Build-up Cap Research Articles

Angular distributions of absorbed dose of Bremsstrahlung and secondary electrons induced by18-, 28- and 38-MeV electron beams in thick targets

Angular distributions of absorbed dose of Bremsstrahlung photons and secondary electrons at a wide range of emission angles from 0 to 135°, were experimentally obtained using an ion chamber with a 0.6 cm(3) air volume covered with or without a build-up cap. The Bremsstrahlung photons and electrons were produced by 18-, 28- and 38-MeV electron beams bombarding tungsten, copper, aluminium and carbon targets. The absorbed doses were also calculated from simulated photon and electron energy spectra by multiplying simulated response functions of the ion chambers, simulated with the MCNPX code. Calculated-to-experimental (C/E) dose ratios obtained are from 0.70 to 1.57 for high-Z targets of W and Cu, from 15 to 135° and the C/E range from 0.6 to 1.4 at 0°; however, the values of C/E for low-Z targets of Al and C are from 0.5 to 1.8 from 0 to 135°. Angular distributions at the forward angles decrease with increasing angles; on the other hand, the angular distributions at the backward angles depend on the target species. The dependences of absorbed doses on electron energy and target thickness were compared between the measured and simulated results. The attenuation profiles of absorbed doses of Bremsstrahlung beams at 0, 30 and 135° were also measured.

Reconsideration of the Iwasaki–Waggener iterative perturbation method for reconstructing high-energy X-ray spectra

We have reviewed applicable ranges for attenuating media and off-axis distances regarding the high-energy X-ray spectra reconstructed via the Iwasaki-Waggener iterative perturbation method for 4-20 MV X-ray beams. Sets of in-air relative transmission data used for reconstruction of spectra were calculated for low- and high-Z attenuators (acrylic and lead, respectively) by use of a functional spectral formula. More accurate sets of spectra could be reconstructed by dividing the off-axis distances of R = 0-20 cm into two series of R = 0-10 cm and R = 10-20 cm, and by taking into account the radiation attenuation and scatter in the buildup cap of the dosimeter. We also incorporated in the reconstructed spectra an adjustment factor (f (adjust) ≈ 1) that is determined by the attenuating medium, the acceleration voltage, and the set of off-axis distances. This resulted in calculated in-air relative transmission data to within ±2 % deviation for the low-Z attenuators water, acrylic, and aluminum (Al) with 0-50 cm thicknesses and R = 0-20 cm; data to within ±3 % deviation were obtained for high-Z attenuators such as iron (Fe), copper (Cu), silver (Ag), tungsten (W), platinum (Pt), gold (Au), lead (Pb), thorium (Th), and uranium (U) having thicknesses of 0-10 cm and R = 0-20 cm. By taking into account the radiation attenuation and scatter in the buildup cap, we could analyze the in-air chamber response along a line perpendicular to the isocenter axis.

SU-E-T-176: Improved Collimator Scattering Factor (Sc) Measurements for Small Fields Using Build-Up Caps in Robotic Radiosurgery.

The Monte Carlo calculation algorithm in the MultiPlan (Accuray, Palo Alto, CA) treatment planning system used for CyberKnife (Accuray) robotic radiosurgery requires in-air measurements. In this study, results were compared for the impact of build-up caps using a diode detector for small field in-air measurements. Two acrylic caps custom-made for the SFD diode detector (IBA, Germany) of two thicknesses were compared against free-in-air measurements. The 1.5 and 5 cm thicknesses correspond to Dmax and a depth beyond the range of electron contamination, respectively. A Blue Phantom (IBA) was used to position the diode 80 cm SAD. Measurements were made for all 12 fixed cones ranging from 5 to 60 mm. For the 5 mm cone, there is a 15.6% and 20.0% difference in the Sc factor between the 1.5 cm and 5 cm buildup caps, respectively, versus the free-in-air measurement. For the 7.5 mm cone, the difference is 6.3% and 10.4% for the 1.5 cm and 5 cm buildup caps, respectively, versus the free-in-air measurement. While generally decreasing, the Sc factor for either buildup cap does not agree within 2% to free-in-air measurements until the cone used is larger than 40 mm. Overall, the two buildup caps yield similar Sc factors and the differences observed are attributed to electron contamination. The Monte Carlo calculation algorithm used by the CyberKnife planning manual does not refer to any build-up caps. This study has shown that, particularly for small fields, there are large differences in Sc factors measured with and without buildup caps. In general, for fields smaller than 10 mm, build-up caps should be carefully chosen, especially in commissioning data.

SU-E-T-536: Is BJR Supplement #25 Recommendation for Megavoltage Energy Independent Scatter Factor Still Valid for Flattening Filter Free Photon Beams?

In the process of measuring and validating fundamental dosimetry data prior to the clinical use of a treatment unit, it is prudent to compare measurements with previously published equivalent data. During the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian True Beam 6 MV FFF and 10 MV FFF) we compared measured Phantom Scatter Factors (Sp) with the Normalized Peak Scatter Factors (NPSFs) from the British Journal of Radiology Supplement 25 (BJR #25). The purpose of this work was to determine whether the energy independent NPSFs BJR#25 are valid for comparison with FFF photon beams. All measurements were performed using a Varian TrueBeam linear accelerator with photon energies of 6-MV, 6-MV FFF and 10-MV FFF modes. For all measurements, a Scanditronix CC04 ionization chamber was used. Both water and in-air measurements were made to obtain NPSFs normalized to the 10 × 10 cm2 field size. For measurements in water, the chamber was positioned at 100 cm source-to-axis distance at the depth of dose maximum. For in-air measurements, the chamber was positioned at 100 cm source-to-axis distance with appropriate build-up cap. From BJR #25, NPSFs were obtained for comparison with the measurements. The NPSF agreement between the 6-MV and 6-MV FFF with the BJR#25 were all within ±0.5%. The agreement ranged from 0.996 to 1.004 and 0.995 to 1.002 for 6-MV and 6-MV FFF, respectively. We also found that 10-MV FFF showed very similar trend. The scatter factors reported in BJR #25 are valid for comparison for 6-MV, 6-MV FFF, and 10-MV. Additional investigation is needed to further understand the dosimetric characteristics of FFF mode.

An improved physics-based approach for unfolding megavoltage bremsstrahlung spectra using transmission analysis

To develop a physics-based approach to improve the accuracy and robustness of the ill-conditioned problem of unfolding megavoltage bremsstrahlung spectra from transmission data. Spectra are specified using a rigorously-benchmarked functional form. Since ion chambers are the typical detector used in transmission measurements, the energy response of a Farmer chamber is calculated using the EGSnrc Monte Carlo code, and the effect of approximating the energy response on the accuracy of the unfolded spectra is studied. A proposal is introduced to enhance spectral sensitivity by combining transmission data measured with multiple detectors of different energy response and by combining data from multiple attenuating materials. Monte Carlo methods are developed to correct for nonideal exponential attenuation (e.g., scatter effects and secondary attenuation). The performance of the proposed methods is evaluated for a diverse set of validated clinical spectra (3.5-25 MV) using analytical transmission data with simulated experimental noise. The approximations commonly used in previous studies for the ion-chamber energy response lead to significant errors in the unfolded spectra. Of the configurations studied, the one with best spectral sensitivity is to measure four full transmission curves using separate low-Z and high-Z attenuators in conjunction with two detectors of different energy response (the authors propose a Farmer-type ion chamber, once with a low-Z, and once with a high-Z buildup cap material), then to feed the data simultaneously to the unfolding algorithm. Deviations from ideal exponential attenuation are as much as 1.5% for the smallest transmission signals, and the proposed methods properly correct for those deviations. The transmission data with enhanced spectral sensitivity, combined with the accurate and flexible spectral functional form, lead to robust unfolding without requiring a priori knowledge of the spectrum. Compared with the commonly-used methods, the accuracy is improved for the unfolded spectra and for the unfolded mean incident electron kinetic energy by at least factors of three and four, respectively. With simulated experimental noise and a lowest transmission of 1%, the unfolded energy fluence spectra agree with the original spectra with a normalized root-mean-square deviation, %Δ(ψ), of 2.3%. The unfolded mean incident electron kinetic energies agree, on average, with the original values within 1.4%. A lowest transmission of only 10% still allows unfolding with %Δ(ψ) of 3.3%. In the presence of realistic experimental noise, the proposed approach significantly improves the accuracy and robustness of the spectral unfolding problem for all therapy and MV imaging beams of clinical interest.

In vivodosimetry with optically stimulated luminescent dosimeters, OSLDs, compared to diodes; the effects of buildup cap thickness and fabrication material

For external beam in vivo measurements, the dosimeter is normally placed on the patient's skin, and the dose to a point of interest inside the patient is derived from surface measurements. In order to obtain accurate and reliable measurements, which correlate with the dose values predicted by a treatment planning system, a dosimeter needs to be at a point of electronic equilibrium. This equilibrium is accomplished by adding material (buildup) above the detector. This paper examines the use of buildup caps in a clinical setting for two common detector types: OSLDs and diodes. Clinically built buildup-caps and commercially available hemispherical caps are investigated. The effects of buildup cap thickness and fabrication material on field-size correction factors, C(FS), are reported, and differences between the effects of thickness and fabrication material are explained based on physical parameters. Measurements are made on solid water phantoms for 6 and 15 MV x-ray beams. Two types of dosimeters are used: OSLDs, InLight∕OSL Nanodot dosimeters (Landauer, Inc., Glenwood, IL) and a P-type surface diode (Standard Imaging, Madison, WI). Buildup caps for these detectors were fabricated out of M3, a water-equivalent material, and sheet-metal stock of Al, Cu, and Pb. Also, commercially available hemispherical buildup caps made of plastic water and brass (Landauer, Inc., Glenwood, IL) were used with Nanodots. OSLDs were read with an InLight microStar reader (Landauer, Inc., Glenwood, IL). Dose calculations were carried out with the XiO treatment planning system (CMS∕Elekta, Stockholm) with tissue heterogeneity corrections. For OSLDs and diodes, when measurements are made with no buildup cap a change in C(FS) of 200% occurs for a field-size change from 3 cm × 3 cm to 30 cm × 30 cm. The change in C(FS) is reduced to about 4% when a buildup cap with wall thickness equal to the depth of maximum dose is used. Buildup caps with larger wall thickness do not cause further reduction in C(FS). The buildup cap fabrication material has little or no effect on C(FS). The perturbation to the delivered dose caused by placing a detector with a buildup cap on the surface of a patient is measured to be 4%-7%. A comparison between calculated dose and dose measured with a Nanodot and a diode for 6 and 15 MV x-rays is made. When C(FS) factors are carefully determined and applied to measurements made on a phantom, the differences between measured and calculated doses were found to be between ±1.3%. OSLDs and diodes with appropriate buildup caps can be used to measure dose on the surface of a patient and predict the delivered dose to depth dmax in a range of ±1.3% for 100 cGy. The buildup cap: can be fabricated from any material examined in this work, is best with wall thickness dmax, and causes a perturbation to the delivered dose of 4%-7% when the wall thickness is dmax. OSLDs and diodes with buildup caps can both give accurate measurements of delivered dose.

SU-E-T-362: Small Field Dosimetry for Double Scattering Proton Beams

Purpose: This study examines the accuracy of dose calculation in small square fields for double scattering proton beams using an empirical method based on dose per incident fluence. Methods: Fractional Depth doses (FDD) at SSD = 220 cm for open beam and small square fields (s = 1, 2, 3, 5, 10 and 15cm) are renormalized to surface. The resulting pristine peaks depth dose is normalized to the broad beam FDD to determine a fractional FDD, FDDf for selected proton energies (90 – 200 MeV). The head scatter factor (H), defined as the ratio of proton energy fluence at point of interest to 10×10 cm2 at otherwise the same geometry, is measured using a diode in air without buildup cap as a function of square field sizes for proton energies. 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 field size, energy, and SSD. Results: It is found that H is a single function of renormalized equivalent square, sN = s*230/(SSD+d) and is independent of SSD for a given proton energy. Normally, the s dependence of H is a function of SSD. 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%. Conclusions: By normalizing the Fractional depth dose (FDD) to the surface, most characteristics of small field dosimetry can be simplified and adequately described by H, a quantity describing the incident proton energy fluence.

Multichannel Dosimeter and $\alpha$-Al$_{2}$O$_{3}$:C Optically Stimulated Luminescence (OSL) Fiber Sensors for Use in Radiation Therapy—Evaluation With Photon Beams

A multichannel OSL fiber optic dosimeter based on optically stimulated luminescence (OSL) of alumina is proposed for online in vivo dosimetry (IVD) in radiation therapy (RT). Two types of dosimetric-grade Al2O3:C crystals are compared and show different behavior according to manufacturing process. Metrological validations have been performed with a Saturne 43 LINAC in reference conditions at CEA LIST LNHB (French Ionizing Radiation Reference Laboratory). The dose response of OSL integrals under photon beam irradiation (6, 12, and 20 MV) show sublinearity behavior modeled by second-order equations and exhibit a small energy dependence (between 0.7% and 1.4%), explained by a modified intermediate cavity model adapted to a LINAC photon spectrum. Preclinical tests at Institut Gustave Roussy (IGR) prove that a proper design for a PMMA build-up cap leads to a low dependence vs photon beam orientation (± 1.5% and ± 0.9%) and vs field size in view of surface measurements.

Effect of the calibration in water and the build-up cap on the Mg(Ar) ionization chamber measurements

Magnesium-walled argon gas flow ionization chamber (Mg(Ar)) is used for photon dose measurements in the epithermal neutron beam of FiR 1 reactor in Finland. In this study, the photon dose measurements were re-evaluated against calculations applying a new chamber calibration factor defined in water instead of in air. Also, effect of the build-up cap on the measurements was investigated. The new calibration factor provides improved agreement between measured and calculated photon dose. Use of the build-up cap does not affect the measured signal in water in neutron beam.

Effect of radiation-induced charge accumulation on build-up cap on the signal current from an ionisation chamber

The signal current from a thimble ionisation chamber with a build-up cap made of an insulator decreases by about 0.41 % after being irradiated for 17 h at an air kerma rate of 41 Gy h(-1) by a collimated (60)Co gamma-ray beam in air. In contrast, the signal current remains constant when the thimble ionisation chamber is irradiated in a water phantom. During irradiation, positive charge is considered to accumulate near the outer surface of the build-up cap where electron equilibrium is not achieved. Secondary electrons travelling in the build-up cap and the chamber wall toward the ionisation volume are decelerated by the electric field generated by the positive charge. Consequently, the signal current decreases with increasing charge accumulation because some secondary electrons are prevented from entering the ionisation volume. In the water phantom, electron equilibrium is established in and around the ionisation chamber and charge does not accumulate. To confirm this hypothesis, the signal current was measured for an ionisation chamber in air with a build-up cap wrapped with Al foil and covered with PMMA tubes. Electron equilibrium was established over the build-up cap because the tubes were thicker than the secondary electron range. The signal current decreased with increasing positive voltage applied to the Al foil. It was estimated from the results that positive charges equivalent to a voltage of over 6 kV applied to the Al foil accumulated during irradiation. The signal current was also measured for an ionisation chamber with a metal build-up cap and for an ionisation chamber with a wall and build-up cap made of conductive plastic.

Teeth restoration for overdenture support

Introduction. If patients have remained only a few teeth in one jaw, these teeth can be retained and specifically prepared to bear mobile supradental prosthesis i.e. overdenture (OD). The patients are usually very motivated to preserve remained teeth. The aim of this study was to present the plan, the preparation procedure and the treatment protocol for the remaining teeth that will bear full mobile OD. Material and Methods. Study included 12 patients and 23 remaining teeth. Treatment plan considered: radiological analysis and selection of the remaining teeth, reduction of the clinical crown of the teeth, endodontic and periodontal treatment and finally conservative or prosthodontic restoration of these teeth. Fifteen teeth were restored using cast crown build-up (cap) and eight teeth were restored with amalgam restorations. Patients who received twelve lower mobile full OD were observed for one year to assess the integrity of the extra alveolar tooth structure. Results. After observation period of one year, the integrity of crown restorations was maintained as well as the alveolar bone height. Conclusion. The restoration of the remaining teeth using cast crown build-up (cap) and amalgam is very important for maintaining extra alveolar tooth structure and successful treatment with overdentures.

Sci—Thur PM: YIS — 01: Computational and Experimental Methods to Address the Limitations of Reconstructing Linac Photon Spectra from Transmission Measurements

In the past two decades, the ill‐conditioned problem of unfolding linac photon spectra from transmission measurements has been extensively investigated. Previous studies suffer from various limitations that affect the accuracy and robustness of the unfolding. The goal of this study is to address the limitations of unfolding in general, and using parameterization of the spectrum in particular. To this end, the following new and refined methods are implemented. Different attenuator materials and different buildup caps are simultaneously used for better energy differentiation in most of the megavoltage energy range. The spectra are described using a new flexible and physics‐based functional form validated against more than 70 realistic spectra. The radiation detection system is modeled accurately using the EGSnrc usercodes BEAMnrc and cavity. Forward Compton scatter in the attenuators is modeled and corrected for. The proposed methods are validated experimentally on NRC research linac whose incident electron beam parameters are accurately known to 0.5% and its spectra have been independently measured using a NaI detector. A linear system was built to automatically drive attenuator lengths one at a time into the beam, which reduces acquisition time and beam instability uncertainties. Causes for experimental Type B uncertainties (leakage, polarity and scatter) are being investigated, particularly for small transmission signals. Results show that the proposed methods significantly improve the accuracy and robustness of unfolding in the presence of realistic experimental noise. Improvements by factors of 4 and 8 have been achieved in the accuracy of spectral unfolding and maximum energy estimation, respectively.

SU-GG-T-611: Investigation of Different Carbon Fiber Tabletops Used in Radiation Therapy

Purpose: The dose changes in the buildup region and beam attenuation by several carbon fiber tabletops were investigated for 6 and 10 MV photon beams. Method and Materials: Measurements were performed for 3 × 3, 10 × 10, and 25 × 25 cm2 field sizes. The surface dose and percentage depth doses (PDD) were measured using a Markus parallel plate ionization chamber. A Farmer type ionization chamber with appropriate buildup caps have been used for attenuation measurement at several gantry angles (from 0° to 180°). Oncor's couch (Siemens schematic G5499), Primus' couch (MED-TEC model IL3005), and CT's couch (MED-TEC model IL3302) have been studied. Results: For a 6 MV beam Oncor couch increases the surface dose from 16.8% to 77.6% for small fields. Attenuation varies from 1% to 4.7% for 180° and 120° gantry angles. At 6 and 10 MV beams respectively, Siemen's couch increases the dose from 17.8% to 43.2% and from 10.0% to 28.5%, the beam attenuation varies from 0.1% to 8.6% and 0.2% to 3.1% for 180° and 120° gantry angles. The increase of the surface dose for the CT couch is from 17.5% to 77.8% and 9.6% to 58.9% for 6 and 10 MV beams respectively, and beam attenuation varies from 1% to 4.7% and 1% to 2.4% for 180° and 120° gantry angles. Conclusion: The carbon fiber tabletops significantly decrease the skin sparring effect. Our data show that the surface dose increase in the buildup region is about the same for the three couches. The dosimetric effect of the tabletop may be higher, especially for intensity-modulated radiation therapy depending on the beam orientation. Attenuation should be considered and corrected such as any material under the patient at the treatment planning stage. Keywords: Carbon fibre tabletops, Oncor, Primus, CT, PDD

SU-GG-T-95: The Effect of Cloth and Paper Gowns on Skin Dose for 6 MeV Total Skin Electron Treatments

Purpose: Traditionally, patients treated with total skin electrons are required to remove all clothing. This is assumed to be best for the treatment of superficial skin lesions out of concern clothing may significantly perturb the electron dose delivered. However, no known peer‐reviewed studies have been done to validate this. We investigate what effect cloth and paper gowns will have on dose near the skin surface. Method and Materials: Using a Markus ion chamber and Gafchromic‐EBT2 film, dose to a cylindrical phantom was measured with a cloth hospital gown, a paper gown, and a tri‐layer cloth (to simulate folds and seams) compared to no covering. A 6‐MeV electron beam with spoiler accessory was used at a distance of about 4‐meters, with gantry angled at 248° and 292°. The materials covered the films which were placed onto the phantom at 1000‐MU per shot. The phantom was rotated at −60°, 0°, and 60° relative to the beam's central axis simulating the six‐field Stanford technique. Similar procedure was followed for Markus chamber with 1‐mm buildup cap in a Lucite phantom. Effect of air gap of 0, 3, and 5‐cm between full‐size gowns and ion chamber also investigated. Results: Compared to no covering, the films revealed a 0.8% increase in dose for cloth, 1.8% for tri‐layered cloth, and 0.7% for paper. Markus chamber readings revealed at 0°, 60° orientation −0.03%, −1.5% for cloth, 1.4%, −4.1% for tri‐layered cloth, and 0.2%, −0.2% for paper, respectively. Air gaps had differences by less than 0.7% for cloth and 0.2% for paper. Conclusion: For cloth and paper gowns there was a variability of less than 1.5% in dose delivered. Air gaps did not substantially affect dose but folds and seams in cloth may perturb dose more than 4%. For patient comfort, paper gowns may be most appropriate covering option.

TU‐B‐BRA‐01: Total Body Irradiation (TBI): Looking from Differrent Angles

Purpose: To explore the new degree of freedom of VMAT to design a novel technique for TBI, in which patient is positioned supine/prone on the floor and an arc field is delivered with variable dose rate and/or dynamic collimators. Method and Materials: A formalism of designing an arc field with variable dose rate and/or dynamic jaw/MLC to deliver an arbitrary one‐dimensional dose profile is developed. The dose rate and/or jaw/MLC position are obtained by minimizing the difference between prescribed and calculated dose at each spatial point. The formalism is applied to design TBI treatment plan to achieve (1) a uniform dose over the entire body, and (2) a reduced dose in the lung region. MATLab codes were developed to solve the optimization problem using gradient decent method. Three types of treatment plans were generated: (1) variable dose rate (VDR) only, (2) VDR with dynamic MLC (DMLC), and (3) DMLC only. The optimized treatment plans were converted into DICOM‐RT format and delivered in a Varian 21‐EX machine capable of Rapidarc in DICOM‐RT mode. Measurements with farmer chamber in air with buildup cap were performed and compared with the optimization results. Results: For a patient with 200cm height placed at 100cm below the linac iso‐center, the differences between measured profile from plans with VDR only for uniform/lung sparing case are (−1.4±1.9)% and (−0.2±2.4)%, respectively. They improved to (−0.4±1.0)% and (−0.5±2.2)% with VDR+DMLC. The latter also had 40% higher efficiency. The results for DMLC only were less desirable with reduced range of (−85cm, 85cm) and slightly higher discrepancies. Conclusion: It is feasible to deliver TBI using VMAT, thereby eliminating the need for large treatment room size or hand‐made compensators. Regular linac operating in arc therapy with dynamic MLC alone can be also used, although with less efficiency.

Quantification and reduction of peripheral dose from leakage radiation on Siemens primus accelerators in electron therapy mode

In this work, leakage radiation from EA200 series electron applicators on Siemens Primus accelerators is quantified, and its penetration ability in water and/or the shielding material Xenolite‐NL established. Initially, measurement of leakage from 10×10−25×25 cm2 applicators was performed as a function of height along applicator and of lateral distance from applicator body. Relative to central‐axis ionization maximum in solid water, the maximum leakage in air observed with a cylindrical ion chamber with 1 cm solid water buildup cap at a lateral distance of 2 cm from the front and right sidewalls of applicators were 17% and 14%, respectively; these maxima were recorded for 18 MeV electron beams and applicator sizes of ≥20×20 cm2. In the patient plane, the applicator leakage gave rise to a broad peripheral dose off‐axis distance peak that shifted closer to the field edge as the electron energy increases. The maximum peripheral dose from normally incident primary electron beams at a depth of 1 cm in a water phantom was observed to be equal to 5% of the central‐axis dose maximum and as high as 9% for obliquely incident beams with angles of obliquity ≤ 40°. Measured depth‐peripheral dose curves showed that the “practical range” of the leakage electrons in water varies from approximately 1.4 to 5.7 cm as the primary electron beam energy is raised from 6 to 18 MeV. Next, transmission measurements of leakage radiation through the shielding material Xenolite‐NL showed a 4 mm thick sheet of this material is required to attenuate the leakage from 9 MeV beams by two‐thirds, and that for every additional 3 MeV increase in the primary electron beam energy, an additional Xenolite‐NL thickness of roughly 2 mm is needed to achieve the aforementioned attenuation level. Finally, attachment of a 1 mm thick sheet of lead to the outer surface of applicator sidewalls resulted in a reduction of the peripheral dose by up to 80% and 74% for 9 and 18 MeV beams, respectively. This sidewall modification had an insignificant effect on the clinical depth dose, cross‐axis beam profiles, and output factors.PACS numbers: 87.53.Bn, 87.56.bd, 87.56.J‐

SU‐GG‐T‐332: Dose to Contralateral Breast: A Comparison of Traditional Linac and Tomotherapy Techniques

Purpose: To characterize the dose and its distribution to the contralateral breast in patients undergoing whole‐breast radiotherapy using helical tomotherapy, and traditional linac with enhanced dynamic wedges or field‐in‐field techniques. Method and Materials: At the start of this IRB approved study, patients were selected and offered consent and enrollment. Dosimetry was performed using thermoluminescent dosimeters (TLDs) which were individually calibrated and tracked to generate a linear dose response curve for each. Positions for measurement were marked on the patient for reproducibility. The measurement positions were 5 cm superior, inferior, medial, and lateral to the breast center of the contralateral breast and 2 cm from center of sternum toward the contralateral breast. TLDs were placed in buildup caps to ensure buildup to electronic equilibrium. TLDs were placed daily for five consecutive treatments and then read 10 days after the last treatment. Calibration curves were applied to the TLD readings. Results: To date, three patients have been treated on Tomotherapy and five patients with traditional tangents. The contralateral breast doses, presented as a percentage of the prescription dose ± 1 standard deviation, are as follows: Tomotherapy; two cm from sternum 36 ± 8%, five cm medial 17 ± 6%, five cm superior 6 ± 1%, five cm lateral 6 ± 1%, five cm inferior 6 ± 1%. Traditional tangents; two cm from sternum 12 ± 5%, five cm medial 5 ± 2%, five cm superior 2 ± 1%, five cm lateral 2 ± 1%, five cm inferior 2 ± 1%. The average breast dose, calculated as the arithmetic mean of the doses five cm from the breast center, were 9 ± 6% for Tomotherapy and 3 ± 2% for traditional tangents. Conclusion: At our institution, helical tomotherapy produces doses in the contralateral breast that are approximately three times that of traditional tangents.

Photon beam quality variations of a flattening filter free linear accelerator

Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams. Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm). For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta = 10 degrees. These results agree within +/- 2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV. Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified.

Poster — Wed Eve—32: Internal Target Volume Dose Coverage Measurement for Respiratory Tumor Motion

The purpose of this study was to evaluate the difference between planned and delivered doses within the ITV generated by the co‐alignment of the standard and slow CT scan. A QUASAR™ phantom with a cedar insert (to simulate the lung) and an ionization chamber with buildup cap was scanned twice using modified slow CT scan technique. The respiratory cycle used for CT scan was 4 seconds/cycle and 10.0 mm amplitude along the longitudinal axis of the phantom. The two image sets were co‐aligned using the same reference points. The volume of the ionization chamber was used as a gross tumor volume (GTV) and was contoured on each scan. The ITV was generated by enveloping two GTVs from the two CT scans. The PTV was created by adding a 5mm margin around the ITV. If the chamber movement is within 1 cm, the minimum ITV dose coverage is 98.5%. The ITV dose coverage drops dramatically when the movement is larger than 1cm. The ITV dose coverage drops from 96.3% in 1.1cm motion to 90.7% in 1.5 cm motion. This study evaluated the difference between planned and delivered dose to the GTV using an ITV generated by the co‐alignment of the standard and slow CT scan. Co‐alignment of a slow CT and a standard CT underestimates the amplitude of GTV motion by 6mm. This results in under‐dosing of the ITV by 1.5%. The discrepancy between the TPS and measurement was 0.8%, for a total discrepancy of 2.3 %.

SU-FF-T-641: Testing the Sc, Sp, Scp Formulism for Elekta Beam Modulator

Purpose: The Elekta Beam Modulator™ (EBM) has a unique design in the sense that no movable jaws exist. The MLC has 40 leaf pairs with 4 mm wide leaves. A fixed diaphragm is beneath the MLC to form an aperture of 21 cm by 16 cm. The radiation fields are shaped primarily by the MLC. The goal of this work is to test whether the conventional Sc, Sp, Scp formalism holds true for this accelerator head configuration. Method and Materials: We have postulated two methods for determining Sc for the EBM: Method A: set Sc = 1 for all fields because there are no movable jaws; and Method B: regard the MLC as the “jaws”, since the apertures are formed by the MLC. We measured Sc for Method B using Wellhofer CC13 and PTW 30010 ion chamber with build-up caps. Both setups were configured in RadCalc and calculations compared to measurements. Results: 6 MV results are reported. For 100 MU, dose was measured for standard square, rectangle, and off-axis irregular shaped fields. For the standard fields, both methods agree with measurements to within 0.5%, except for the smallest field 2.4×2.4 cm2. For the off-axis and irregular fields, both calculations differ from the measured dose by 3∼7%. The two methods agree within 0.5% for most standard fields, and the difference between them tends to increase for the off-axis and irregular fields. Conclusions: Two methods to calculate Elekta Beam Modulator (EBM) point doses using the Sc, Sp, Scp formalism were tested. Both agree well with the measurement for the standard fields. However, they start to deviate from the measurements for off-axis and irregular fields. More testing, including 10MV and 15MV, is planned and will be presented for this unique machine.