Build-up Cap Research Articles

Dataset for Beam Commissioning of the Vero4DRT System

We present the results of measurements made using the Vero4DRT radiation therapy system, which is not yet widely used, to assist technicians in achieving reliable and safe radiotherapy to the patient. We measured percent depth dose, beam profile, and relative scatter factor under water and air conditions. The Vero4DRT system has a 150 × 150-mm fixed secondary collimator. Its multileaf collimator (MLC) design is a single-focus type, with 30 pairs of 5 mm thick leaves at the isocenter, and produces a maximum field size of 150 × 150 mm. Profile measurements were performed using a 0.016-cm3 ionization chamber (PTW31016 pinpoint chamber; PTW, Freiburg GmbH Germany). A brass build-up cap was used for measurements obtained in air conditions. We present a useful measurement dataset for users of the Vero4DRT system.

SU-E-T-271: Direct Measurement of Tenth Value Layer Thicknesses for High Density Concretes with a Clinical Machine

Purpose: Use of high density concrete for radiation shielding is increasing, trading cost for space savings associated with the reduced tenth value layer (TVL). Precise information on the attenuation properties of high-density concretes is not readily present in the literature. A simple approximation is to scale the TVLs from NCRP 151 according relative increase in density. Here we present measured TVLs for heavy concretes of various densities using a built-in shielding test port. Methods: Concrete densities tested range from 2.35 g cc−1 (147 pcf) to 5.6 g cc−1 (350 pcf). Measurements were taken using 6MV, 6FFF, and 10FFF on a Varian Truebeam linear accelerator. Field sizes of 4x4, 9x9 and 30x30 cm2 were measured. A PTW 31013 Farmer chamber with a buildup cap was positioned 5.5 m from isocenter along the beam CAX. Concrete thicknesses were incremented in 5 cm intervals. Comparison TVLs were determined by scaling the NCRP 151 TVLs by the density ratio between the sample and standard density. Results: The trend from the first to equilibrium TVL was an increase in thickness, compared with MC modeling, which predicted a decrease. Measured TVLs for 6 MV were reduced by as much as 8.9 cm for TVL₁ and 3.4 cm for TVLE compared to values scaled from NCRP 151. There was 1–3 mm difference in TVL between measurements done at 4x4 versus 30x30 cm2. TVL₁ for 6FFF was 1.1 cm smaller than TVL₁ for 6MV, but TVLE was consistent to within 4 mm. TVL₁ and TVLE for 10FFF were reduced by 8.8 and 3.7 cm from scaled NCRP values, respectively. Conclusions: We have measured the TVL thicknesses for various concretes. Simple density scaling of the values in NCRP 151 is a conservatively safe approximation, but actual TVLs may be reduced enough to eliminate some of the expense of installation. Daniel Harrell and Jim Noller are employees of Shielding Construction Solutions, Inc, the shielding construction company that built the vault discussed in this abstract. Manjit Chopra is an employee of Universal Minerals International, Inc, the company that provided the aggregates for the high density concretes used in the vault construction.

Technical Note: Response measurement for select radiation detectors in magnetic fields

Dose response to applied magnetic fields for ion chambers and solid state detectors has been investigated previously for the anticipated use in linear accelerator-magnetic resonance devices. In this investigation, the authors present the measured response of selected radiation detectors when the magnetic field is applied in the same direction as the radiation beam, i.e., a longitudinal magnetic field, to verify previous simulation only data. The dose response of a PR06C ion chamber, PTW60003 diamond detector, and IBA PFD diode detector is measured in a longitudinal magnetic field. The detectors are irradiated with buildup caps and their long axes either parallel or perpendicular to the incident photon beam. In each case, the magnetic field dose response is reported as the ratio of detector signals with to that without an applied longitudinal magnetic field. The magnetic field dose response for each unique orientation as a function of magnetic field strength was then compared to the previous simulation only studies. The measured dose response of each detector in longitudinal magnetic fields shows no discernable response up to near 0.21 T. This result was expected and matches the previously published simulation only results, showing no appreciable dose response with magnetic field. Low field longitudinal magnetic fields have been shown to have little or no effect on the dose response of the detectors investigated and further lend credibility to previous simulation only studies.

Optimized Construction of Multi-Channel Brachytherapy Cylinder Applicators

The purpose of this work is to identify an optimal construction for rigid multi-channel brachytherapy cylinder applicators, such that the clinical user can maximize lateral dose coverage while minimizing surface and normal tissue doses. Cylindrical multi-channel applicators are commonly used in HDR gynecological brachytherapy to treat the post-hysterectomy vaginal cuff. Literature suggests that adjuvant vaginal brachytherapy has high efficacy and low morbidity for intermediate-risk endometrial cancer [1]. Commercial vaginal cylinders may not have fully considered dosimetric consequences of the cylinder’s construction. Therefore we seek to provide manufacturers with guidance on the optimal radial offsets for peripheral channels within multi-channel cylinder applicators. Commercial multi-channel applicators typically consist of a cylindrical obturator with various diameter build-up caps. The obturator contains one central channel and two or more peripheral channels positioned at a fixed radial distance from the central channel. The radial distance from the central channel to peripheral channels affects the radioactive loading/weighting and usability of peripheral channels. For example, peripheral channels that are located very close to the patient surface cannot be loaded without quickly exceeding a mucosal surface dose limit (e.g. 200%). An ideal radial offset for peripheral channels can maximize asymmetric peripheral dose coverage of the target while maintaining surface dose <200% of prescription dose. This work simulated a multichannel cylinder consisting of a central channel and two lateral channels. A point source approximation was used. Initially, three Ir-192 VS200 sources were simulated for a vaginal cuff treatment, one source in each channel. Optimal dwell times were determined using the methods of Deufel and Furutani [2] to achieve 100% prescription dose 5 mm apically and 5 mm posteriorly from the cylinder surface, with mucosal surface dose <200%. (See Fig1 (a)) The radial distance of peripheral channels was varied from 0 to Rapplicator. The optimal radial distance, dlat, is the distance where lateral 100% dose coverage is maximum. This method was then extended to a vaginal cuff plus vault treatment for a cylinder with 6.5cm treatment length (Fig1 (b)). Dose optimization points were placed at the apex, along the cylinder surface and posterior 5mm along the treatment length to produce a homogenous dose. Results were obtained for cylinders with 3.0, 3.5, and 4.0 cm diameters. A comparison of point and line source results was also performed. Fig 1(c) demonstrates the lateral coverage as a function of dlat for a 3.5 cm diameter cylinder. The optimal lateral channel position is located at 2.8 mm, 4.1 mm and 5.2 mm away from the central channel for the cylinder in diameters of 3, 3.5 and 4cm and provides maximal lateral coverage of 5.7 mm, 6.3 mm and 7 mm from the applicator surface, respectively. Doses 5mm apically, 5mm posteriorly, and at the lateral surface are shown in Fig 1(d). The optimal radial distance for peripheral channels increases and maximum lateral coverage distance increases as cylinder diameter increases. Figs 1(e) and (f) provides results for cuff plus vault treatment. Compared to three-source loading, cuff plus vault treatment with more inferior loading can push dose more laterally (e.g. 7.4 mm vs. 4.1 mm for the 3.5cm applicator) This work proposes a method for optimized construction geometry of multi-channel vaginal cylinders. Manufacturers may use results to construct an applicator that provides maximum lateral coverage while maintaining normal tissue constraints. Furthermore, this work shows that a library of Atlas plans for vaginal cuff treatment may be efficiently and optimally generated using the methods of Deufel and Furutani.

SU-E-T-59: Calculations of Collimator Scatter Factors (Sc) with and Without Custom-Made Build-Up Caps for CyberKnife

Purpose: To investigate the impact of custom-made build-up caps for a diode detector in robotic radiosurgery radiation fields with variable collimator (IRIS) for collimator scatter factor (Sc) calculation. Methods: An acrylic cap was custom-made to fit our SFD (IBA Dosimetry, Germany) diode detector. The cap has thickness of 5 cm, corresponding to a depth beyond electron contamination. IAEA phase space data was used for beam modeling and DOSRZnrc code was used to model the detector. The detector was positioned at 80 cm source-to-detector distance. Calculations were performed with the SFD, with and without the build-up cap, for clinical IRIS settings ranging from 7.5 to 60 mm. Results: The collimator scatter factors were calculated with and without 5 cm build-up cap. They were agreed within 3% difference except 15 mm cone. The Sc factor for 15 mm cone without buildup was 13.2% lower than that with buildup. Conclusion: Sc data is a critical component in advanced algorithms for treatment planning in order to calculate the dose accurately. After incorporating build-up cap, we discovered differences of up to 13.2 % in Sc factors in the SFD detector, when compared against in-air measurements without build-up caps.

SU‐E‐T‐07: A Comparison of SSD and Field Size Dependence of Output for Megavoltage Electron Beams for Different Manufacturers

Purpose:The purpose of this study was to compare the electron beam in‐air output factor (H) for Varian and Siemens accelerators.Methods:Measurements were performed among four accelerators: Varian CL2300ix, Varian TrueBeam, Siemens Oncor, and Siemens Primus. The nominal electron energy measured ranges from 6 to 21 MeV. Measurements were made in air using a diode detector for different square cone sizes (6 – 25cm) with circular field inserts with radius r between 1 cm and 8 cm and for different SSD (100 – 120 cm). H is defined asH(r,SSD)= X(r,SSD)/X(open,SSD), where X is the charge measured by the diode detector without buildup cap in air.Results:For the same nominal energy, cone size, radius of circular cutout, and SSD, H agreed within a maximum deviation of 2.3% and standard deviation of 1% among Varian accelerators but can be as much as 8.7% different between the Varian and the Siemens accelerators. H is a function of electron energy and cutout radius only when the field radius for H measured at different SSD is renormalized as r = r0*5/(SSD‐95), where r0 is the physical field radius at the end of the cone. The radius dependence of H for five cone sizes (6×6, 10×10, 15×15, 20×20 and 25×25cm2) and three SSD(100, 110 and 120cm) for three energies was measured.Conclusion:The in air output factor (H) measured for the same manufacturer can be treated as identical to within 1.1% for Varian machines and to within 1.6% for Siemens machines, but the difference in H measured for the different manufacturers is large enough (&gt;2%) to require machine specific data set.

SU-E-T-98: Dependence of Radiotherapy Couch Transmission Factors On Field Size and Couch-Isocenter Distance

Purpose: The dosimetric effect of the treatment couch is non-negligible in today's radiotherapy treatment. To accurately include couch in dose calculation, we investigated the dependence of couch transmission factors on field size and couch-isocenter distance. Methods: Couch transmission factors for Varian Exact Couch were determined by taking the ratios of ionization of a posterior-anterior beam with and without the couch in the beam path. Measurements were performed at the isocenter using a PTW cylindrical ionization chamber (Model 31030) with an Aluminum buildup cap of 1.1 cm thick for the 6 MV photon beam. Ionization readings for beam sizes ranging from 2 × 2 cm2 to 40 × 40 cm2 were taken. Transmission factors for couch-isocenter distances ranging from 3 cm to 20 cm were also investigated. Results: The couch transmission factors increased with the field size approximately in an exponential manner. For the field sizes that we tested, the transmission factor ranged from 0.976 to 0.992 for couch-isocenter distance of 3 cm. The transmission factor was also monotonically dependent on couch-isocenter separation distance, but in a lighter magnitude. For the tested couch heights, the transmission factor ranged from 0.974 – 0.972 for 2 × 2 cm2 field size and 0.992 – 0.986 for 40 × 40 cm2 field size. The dependence on couch-isocenter distance is stronger for larger field size. Conclusions: The transmission factor of a radiotherapy treatment couch increases with field size of the radiation beam and its distance from the isocenter. Such characterization of the couch transmission factor helps improve the accuracy of couch modeling for radiotherapy treatment planning.

In-vivo dosimetry for field sizes down to 6 × 6 mm2 in shaped beam radiosurgery with microMOSFET

The aim of this study is to evaluate microMOSFET as in-vivo dosimeter in 6 MV shaped-beam radiosurgery for field sizes down to 6 × 6 mm2. A homemade build-up cap was developed and its use with microMOSFET was evaluated down to 6 × 6 mm2. The study with the homemade build-up cap was performed considering its influence on field size over-cover occurring at surface, achievement of the overall process of electronic equilibrium, dose deposition along beam axis and dose attenuation. An optimized calibration method has been validated using MOSFET in shaped-beam radiosurgery for field sizes from 98 × 98 down to 18 × 18 mm2. The method was detailed in a previous study and validated in irregular field shapes series measurements performed on a head phantom. The optimized calibration method was applied to microMOSFET equipped with homemade build-up cap down to 6 × 6 mm2. Using the same irregular field shapes, dose measurements were performed on head phantom. MicroMOSFET results were compared to previous MOSFET ones. Additional irregular field shapes down to 8.8 × 8.8 mm2 were studied with microMOSFET. Isocenter dose attenuation due to the homemade build-up cap over the microMOSFET was near 2% irrespective of field size. Our results suggested that microMOSFET equipped with homemade build-up cap is suitable for in-vivo dosimetry in shaped-beam radiosurgery for field sizes down to 6 × 6 mm2 and therefore that the required build-up cap dimensions to perform entrance in-vivo dosimetry in small-fields have to ensure only partial charge particle equilibrium.

On the Influence of HDR Ir-192 Source Position Uncertainty on Dose Variation within the Varian Ring Applicator

The tandem and ring (T&R) applicator is used as an alternate to tandem and ovoid (T&O) applicators to deliver HDR Ir-192 brachytherapy for cervical cancer. Since the ring interior lumen is larger than the HDR Ir-192 source capsule outer diameter, the source path within the ring is hard to predict. Some studies have shown that the actual source positions could be several millimeters different from planned positions, with potential for inducing significant dose deviation. However, other studies have demonstrated minimal dosimetric impact (Craciunescu et al 2012). Though most centers choose to implement a global source shift during commissioning (McMahon et al 2011) to limit the effect of inherent offsets, potential dose variations due to source position uncertainty are not yet well quantified. The purpose of this study was to investigate these dose variations for the worst geometry condition, and also to assess the validity of implementing a global offset using Monte Carlo (MC) simulations for dose calculation. The MCNP5 radiation transport code was used for all MC simulations. The GammaMedPlus HDR Ir-192 source core was simulated as a 0.09 cm diameter and 0.35 cm long right cylinder. Dose distributions were simulated in water (12 x 20 cm3 right cylinder) using the *F4 FMESH tally in both transverse (X-Y) and coronal (X-Z) plane. With a (0.1 cm)3 grid size, 109 photon histories gave a typical statistical error (k=1) of <1%. The Varian ring was simulated as a torus (0.29 cm and 0.35 cm inner and outer diameters) with a 0.03 cm thick titanium wall. The black buildup cap (0.5 cm) was simulated with polyoxymethylene (1.425 g/cm3) and dwell positions started from 129.5 cm with 0.5 cm step size in the ring lumen center. Dose variations for eccentric source positions were investigated by simulating dwell positions ± 0.007 cm (largest distance) from the lumen center along the Z-direction as shown in Figure 1a-c. To mimic a T&O geometry, dwell positions were simulated only in 135° to 225° and 315° to 45° as shown in Figure 1.e. To assess the influence of a global shift, ± 0.1, ± 0.3, ± 0.5 cm (Figure 1.d and 1.f), shifts were simulated distal and proximal from the ring end. Dose was normalized to 100% at 0.5 cm from the cap on the X-axis. For dose comparison, Point A was defined 2 cm up from the buildup cap and 2 cm laterally from the ring CAX on right (A_RT) and left side (A_LT). A typical dose distribution (X-Z plane) for the source in the ring lumen center was shown in Figure 1.g. Compared to sources in the ring lumen center, dose to points A_LT and A_RT changed ˗3.2% and ˗3.8%, respectively, for sources 0.007 cm away from Point A, and by 2.4% and 4.0% toward Point A, respectively. Compared to no shift, average dose to Point A was 1.6%, 2.3%, and 3.8% lower for the 0.1, 0.3, and 0.5 cm distal global source shifts, respectively. Doses were 1.2%, 1.4%, and 2.6% lower for the proximal shifts. Neither source position uncertainty within ring lumen nor global source shift caused significant dose variation to either point A_RT or point A_LT. Dose variation due to source position uncertainty was small (< 4%), and can be attributed mostly to inverse square law. Dose variations due to global source shifts also were not substantial. Applying a global shift will improve the accuracy of the dose distribution, but not applying the shift or applying a shift in wrong direction may not cause substantial dose calculation errors.

An analytical formalism to calculate phantom scatter factors for flattening filter free (FFF) mode photon beams

Phantom Scatter Factors, Sp in the Khan formalism (Khan et al 1980 J. Radiat. Oncol. Biol. Phys. 6 745–51) describe medium-induced changes in photon-beam intensity as a function of size of the beam. According to the British Journal of Radiology, Supplement 25, megavoltage phantom scatter factors are invariant as a function of photon-beam energy. However, during the commissioning of an accelerator with flattening filter free (FFF) photon beams (Varian TrueBeamTM 6-MV FFF and 10-MV FFF), differences were noted in phantom scatter between the filtered beams and FFF-mode beams. The purpose of this work was to evaluate this difference and provide an analytical formalism to explain the phantom scatter differences between FFF-mode and the filtered mode. An analytical formalism was devised to demonstrate the source of phantom scatter differences between the filtered and the FFF-mode beams. The reason for the differences in the phantom scatter factors between the filtered and the FFF-mode beams is hypothesized to be the non-uniform beam profiles of the FFF-mode beams. The analytical formalism proposed here is based on this idea, taking the product of the filtered phantom scatter factors and the ratio of the off-axis ratio between the FFF-mode and the filtered beams. All measurements were performed using a Varian TrueBeamTM linear accelerator with photon energies of 6-MV and 10-MV in both filtered and FFF-modes. For all measurements, a PTW Farmer type chamber and a Scanditronix CC04 cylindrical ionization were used. The in-water measurements were made at depth dose maximum and 100 cm source-to-axis distance. The in-air measurements were done at 100 cm source-to-axis distance with appropriate build-up cap. From these measurements, the phantom scatter factors were derived for the filtered beams and the FFF-mode beams for both energies to be evaluated against the phantoms scatter factors calculated using the proposed algorithm. For 6-MV, the difference between the measured and the calculated FFF-mode phantom scatter factors ranged from −0.34% to 0.73%. The average per cent difference was −0.17% (1σ = 0.25%). For 10-MV, the difference ranged from −0.19% to 0.24%. The average per cent difference was −0.17% (1σ = 0.13%). An analytical formalism was presented to calculate the phantom scatter factors for FFF-mode beams using filtered phantom scatter factors as a basis. The overall differences between measurements and calculations were within ±0.5% for 6-MV and ±0.25% for 10-MV.

The FiR 1 photon beam model adjustment according to in-air spectrum measurements with the Mg(Ar) ionization chamber

The mixed neutron–photon beam of FiR 1 reactor is used for boron–neutron capture therapy (BNCT) in Finland. A beam model has been defined for patient treatment planning and dosimetric calculations. The neutron beam model has been validated with an activation foil measurements. The photon beam model has not been thoroughly validated against measurements, due to the fact that the beam photon dose rate is low, at most only 2% of the total weighted patient dose at FiR 1. However, improvement of the photon dose detection accuracy is worthwhile, since the beam photon dose is of concern in the beam dosimetry. In this study, we have performed ionization chamber measurements with multiple build-up caps of different thickness to adjust the calculated photon spectrum of a FiR 1 beam model.

Measurement of air kerma rates for 6- to 7-MeV high-energy gamma-ray field by ionisation chamber and build-up plate

The 6- to 7-MeV high-energy gamma-ray calibration field by the (19)F(p, αγ)(16)O reaction is to be served at the Japan Atomic Energy Agency. For the determination of air kerma rates using an ionisation chamber in the 6- to 7-MeV high-energy gamma-ray field, the establishment of the charged particle equilibrium must be achieved during measurement. In addition to measurement of air kerma rates by the ionisation chamber with a thick build-up cap, measurement using the ionisation chamber and a build-up plate (BUP) was attempted, in order to directly determine air kerma rates under the condition of regular calibration for ordinary survey meters and personal dosemeters. Before measurements, Monte Carlo calculations were made to find the optimum arrangement of BUP in front of the ionisation chamber so that the charged particle equilibrium could be well established. Measured results imply that air kerma rates for the 6- to 7-MeV high-energy gamma-ray field could be directly determined under the appropriate condition using an ionisation chamber coupled with build-up materials.

Characteristics of mobile MOSFET dosimetry system for megavoltage photon beams.

The characteristics of a mobile metal oxide semiconductor field effect transistor (mobile MOSFET) detector for standard bias were investigated for megavoltage photon beams. This study was performed with a brass alloy build-up cap for three energies namely Co-60, 6 and 15 MV photon beams. The MOSFETs were calibrated and the performance characteristics were analyzed with respect to dose rate dependence, energy dependence, field size dependence, linearity, build-up factor, and angular dependence for all the three energies. A linear dose-response curve was noted for Co-60, 6 MV, and 15 MV photons. The calibration factors were found to be 1.03, 1, and 0.79 cGy/mV for Co-60, 6 MV, and 15 MV photon energies, respectively. The calibration graph has been obtained to the dose up to 600 cGy, and the dose-response curve was found to be linear. The MOSFETs were found to be energy independent both for measurements performed at depth as well as on the surface with build-up. However, field size dependence was also analyzed for variable field sizes and found to be field size independent. Angular dependence was analyzed by keeping the MOSFET dosimeter in parallel and perpendicular orientation to the angle of incidence of the radiation with and without build-up on the surface of the phantom. The maximum variation for the three energies was found to be within ± 2% for the gantry angles 90° and 270°, the deviations without the build-up for the same gantry angles were found to be 6%, 25%, and 60%, respectively. The MOSFET response was found to be independent of dose rate for all three energies. The dosimetric characteristics of the MOSFET detector make it a suitable in vivo dosimeter for megavoltage photon beams.

Comparison of Head Scatter Factor for 6MV and 10MV flattened (FB) and Unflattened (FFF) Photon Beam using indigenously Designed Columnar Mini Phantom

To measure and compare the head scatter factor for flattened (FB) and unflattened (FFF) of 6MV and 10MV photon beam using indigenously designed mini phantom. A columnar mini phantom was designed as recommended by AAPM Task Group 74 with low and high atomic number materials at 10 cm (mini phantom) and at approximately twice the depth of maximum dose water equivalent thickness (brass build-up cap). Scatter in the accelerator (Sc) values of 6MV-FFF photon beams are lesser than that of the 6MV-FB photon beams (0.66-2.8%; Clinac iX, 2300CD) and (0.47-1.74%; True beam) for field sizes ranging from 10 × 10 cm2 to 40 × 40 cm2. Sc values of 10MV-FFF photon beams are lesser (0.61-2.19%; True beam) than that of the 10MV-FB photons beams for field sizes ranging from 10 × 10 cm2 to 40 × 40 cm2. The SSD had no influence on head scatter for both flattened and unflattened beams and irrespective of head design of the different linear accelerators. The presence of field shaping device influences the Sc values. The collimator exchange effect reveals that the opening of the upper jaw increases Sc irrespective of FB or FFF photon beams and different linear accelerators, and it is less significant in FFF beams. Sc values of 6MV-FB square field were in good agreement with that of AAPM, TG-74 published data for Varian (Clinac iX, 2300CD) accelerator. Our results confirm that the removal of flattening filter decreases in the head scatter factor compared to flattened beam. This could reduce the out-of-field dose in advanced treatment delivery techniques.

SU-E-T-115: Collimator Scatter Factor (Sc) Measurements for IRIS in CyberKnife Using Build-Up Caps

Purpose: To investigate the impact of custom-made build-up caps for two different diode detectors in robotic radiosurgery radiation fields with variable collimator (IRIS) for collimator scatter factor (Sc) measurement. Methods: Two acrylic caps were custom-made to fit our SFD or PFD (IBA Dosimetry, Germany) diode detectors. The two caps have thicknesses of 1.5 and 5 cm, corresponding to the dmax of the 6 MV CyberKnife and a depth beyond electron contamination, respectively. A watertank (Blue Phantom, IBA Dosimetry) was used to position the detector at 80 cm source-to-detector distance. Measurements were performed with the SFD and PFD, with and without the build-up caps, for all 12 clinical IRIS settings ranging from 5 to 60 mm. Result: As expected, the biggest discrepancy was found in the smallest (5mm) IRIS field size. The Sc factor with 5 cm build-up cap was 6.6% lower than that without build-up using the SFD detector while the PFD differed by 13%. However, when the 1.5 cm build-up cap was used, the largest difference (4%) was found using the 10 mm IRIS field size using the SFD while the maximum discrepancy using the PFD was still using the 5 mm IRIS field size (7.6%). For IRIS field sizes larger than 10 mm, maximum discrepancies were less than 1.3% and 3.7% for SFD and PFD, respectively. Conclusion: Sc measurement data are critical components in advanced algorithms for treatment planning, such as Monte Carlo, in order to calculate the dose accurately. After incorporating build-up caps, we discovered differences of up to 6.6% and 13% in Sc factors in the SFD and PFD detectors, respectively, when compared against in-air measurements without build-up caps. These are significant differences and were obtained with IRIS settings routinely used for clinical treatment.

SU-E-T-85: Total Body Irradiation (TBI) Optically Stimulated Luminescence In Vivo Dosimetry

Purpose: To adequately monitor the doses for Total Body Irradiation (TBI) patients using opposed fields, we carry out both active and passive in vivo dosimetry, with the passive dosimeter being Optically Stimulated Luminescence (OSL) detectors. The OSLs are embedded in hemispherical brass buildup caps with the OSL/flat surface placed against patient skin. The in vivo dosimetry devices are not repositioned when the patient is rotated, so both entrance and exit dosimetry are carried out, which arguably gives a better midline dose estimate, and improves marrow graft acceptance by minimizing treatment time. The OSLs were calibrated using a 100cm SAD entrance method with brass buildup caps. The buildup caps have a maximum density thickness of 6.6g/cm2; dmax for our 18MV beam is 3.4g/cm2. This method results in an overestimation of exit dose due to the density thickness differences, requiring a correction factor for exit measured doses. Methods: To determine the exit dosimetry correction factor, OSLs were placed on the entrance and exit sides of solid water phantoms under TBI conditions having thicknesses of 10, 30, and 49.2cm. Combinations of OSLs included entrance-only, exit-only and combined entrance and exit measured doses. The entrance and exit dose to the dosimeter position was calculated. Results: The exit dose correction factor calculated from the ratio of calculated to measured values was larger than anticipated (∼17%). The correction factor was applied to the exit dose calculations and compared to the combined measurement for verification. Results were within 1.2% (Table 1).The method was tested on ten patients, giving agreement within 1.1±4% (Table 2). Acceptable TBI tolerance is ±10%. Conclusion: While this is acceptable, we will consider using non-brass buildup caps to obtain smaller correction factors.

SU‐E‐T‐01: Comparison of Head Scatters in Flattening‐Filter‐Free and Flattened Photon Beams

Purpose: To decouple and quantify scatter contributions in air and in phantom for flattening‐filter‐free (FFF) photon beams in comparison with those for flattened beams from a medical linac in radiotherapy. Methods: The 7 UF FFF beam on a Siemens Artiste linac shows equivalent depth dose distribution as the 6 MV flat beam. An ionization chamber was used to measure doses along central axis in air (with a 3 cm diameter buildup cap) and in phantom (SSD = 100 cm and depth = 1.5 cm) for both beams in a range of field size, yielding head scatter factors (Sc) and output factors (Scp). A third‐order polynomial function was used to fit the data within the fields of 3×3 to 12×12 cm2 for each Sc or Scp distribution. An extrapolation of the fitted function to 0×0 field yielded the dose component due to primary radiation. Scatter components for different field sizes can be derived by subtracting this primary component from measured total doses. Results: A primary FFF beam produces only 5% scatters in 10× 10 cm2 field through the linac head while 43% scatters are produced by a flat primary beam, resulting in a reduction of scatter contributions by a factor of 6 in the measured doses in air. Comparable scatter contributions (∼10% in 10× 10 field) produced in the phantom at the same depth were observed for both primary beams as expected. Overall, the significantly reduced head scatter for the FFF beam leads to a total scatter reduction of 60% in phantom at 1.5 cm depth (14% and 34% scatter contributions in 10× 10 field for the FFF and flat beams, respectively). Conclusion: Our method works well in decoupling scatter components from measured total doses. The significant reduction of head scatters for FFF beams would provide dosimetric benefits to patients with reduced shallow‐dose. This work is supported in part by a Siemens research fund.

Effects on the photon beam from an electromagnetic array used for patient localization and tumor tracking

One of the main components in a Calypso 4D localization and tracking system is an electromagnetic array placed above patients that is used for target monitoring during radiation treatment. The beam attenuation and beam spoiling properties of the Calypso electromagnetic array at various beam angles were investigated. Measurements were performed on a Varian Clinac iX linear accelerator with 6 MV and 15 MV photon beams. The narrow beam attenuation properties were measured under a field size of 1 cm×1 cm, with a photon diode placed in a cylindrical graphite buildup cap. The broad beam attenuation properties were measured under a field size of 10 cm×10 cm, with a 0.6 cc cylindrical Farmer chamber placed in a polystyrene buildup cap. Beam spoiling properties of the array were studied by measuring depth‐dose change from the array under a field size of 10 cm×10 cm cm in a water‐equivalent plastic phantom with an embedded Markus parallel plate chamber. Change in depth doses were measured with the array placed at distances of 2, 5, and 10 cm from the phantom surface. Narrow beam attenuation and broad beam attenuation from the array were found to be less than 2%–3% for both 6 MV and 15 MV beams at angles less than 40°, and were more pronounced at more oblique angles. Spoiling effects are appreciable at beam buildup region, but are insignificant at depths beyond dmax. Dose measurements in a QA phantom using patient IMRT and VMAT treatment plans were shown to have less than 2.5% dose difference with the Calypso array. The results indicate that the dose difference due to the placement of Calypso array is clinically insignificant.PACS number: 87.56.‐v

Accumulation and dissipation of positive charges induced on a PMMA build-up cap of an ionisation chamber by 60Co gamma-ray irradiation

The signal current from an ionisation chamber with a PMMA build-up cap decreases with irradiation time due to electric fields produced by positive charges induced on the cap. In the present study, it was confirmed that the signal current decreases faster for irradiation using narrower (60)Co gamma-ray beams. This is because the number of secondary electrons that are emitted from surrounding materials and penetrate the build-up cap is smaller in a narrower gamma-ray beam, so that fewer positive charges are neutralised. The ionisation chamber was first subjected to continuous gamma-ray irradiation for 24 h, following which it was irradiated with shorter periodic gamma-ray bursts while measuring the current signal. This allowed the coefficients of positive charge accumulation and dissipation to be determined. It was found that the dissipation coefficient has a large constant value during gamma-ray irradiation and decreases asymptotically to a small value after irradiation is stopped. From the coefficients, the minimum signal current was calculated, which is the value when accumulation and dissipation balance each other under continuous irradiation. The time required for the signal current to recover following irradiation was also calculated.

Detailed high‐accuracy megavoltage transmission measurements: A sensitive experimental benchmark of EGSnrc

There are three goals for this study: (a) to perform detailed megavoltage transmission measurements in order to identify the factors that affect the measurement accuracy, (b) to use the measured data as a benchmark for the EGSnrc system in order to identify the computational limiting factors, and (c) to provide data for others to benchmark Monte Carlo codes. Transmission measurements are performed at the National Research Council Canada on a research linac whose incident electron parameters are independently known. Automated transmission measurements are made on-axis, down to a transmission value of ∼1.7%, for eight beams between 10 MV (the lowest stable MV beam on the linac) and 30 MV, using fully stopping Be, Al, and Pb bremsstrahlung targets and no fattening filters. To diversify energy differentiation, data are acquired for each beam using low-Z and high-Z attenuators (C and Pb) and Farmer chambers with low-Z and high-Z buildup caps. Experimental corrections are applied for beam drifts (2%), polarity (2.5% typical maximum, 6% extreme), ion recombination (0.2%), leakage (0.3%), and room scatter (0.8%)-the values in parentheses are the largest corrections applied. The experimental setup and the detectors are modeled using EGSnrc, with the newly added photonuclear attenuation included (up to a 5.6% effect). A detailed sensitivity analysis is carried out for the measured and calculated transmission data. The developed experimental protocol allows for transmission measurements with 0.4% uncertainty on the smallest signals. Suggestions for accurate transmission measurements are provided. Measurements and EGSnrc calculations agree typically within 0.2% for the sensitivity of the transmission values to the detector details, to the bremsstrahlung target material, and to the incident electron energy. Direct comparison of the measured and calculated transmission data shows agreement better than 2% for C (3.4% for the 10 MV beam) and typically better than 1% for Pb. The differences can be explained by acceptable photon cross section changes of ≤0.4%. Accurate transmission measurements require accounting for a number of influence quantities which, if ignored, can collectively introduce errors larger than 10%. Accurate transmission calculations require the use of the most accurate data and physics options available in EGSnrc, particularly the more accurate bremsstrahlung angular sampling option and the newly added modeling of photonuclear attenuation. Comparison between measurements and calculations implies that EGSnrc is accurate within 0.2% for relative ion chamber response calculations. Photon cross section uncertainties are the ultimate limiting factor for the accuracy of the calculated transmission data (Monte Carlo or analytical).