blocking factor in water at reference depth and is measured as a function of radius, electron energy, and SSD (100, 120 and 140cm). Results: It is found that H is a single function of renormalized radius, rN = r*100/(SSD+d) and is independent of SSD for a given electron energy and cone size. Normally, the radius dependence of H is a function of both the SSD and cone size. The extrapolated value of PSF from percent depth dose (PDD) renormalized to the surface is remarkably close to the measured PSF data, to within 2%. The calculated FDDf has a bowl‐shaped dependence on the depth with the height increasing with increasing electron energy. Conclusion: By normalizing the Fractional depth dose (FDD) to the surface, most characteristics of small field electron dosimetry can be simplified and adequately described by H, a quantity describing the incident electron energy fluence.
SU‐FF‐T‐346: Measurement of Surface Dose Increase From Patient Immobilization Thermoplastic Masks Using Gafchromic EBT Films
Authors: Ali Imad , Matthiesen Chance , Algan Ozer , Thompson Spencer , Bogardus Carl , Herman Terence , Ahmad Salahuddin
Published: April 2010

Purpose: To evaluate setup accuracy of the BrainLAB stereotactic radiotherapy head immobilization mask using orthogonal projections from on‐board imaging. Materials and Methods: Eight patients were simulated and treated with the BrainLAB stereotactic head immobilization masks and CT‐localizers for brain lesions using single and hypo‐fractions. Orthogonal pairs of kV projections were acquired using Varian Trilogy's on‐board imager. The kV projections were then registered with digitally‐reconstructed‐radiographs (DRR) obtained from treatment planning. Shifts between the kV images and reference DRR's were calculated: anterior‐posterior (A‐P), left‐right (L‐R) and superior‐inferior (S‐I). If the shifts were > 2 mm in any direction, the patient was reset within the immobilization mask until satisfying accuracy was achieved. Results: Shifts as large as 5 mm in the A‐P, L‐R and S‐I directions between kV projections and DRR's were found. These shifts represent offsets between the treatment and simulation setup using immobilization mask. The mean offsets of 0.1 mm, 0.7 mm, and −1.6 mm represent systematic errors in the A‐P, L‐R and S‐I directions, respectively, using the BrainLAB localizer. The mean of the radial shifts is about 1.7 mm. The standard deviations of shifts 2.2 mm, 2.0 mm, and 2.6 mm represent patient setup accuracy with the BrainLAB mask in A‐P, L‐R and S‐I directions. Conclusions: The BrainLAB mask provides less invasive immobilization system than the head ring. Relying on this system for patient setup might be associated with errors as large as 5 mm. Image‐guidance with the kV on‐board imager provides shifts that are required to setup patient prior to or during patient treatment. The patient may relax or move during treatment, and thus uncontrolled and undetected setup errors may be produced with patients that are not well immobilized. The combination of stereotactic immobilization and image guidance will achieve more controlled and accurate patient setup within 2 mm.
SU‐FF‐T‐347: Utility of Serpentine Curve for Smoothing and Extrapolating Depth Dose Scans
Authors: Lightfoot D
Published: April 2010

Purpose: To smooth measured data to a function that will permit well behaved extrapolation beyond the limits of measurement. This is of particular interest in extrapolating typical ion chamber scans of depth dose profiles to surface dose for comparison with other results such as Monte Carlo calculations, extrapolation chambers, film, etc. Method and Materials: The serpentine curve was chosen as a potential function because it has a general shape in the first quadrant that is similar to a photon depth dose curve. However, it cannot be fit to the data with a simple scaling of axes. The method chosen for variable scaling was to convert the measured percent dose at each depth to a serpentine depth value. A polynomial fit of the relationship between serpentine depth and physical depth was obtained from a Microsoft Excel spreadsheet. The depth dose curve was recomputed based on the polynomial conversion of depth to serpentine depth. Results: Depth dose curves ranging from 1×1 to 40×40 cm were analyzed. For extrapolating to surface data up to 5 cm depth was used. Data less than a 5 mm depth was not used to avoid the distortions that occur as the ion chamber becomes only partially rather than fully submerged. Data from 5 cm to 35 cm was separately fit to permit extrapolation to 40 cm depth. The recomputed curves exactly overlay the measured data except for the noise in the measured data and a slight difference at the 5 cm point. They have well behaved extrapolation to surface and deep depth values that compare well with alternate measurements. Conclusion: A variable scaling of depth to serpentine depth yields excellent smoothing of data and yields well behaved extrapolation. It may also serve as a good method of interpolating sparse data.
SU‐FF‐T‐354: Evaluation of Dosimetric Characteristics of Electron Beam Arc Therapy Using Gafchromic EBT Film
Authors: Swamidas J , Upreti R , Sharma P , Sharma S , Pai R , Deshpande D
Published: April 2010

Purpose: Evaluation of dosimetric characteristics of therapeutic electron beam arc therapy using GAFCHROMIC‐EBT film. Method and Materials: Output, PDDs and profiles were obtained using EBT film for 6 and 9MeV static and arc beam from Clinac2100CD linear accelerator using slab and cylindrical solid water phantom with gel bolus of thickness 1.5/2.0cm. For static beams, the measurements were carried out at SSD 100cm while for arc various SSDs of 80, 82, 85, 87 and 90cm were used. Measurements of EBT film were validated with ion chamber (PPC05 and FC65G). The effect of isocenter depth and the dose rate dependence were evaluated. Films were scanned with the Vidar dosimetri‐pro(red) scanner and analyzed with Omni‐PRO software. Results: Relationship of the dose vs optical density was linear within ±1% for 6 and 9MeV arc beam. PDD of arc beams show higher penetration and the dose‐maximum of 6 and 9MeV beam occurs at a slightly greater depth, 2 and 4mm respectively as compared to static beam. Arc beam resulted in higher x‐ray contamination towards the center of rotation and lesser surface dose, however, surface dose was found to be decreasing with increase in SSD as compared to static beam. An increase in relative dose was observed with SSD, however for larger SSDs (>85cm) the increase was rapid. Ion chamber measurements showed a higher relative output (6%) for the lower dose rates (100–300MU/min), whereas less dependent(<1%) for higher dose rates (400–1000MU/min). The response of EBT film was found to be dose rate dependent (3%) for lower dose rate in the range of 100–200MU/min, however, profiles obtained for various dose rates were independent of the dose rate. Conclusion: All the parameters evaluated in this study were found to be within acceptable limits, hence Gafchromic film can be a valid choice of dosimeter for electron beam arc therapy.
SU‐FF‐T‐355: Investigation of Doped Quartz Fibre Detectors for Radiotherapy Dosimetry Applications
Authors: MacKay R , Aitkenhead A , Hardy M
Published: April 2010

Purpose: The measurement of radiation dose and dose distributions is key to the delivery of safe radiotherapy treatments. The effectiveness of new radiation dosimeters manufactured by doping quartz fibres with copper and terbium has been investigated for a range of applications. Method and Materials: Doped quartz fibres exhibit spontaneous phosphorescence when irradiated. A prototype commercial system has been developed by splicing such a fibre has been to a commercial fibre optic cable and measuring the signal using a photomultiplier tube. The collection of the phosphorescence can be triggered using a number of methods such as using the linear accelerator current pulse or the detection of scattered radiation. Results: The gated signal from the fibre was averaged to determine the signal from the copper and terbium doped detectors. The half‐lives of the decay were found to be 0.08ms and 1.75ms for the copper and terbium detectors retrospectively. The slower decay of the terbium allows a higher signal to be collected but at standard accelerator pulse rates the signal of the fibre has not fully decayed before the next current pulse. The integrated signal from the fibres gives a repeatable measure of radiation dose. Fibres have been tested for a number of applications including IMRT point dosimetry for which there small volume is an advantage. The property of the dosimeters is dependant on the doping used. Terbium detectors will require correction of the signal when used at default dose rates for linear accelerators. Variation of the signal property with field size and energy will be presented. Conclusion: Doped quartz fibres have the potential to be an effective small volume dosimeter for a range of radiotherapy applications.

Conflict of Interest: The detectors used in these experiments have been supplied by Global Dosimetry.
SU‐FF‐T‐361: Dose Response Characteristics of the New EBT‐2 Film for Different Megavoltage Beams
Authors: Chiu‐Tsao S , Chan M
Published: April 2010

Purpose: We studied the dose response characteristics of the newly introduced EBT‐2 radiochromic films for megavoltage photon and electron beams of different energies. Method and Materials: EBT‐2 films (ISP prototype lot♯020609), 2.5×3.2cm2, were individually irradiated by 6MV and 15MV photons, and 6MeV and 20MeV electrons (Varian Cl‐iX linac) at dmax and the center of 10×10cm2 field size in a Solid Water phantom (RMI457). The dose range was up to 50 Gy for 6MV photon and 20 Gy for the other three energy modalities. The dose was calibrated against an ion chamber with ADCL calibration. All the films were scanned using Epson 4870 and 10000XL flatbed scanners with 48 bit color (RGB) at least one day after irradiation. The pixel values in a central region of 0.5×0.5cm2 in each film were sampled and the mean value was assigned to each film. The optical density and net optical density (NOD) values for each film were determined from the mean pixel value. Results: The NOD values for the red, green and blue channels were plotted against the doses for each energy modality. The curves from the Epson 10000XL and 4870 scanners were also compared. The dose response curves of the new EBT‐2 films were found to be quite similar for the four megavoltage beams and similar to those of EBT film. For dose of 20 Gy, the NOD values from Epson 10000XL were about 1.6, 1.4 and 0.8 for red, green, and blue channels, respectively. Conclusion: The dose response characteristics of the newly introduced EBT‐2 films have been studied for four megavoltage photon and electron beams, and found to be similar to the predecessor, EBT films. The new EBT‐2 films are expected to be feasible for 2D dosimetry applications. Conflict of Interest: Prototype EBT‐2 films were provided by the International Specialty Products.
SU‐FF‐T‐381: Design and Clinical Implementation of a TG‐106 Compliant Linear Accelerator Data Management System
Authors: Adnani N
Published: April 2010

Purpose: Minimize errors in linear accelerators beam dosimetry by designing and clinically implementing a computerized commissioning data management system (CDMS). Method and Materials: The Microsoft Visual Basic programming language was used to develop CDMS. A user friendly GUI allows users to access all of the system's features. These are organized in modules that mimic all the steps behind the commissioning of a linear accelerator. The process begins by creating a new machine and configuring the commissioning project. Output factor are acquired by radiation type, energy and accessory. PDDs, TMRs and OARs tables are imported directly from the scanning software. Beam modeling tools such as a dose rate table generator are provided. Treatment machine data can be compared for purposes of verification or annual calibration. MU calculations can be performed manually by entering the corresponding beam parameters or through the use of CDMS' DICOM RT import filter. Tools for TG‐51 and Monthly Calibrations are provided as well as a documents manager to handle associated documentation. Results: A total of 22 commissioning projects were included in the study. Out of the 22, 12 were completed without, and 10 with, the use of CDMS. As expected, errors in collected data were reduced by a factor of 3.0. Beam modeling errors have all but been eliminated. Data books errors were reduced drastically (a factor of 7.0) while completions on time were improved 6 times. As a result, the overall satisfaction of the clinic with the commissioning task improved by a factor of 2. Conclusion: The introduction of CDMS for clinical use appears to have achieved its primary goal of reducing errors in the physics data during the commissioning of Linear accelerators. Finally, although CDMS was developed independently of the Radiation Therapy Committee of the AAPM, it complies with most of TG‐106 recommendations.
SU‐FF‐T‐383: Practical Use of Gafchromic® EBT Films with Electron Beams — Application to TPS Quality Assurance Tests
Authors: Barouky J El , Fournier‐Bidoz N , Fares G , Mazal A , Rosenwald J
Published: April 2010

Purpose: To validate dosimetry using Gafchromic® EBT films parallel to electron beams in order to facilitate electron TPS Quality Assurance tests. Method and Materials: 2D dose distributions were performed using EBT films with 6, 9 and 12 MeV electron beams and 10×10, 5×10, 4×10 and 3×10 cm2 fields. The films were exposed parallel to the incident beam in a polystyrene phantom. An experimental setup was suggested to perform irradiation with EBT films parallel to the incident beam (Fig.1, 2). The CARPET® phantom was used to perform a QA test with the presence of a shaped bolus applied on the surface. An EBT film was positioned parallel to the beam in the centre of the phantom and another one under the bolus in order to measure the skin dose. This setup combines 5 different quality assurance tests: oblique incidence, complex surface shape, extended SSD, skin dose test and presence of a shaped bolus (Fig.3). Results: Comparison with IC15 ionisation chamber measurements showed that differences in absolute dose for the standard fields ranged from 0.5 to 1%, and mean difference for small fields was 2.4%. The mean offset in the PDD curves was 0.8 mm and the maximal difference in the profiles widths was 1.4 mm (TableI). The isodoses matching between calculated (Eclipse TPS) and measured data for the CARPET® phantom test showed very good agreement. The γ index map showed that almost 99% of the points satisfy the γ < 1 criteria with a 3% – 3 mm tolerance in dose and distance respectively (Fig.4). Conclusion: This study validates Gafchromic® EBT film dosimetry for standard and small electron fields. Parallel film dosimetry in electron beams greatly facilitates TPS QA tests which is very important in a clinical busy environment.
SU‐FF‐T‐392: Alanine as a Small Field Dosimeter ‐ First Tests in Gamma Knife Radiosurgery Fields
Authors: Novotny J , Desrosiers M , Huq M , Bednarz G , Puhl J , Seltzer S
Published: April 2010

Introduction and purpose: An international working group on reference dosimetry of small and nonstandard fields has published a new formalism for the dosimetry of small and composite fields (Med. Phys. 35, 2008). Ionization chambers, which are the backbones of radiotherapy dosimetry, are not always suitable for measurements in regions of high dose gradients or where non‐uniform beam distributions are encountered. It is important to identify and evaluate new dosimeters that are suitable for measurements of absolute dose in small and non‐standard fields. The purpose of this study was to investigate whether alanine can be used as an absolute dosimeter for small field dosimetry of the Leksell Gamma Knife (LGK). Methods and Materials: Alanine pellets (4.8 mm in diameter and 2.0 mm in height) were irradiated to 40, 100 and 150 Gy in the center of a LGK spherical polystyrene phantom (diameter 160 mm) using 16 mm collimator on the LGK Perfexion unit and 18 mm collimator on the LGK 4C unit. For each dose point three pellets were irradiated separately in three separate irradiations. The alanine dosimeters were measured with a Bruker ECS106 Electron Paramagnetic Resonance spectrometer using the protocol described in the NIST Division Quality System Manual. Results of alanine dosimetry were compared with ion chamber measurements performed with A16 Exradin ion chamber following AAPM TG21 protocol. Results and Conclusions: Very good agreement was observed between the results obtained by alanine dosimetry and ionization chamber dosimetry. Mean deviations between both dosimetric methods for the LGK Perfexion were found to be −0.6%, 1.2% and 1.7% for doses 40, 100 and 150 Gy, respectively. Similarly, mean observed deviations for the LGK 4C were −0.2%, 0.7% and 1.2% for doses 40, 100 and 150 Gy, respectively. Measurements for the 8 mm collimator and composite field dosimetry are in progress.
SU‐FF‐T‐395: The Effect of Lung Heterogeneity and Respiration On Proton Therapy Treatment Margins
Authors: Tafo A Guemnie , Veltchev I , Li J , Fan J , Fourkal E , Ma C
Published: April 2010

Purpose: A critical parameter in lung proton therapy is the range determination, which is related to the treatment margins required to build the PTV. Factors affecting treatment margins include intrafractional tumor motion, interfractional tumor deformation, and anatomic changes. This work investigates the effect of lung heterogeneity and respiratory motion on proton dose distributions using CT scans at different respiratory phases. Method and Materials: Ten previously treated lung patients were recruited for this study. For each patient 4D CT scans were obtained with 10 respiratory phases. The variation of lung density at different respiratory phases and the effective distance from the patient surface to the back of the target volume were analyzed using specially designed software. Monte Carlo simulations were performed to calculate proton dose distributions with different energies and field sizes. The average lung density for a 1cm beamlet was computed for different target locations and beam orientations to provide useful information for advanced proton therapy treatment planning. The density calculation was performed in lung regions without major heterogeneities. Tumor motion and setup uncertainties were not included in this study. Results: Results showed a density difference between the inhale and the exhale phases of up to 0.25g/cm3 (the lung average density is 0.17g/cm3 for the inhale phase). Taking into account the real lateral size of the lung for each patient, this difference leads to a 1.3cm difference between the two extreme phases. The effects of lung heterogeneity vary based on the target location and beam orientation. Monte Carlo calculated dose distributions showed variable distal edge shifts and SOBP plateau deformation. Conclusion: The effects of lung heterogeneity and respiration vary significantly, which require precise margin determination for individual patients in advanced proton therapy with beam scanning and intensity modulation. Additional margins will be needed for population‐based PTV.
SU‐FF‐T‐409: Simulation of the Risk of Developing a Second Cancer Due to the Scattered Radiation for Different Treatment Modalities at Different Sites
Authors: Athar B , Seco J , Hancox C , Paganetti H
Published: April 2010

Purpose: Pediatric patients undergoing radiation therapy receive higher doses at organs and tissues lateral to the primary field edge and stand a higher risk of developing secondary cancers during the span of their longer post treatment lives. There is a concern in particular for modern treatment techniques, like IMRT or proton therapy. Consequently, we did extensive simulations on organ specific equivalent doses. Method and Materials: Age and gender specific whole‐body phantoms (an adult, a 9 month old male, a 4‐year old female, an 8‐year old female, an 11‐year old male, and a 14‐year old male) have been implement in Geant4 in order to determine the doses from the secondary radiation in patients undergoing proton and IMRT treatments. We have used several fields planned for different treatment sites (head & neck, abdominal) to determine the secondary organ doses. Lifetime attributable risk (LAR) is also determined based on two risk models, namely, excess relative risk and excess absolute risk. Results: For passive scattered proton therapy, neutron equivalent doses to various organs are dominated by the treatment head contribution for small fields and by the patient contribution for large fields. Passive scattered proton therapy and scanned beam proton therapy were simulated separately. The maximum dose organs received in the spinal field was 8.5mSv/Gy. Simulated LAR for passive scattered proton therapy for an 8‐year old female and 11‐year old male patients for the lung are found to be 1.05% and 0.38%, respectively. Conclusions: We find the scattered photon and neutron doses to various organs and tissues decrease with phantom age and it depends on the field size. Breast in female and lung/bronchi in male patients show the largest risk but also the baseline values for these organs are the highest. For most organ the risks are below the baseline risk.