Electron Arc Research Articles

SU-GG-T-395: Monitor Unit Calculation for Electron Arc Therapy Using Monte Carlo Simulations

Purpose: This study reported a monitor unit (MU) calculation method for electron arc therapy. This MU calculation was carried out using Monte Carlo simulation and verified by measurement. Methods and Materials: The 9 MeV phase space electron beam (2.6 × 21 cm2) attached to the block tray holder of a Varian 21 EX linac was generated using the EGSnrc‐based BEAMnrc code, and was verified by dosimetric measurements using ionization chamber and radiographic film. The relative output factor (ROF), source‐to‐surface (SSD) offset and dwell factor (DF), required in the MU calculation, were determined using measurement and Monte Carlo simulation. Solid water rectangular and cylindrical phantom made of polystyrene were used in the measurements. Results: Deviations of ROF, SSD offset and DF between measured and Monte Carlo results were 1.2%, 0.18% and 1.5% respectively. Our results show that the DF decreased with an increase of the treatment arc angle (α), and such decrease of DF was more significant in the α range of 0 – 60 deg than 60 – 120 deg. Comparing the DF determined using the rectangular and cylindrical phantom through measurement and Monte Carlo simulation, we found that the DF determined by the rectangular agreed well with that by the cylindrical phantom within ±1.2%. It shows that a simple setup of solid water rectangular phantom was sufficient to replace the cylindrical phantom to determine the DF associated with the electron arc. Conclusions:Monte Carlo simulation was proved to be an alternative way to determine the MU for electron arc therapy. Since Monte Carlo simulation can generate pre‐calculated database of ROF, SSD offset and DF for the MU calculation with a reduction of human effort and linac beam‐on time, it is suggested to integrate Monte Carlo simulation into the commissioning of electron arc therapy.

Monte Carlo calculation of monitor unit for electron arc therapy

Monitor unit (MU) calculations for electron are therapy were carried out using Monte Carlo simulations and verified by measurements. Variations in the dwell factor (DF), source-to-surface distance (SSD), and treatment are angle (a) were studied. Moreover, the possibility of measuring the DF, which requires gantry rotation, using a solid water rectangular, instead of cylindrical, phantom was investigated. A phase space file based on the 9 MeV electron beam with rectangular cutout (physical size = 2.6 x 21 cm2) attached to the block tray holder of a Varian 21 EX linear accelerator (linac) was generated using the EGSnrc-based Monte Carlo code and verified by measurement. The relative output factor (ROF), SSD offset, and DF, needed in the MU calculation, were determined using measurements and Monte Carlo simulations. An ionization chamber, a radiographic film, a solid water rectangular phantom, and a cylindrical phantom made of polystyrene were used in dosimetry measurements. Percentage deviations of ROF, SSD offset, and DF between measured and Monte Carlo results were 1.2%, 0.18%, and 1.5%, respectively. It was found that the DF decreased with an increase in a, and such a decrease in DF was more significant in the a range of 0 degrees-60 degrees than 60 degrees-120 degrees. Moreover, for a fixed a, the DF increased with an increase in SSD. Comparing the DF determined using the rectangular and cylindrical phantom through measurements and Monte Carlo simulations, it was found that the DF determined by the rectangular phantom agreed well with that by the cylindrical one within +/- 1.2%. It shows that a simple setup of a solid water rectangular phantom was sufficient to replace the cylindrical phantom using our specific cutout to determine the DF associated with the electron arc. By verifying using dosimetry measurements, Monte Carlo simulations proved to be an alternative way to perform MU calculations effectively for electron are therapy. Since Monte Carlo simulations can generate a precalculated database of ROF, SSD offset, and DF for the MU calculation, with a reduction in human effort and linac beam-on time, it is recommended that Monte Carlo simulations be partially or completely integrated into the commissioning of electron are therapy.

Serial Batch Elution of Electric Arc Furnace Oxidizing Slag Discharged from Normal Steelmaking Process into Fresh Water

The serial batch leaching test of an electronic arc furnace oxidizing slag was performed on the basis of JIS K 0058-1 to investigate the safety of the slag and to clarify the elution mechanism. The slag which was discharged at the time of refining of normal steel was used for this experiment. The slag was dissolved in water whose initial pH was 6.0. After that, the slag was naturally dried. This operation was performed 12 times successively.The dissolution behaviors of Ca and Mg were expressed by the parabolic law. That of Si was expressed by the linear law. The dissolutions of them from the slag were controlled by the diffusion through pore and the surface layer through the surface layer where compositions of FeO and Al2O3 were high. Generally, the dissolve concentrations decreased with the increase in the number of the elution times. The environmentally regulated substances were not detected or less than the levels of the environmental quality standards for soil. Ba, Mn, Mo, K, Na, Sr and W were detected as minor element except for the environmentally regulated elements. Generally, the detected environmentally regulated substances and the other minor elements decreased with the number of the elution times. During the elution of the slag, the pH of the aqueous solution steeply increased initially, and it decreased afterwards. The pH decreased with the increase in the number of the elution times.

Serial Batch Elution of Electric Arc Furnace Oxidizing Slag Discharged from Normal Steelmaking Process into Fresh Water

The serial batch leaching test of an electronic arc furnace oxidizing slag was performed on the basis of JIS K 0058-1 to investigate the safety of the slag and to clarify the elution mechanism. The slag which was discharged at the time of refining of normal steel was used for this experiment. The slag was dissolved in water whose initial pH was 6.0. After that, the slag was naturally dried. This operation was performed 12 times successively.The dissolution behaviors of Ca and Mg were expressed by the parabolic law. That of Si was expressed by the linear law. The dissolutions of them from the slag were controlled by the diffusion through pore and the surface layer through the surface layer where compositions of FeO and Al2O3 were high. Generally, the dissolve concentrations decreased with the increase in the number of the elution times. The environmentally regulated substances were not detected or less than the levels of the environmental quality standards for soil. Ba, Mn, Mo, K, Na, Sr and W were detected as minor element except for the environmentally regulated elements. Generally, the detected environmentally regulated substances and the other minor elements decreased with the number of the elution times. During the elution of the slag, the pH of the aqueous solution steeply increased initially, and it decreased afterwards. The pH decreased with the increase in the number of the elution times.

Behavior of CO2 Absorption in Wet-grinding of Electronic Arc Furnace Reducing Slag by Vibration Ball Mill

A fundamental study was carried out in order to fix CO2 on slag. The electronic arc furnace reducing slag was wet-ground by a vibration ball mill under CO2 atmosphere. The effect of grinding conditions on the behavior of CO2 absorption was investigated.The rate of the absorption of CO2 under wet grinding was larger than that under dry grinding. The total amount of CO2 absorption increased with the increase in total amount of the slag and water in case that the weight ratio of the water to the slag was kept constant. The CO2 was fixed as CaCO3 and Ca4Si2O6(CO3)(OH, F)2. Any compounds of Mg were not detected by XRD after the experiment. In the early stage of grinding, the CO2 was absorbed even if the grinding was stopped. The concentration of Ca in the water was larger than the solubility of Ca(OH)2. Hence, the absorption of CO2 was determined by the surface reaction that consisted of chemical reaction and mass transfer in liquid. In the latter stage of grinding, the CO2 was not almost absorbed immediately after the grinding was stopped. The concentration of Ca in water was small. Accordingly, the absorption of CO2 was influenced by the dissolution of slag into water.The amount of the exhausted CO2 was calculated from the electronic power which was necessary for operating this experimental apparatus. The amount of CO2 absorption was larger than that of the exhausted CO2 from a solar, an atomic, a wind and a water power plant.

Position of proton precipitation during auroral intensifications

The unique spectrographic observations of auroras on the Kola Peninsula, simultaneously performed in 1970 at Loparskaya and Kem stations using C-180-S cameras, have been analyzed by up-to-date digital data processing. The position and dynamics of proton precipitation relative to other manifestations of auroral and substorm activity (auroral arcs and electrojets) under moderately and weakly disturbed conditions have been analyzed. Several previously known regularities in the morphology of proton auroras have been confirmed. It has been indicated that the direction of motion of the proton band equatorward boundary in the evening sector changes at a sign reversal of the IMF Z component. Weak breakups affect the poleward boundary of the proton band but do not influence the position of the equatorward boundary of this band, which results in the expansion of the proton emission region. When a disturbance is stronger, the proton emission disappears near an active electron arc and subsequently appears poleward of its position before intensification. Short-term proton precipitation is also observed in the region of active electron precipitation during an intense breakup in the form of N–S structures.

電気炉酸化スラグの淡水への溶出挙動

Leaching test of the electronic arc furnace slag was carried out on the basis of JIS K 0058-1 (2005) to study the dissolution behavior of the environmentally controlled elements and the main elements of which the slag was composed. The used slag was the oxidizing slag that was discharged in the smelting of normal steel. The leaching test was performed changing the initial hydrogen ion exponent (pH) of fresh and sea water and the size of slag. During the test, the concentrations of dissolved elements were measured by ICP-AES. The pH of the solution was also measured.The concentrations of the environmentally controlled substances were below the environmental quality standards for soil and sea water. Generally, the dissolution behavior of Mg and Ca in the slag was expressed by the parabolic law, and that of Si was expressed by the linear law. The elution of those elements was estimated to be controlled be the diffusion through the surface layer of the slag. In the case that the initial pH was below 8, the pH of the solution increased up to approximately 9.5 in the early stage of the dissolution. Subsequently, it decreased to approximately 8.5. In the case that the initial pH was 10, the pH decreased to approximately 8.5. Hence the slag itself showed buffering action. The solubility of each main element of the slag was calculated from the Debye–Hückel theory. The ions in the solution and the solid phases during the elution and the precipitation were discussed from the solubility.

振動ボールミルによる電気炉還元スラグの湿式粉砕時の二酸化炭素吸収挙動

A fundamental study was carried out in order to fix CO2 on slag. The electronic arc furnace reducing slag was wet-ground by a vibration ball mill under CO2 atmosphere. The effect of grinding conditions on the behavior of CO2 absorption was investigated.The rate of the absorption of CO2 under wet grinding was larger than that under dry grinding. The total amount of CO2 absorption increased with the increase in total amount of the slag and water in case that the weight ratio of the water to the slag was kept constant. The CO2 was fixed as CaCO3 and Ca4Si2O6(CO3)(OH,F)2. Any compounds of Mg were not detected by XRD after the experiment. In the early stage of grinding, the CO2 was absorbed even if the grinding was stopped. The concentration of Ca in the water was larger than the solubility of Ca(OH)2. Hence, the absorption of CO2 was determined by the surface reaction that consisted of chemical reaction and mass transfer in liquid. In the later stage of grinding, the CO2 was not almost absorbed immediately the grinding was stopped. The concentration of Ca in water was small. Accordingly, the absorption of CO2 was influenced by the dissolution of slag into water.The amount of the exhausted CO2 was calculated from the electronic power that was necessary for operating this experimental apparatus from various power plants. The amount of CO2 absorption was larger than that of the exhausted CO2 from a solar, an atomic, a wind and a water power plant.

SU‐GG‐T‐519: Commissioning of the Commercial Planning System for the Electron Arc Treatment

Purpose: Most commercial radiation treatment planning systems do not provide the planning mode for the electron arc treatment which most linear accelerators have. We have commissioned a new exclusive physics machine for pseudo electron arc planning and checked the validity with phantom measurements. Method and Materials: With an electron arc aperture tray under Varian 21‐EX, minimum beam data including depth doses, lateral profiles and output factors were measured and transferred into a Pinnacle3 for commissioning. An acrylic cylindrical phantom wrapped with 1 cm‐thick bolus was used for planning and measurement. Multiple static beams with 5 degrees interval were adopted for the pseudo arc planning and delivered via dynamic electron arc mode of the Varian linac. For the evaluation of the pseudo arc planning, measurements were made with MOSFET detectors placed at five different points beneath the bolus and on the surface. Results: Pinnacle3 was commissioned successfully for the electron arc treatments with a fixed field of 6×25 cm2 on the Varian 21‐EX. The extended distance from the beam‐shaping‐edge of the electron arc aperture to the surface makes the beam characteristics totally different from those for the usual electron beams. The MOSFET measurements agreed with the calculation to within 5% except the ends of the arc where 16.5% of discrepancy was obtained. Conclusion: Without the electron arc planning function, multiple static beams can be exploited successfully for the electron arc treatment on the commercial planning system. However, a new physics machine for the exclusive electron arc therapy should be commissioned because of the totally different characteristics of the electron beam for arc therapy. In‐vivo dosimetry is a necessity for the plan evaluation.

Treatment planning comparison of electron arc therapy and photon intensity modulated radiotherapy for Askin’s tumor of chest wall

Background and Purpose A dosimetric study to quantitatively compare radiotherapy treatment plans for Askin’s tumor using Electron Arc (EA) vs. photon Intensity Modulated Radiotherapy (IMRT). Materials and methods Five patients treated with EA were included in this study. Treatment plans were generated for each patient using EA and IMRT. Plans were compared using dose volume histograms (DVH) of the Planning Target Volume (PTV) and Organs at Risk (OAR). Results IMRT resulted in superior PTV coverage, and homogeneous dose distribution compared to EA. For EA, 92% of the PTV was covered to 85% of the dose compared to IMRT in which 96% was covered to 95% of the dose. V 107 that represents the hot spot within the PTV was more in IMRT compared to EA: 7.4(±2)% vs. 3(±0.5)%, respectively. With PTVs located close to the spinal cord (SC), the dose to SC was more with EA, whereas for PTVs located away from the SC, the dose to SC was more with IMRT. The cardiac dose profile was similar to that of SC. Ipsilateral lung received lower doses with IMRT while contralateral lung received higher dose with IMRT compared to EA. For non-OAR normal tissues, IMRT resulted in large volumes of low dose regions. Conclusions IMRT resulted in superior PTV coverage and sparing of OAR compared to EA plans. Although IMRT seems to be superior to EA, one needs to keep in mind the volume of low dose regions associated with IMRT, especially while treating young children.

Po‐Thur Eve General‐27: Analytical model for electron arc beam output determination using an Elekta SL‐25 linear accelerator

The clinical implementation of electron arc therapy requires a large amount of measured dosimetric data. All necessary measurements were done on an Elekta SL‐25 with a 10 MeV electron beam. Beam output and PDD measurements were performed in five different cylindrical acrylic phantoms using TLDs. The phantom radius (di) ranged between 5.1 and 15.2 cm, the total arc angles (α) varied between 60° and 160° and the number of monitor units (MU) per degree between 0.5 and 7. Based on this data bank, an analytical model was developed for monitor unit (MU) calculation. This model estimates arc beam output at the depth of maximum dose (dmax) as a function of di and α for a given field width at isocentre (w). Curve fitting of the complete set of beam output data was done with an asymptotic relationship between the dose rate at dmax and the inverse square of di. The dependence of the beam output on α was introduced by assuming an explicit function of α for each parameter of the model. Results show that the calculated beam output data is a good approximation for all measured data for all phantoms and arc angles: 89% of the calculated values are within ± 3% of the measured ones and all calculated points are within a ± 5% error range. The MU calculation then becomes straightforward, without the need of measuring each clinical case.

Application of the electron pencil beam redefinition algorithm to electron arc therapy

This project investigated the potential of summing fixed-beam dose distributions calculated using the pencil-beam redefinition algorithm (PBRA) at small angular steps (1 degree) to model an electron arc therapy beam. The PRBA, previously modified to model skin collimation, was modified further by incorporating two correction factors. One correction factor that is energy, SSD (source-to-surface distance), and field-width dependent constrained the calculated dose output to be the same as the measured dose output for fixed-beam geometries within the range of field widths and SSDs encountered in arc therapy. Another correction factor (single field-width correction factor for each energy) compensated for large-angle scattering not being modeled, allowing a more accurate calculation of dose output at mid arc. The PBRA was commissioned to accurately calculate dose in a water phantom for fixed-beam geometries typical of electron arc therapy. Calculated central-axis depth doses agreed with measured doses to within 2% in the low-dose gradient regions and within 1-mm in the high-dose gradient regions. Off-axis doses agreed to within 2 mm in the high-dose gradient regions and within 3% in the low-dose gradient regions. Arced-beam calculations of dose output and depth dose at mid arc were evaluated by comparing to data measured using two cylindrical water phantoms with radii of 12 and 15 cm at 10 and 15 MeV. Dose output was measured for all combinations of phantom radii of curvature, collimator widths (4, 5, and 6 cm), and arc angles (0 degrees, 20 degrees, 40 degrees, 60 degrees, 80 degrees, and 90 degrees) for both beam energies. Results showed the calculated mid-arc dose output to agree within 2% of measurement for all combinations. For a 90 degree arc angle and 5 x 20 cm2 field size, the calculated mid-arc depth dose in the low-dose gradient region agreed to within 2% of measurement for all depths at 10 MeV and for depths greater than depth of dose maximum R100 at 15 MeV. For depths in the buildup region at 15 MeV the calculations overestimated the measured dose by as much as 3.4%. Mid-arc depth dose in the high-dose gradient region agreed to within 2.2 mm of measured dose. Calculated two-dimensional relative dose distributions in the plane of rotation were compared to dose measurements using film in a cylindrical polystyrene phantom for a 90 degree arc angle and field widths of 4, 5, and 6 cm at 10 and 15 MeV. Results showed that off-axis dose at the ends of arc (without skin collimation) agreed to within 2% in the low-dose gradient region and to within 1.2 mm in the high-dose gradient region. This work showed that the accuracy of the PBRA arced-beam dose model met the criteria specified by Van Dyk et al. [Int. J. Radiat. Oncol. Biol. Phys. 26, 261-273 (1993)] with the exception of the buildup region of the 15 MeV beam. Based on the present results, results of a previous study showing acceptable accuracy in the presence of skin collimation, and results of a previous study showing acceptable accuracy in the presence of internal heterogeneities, it is concluded that the PBRA arced-beam dose model should be adequate for planning electron arc therapy.

MO‐E‐224C‐04: Motorized Multi‐Leaf Collimator for Electrons: Measurements with a Prototype and Monte Carlo Simulations

Purpose: To develop a motorized multi‐leaf collimator for electrons (eMLC) and to compare preliminary measurements to Monte Carlo simulations. Method and Materials: An eMLC has been developed. It is the first prototype with fully motorized capabilities. The eMLC is remotely controlled by the operator using home brewed and fully graphical software running on a Windows workstation. The control workstation is connected to the eMLC's custom‐built electronic controller which keeps track of the component states and executes various low‐level commands, including leaf displacement orders. There are 36 independent brass leaves on each side of the eMLC and the maximum size of the generated field is 25.2 × 19.5 cm2. The interleaf distance is less than 0.03 mm. The eMLC prototype is an add‐on device for the Elekta Precise linac (Elekta Ltd., England). The actual distance from the source to the bottom of the leaves is 95.3 cm. The eMLC has been modeled using the BEAMnrc Monte Carlo toolkit and every possible leaf positions is reproducible with our Monte Carlo model. Results: Various preliminary measurements were performed: open field, closed field, one leaf profile, interleaf leakage, leaf transmission, and comparison with conventional custom cutouts. For all measurements, comparison to Monte Carlo simulations are carried out. The transmission through the leaves is 2.4% for a 12 MeV field at the surface of a water phantom with SSD=100 cm. The interleaf leakage is negligible as no interleaf pattern is detected on a closed field profile for all the investigated energies (up to 12 MeV). Conclusion: The motorized eMLC prototype is versatile and easy to operate with a computer control from outside the treatment room. Possible applications of the eMLC go from simply replacing the conventional custom cutouts to complex usages like MERT or electron arc therapy.Research sponsored by Elekta Ltd.

SU-FF-T-209: Electron Arc Therapy with An Elekta SL-25 10 MeV Beam Using a Dedicated Short Applicator

Purpose: To implement the electron arc therapy with an Elekta SL‐25 10 Mev beam using a dedicated short applicator. Method and Materials:. All measurements were done on an Elekta SL‐25 with a 10 MeV electron beam. Radial PDD were characterized for 4 different cylindrical acrylic phantoms using TLDs and Kodak X‐Omat‐V radiographic films The phantom radius (di ) ranged between 5.1 and 15.2 cm, the total arc angles (α) varied between 60° and 160° and the number of monitor units (MU) per degree between 0.5 and 7. The field with at isocenter (w) was varied between 3.7 and 9.8 cm. Based on this data bank, an analytical model was developed for monitor unit (MU) calculation. This model estimates arc beam output at the depth of maximum dose (dmax) as a function of di and α for a given w. A verification of the model precision was done with a new cylindrical phantom. Results:Curve fitting of the complete set of beam output data with w fixed at 7.3 cm was done with an asymptotic relationship between the dose rate at d max and the inverse square of di . The dependence of the beam output on α was introduced by assuming an explicit function of α for each parameter of the model. Results show that the calculated beam output data is a good approximation for all measured data for all phantoms and arc angles: 89% of the calculated values are within ± 3% of the measured ones and all points are within a ± 5% error range. Conclusion: From our results, clinical implementation of the electron arc therapy technique is possible and should be facilitated by the use of our predictive model in the treatment planning process.

Modeling skin collimation using the electron pencil beam redefinition algorithm

Skin collimation is an important tool for electron beam therapy that is used to minimize the penumbra when treating near critical structures, at extended treatment distances, with bolus, or using arc therapy. It is usually made of lead or lead alloy material that conforms to and is placed on patient surface. Presently, commercially available treatment-planning systems lack the ability to model skin collimation and to accurately calculate dose in its presence. The purpose of the present work was to evaluate the use of the pencil beam redefinition algorithm (PBRA) in calculating dose in the presence of skin collimation. Skin collimation was incorporated into the PBRA by terminating the transport of electrons once they enter the skin collimator. Both fixed- and arced-beam dose calculations for arced-beam geometries were evaluated by comparing them with measured dose distributions for 10- and 15-MeV beams. Fixed-beam dose distributions were measured in water at 88-cm source-to-surface distance with an air gap of 32 cm. The 6 x 20-cm2 field (dimensions projected to isocenter) had a 10-mm thick lead collimator placed on the surface of the water with its edge 5 cm inside the field's edge located at +10 cm. Arced-beam dose distributions were measured in a 13.5-cm radius polystyrene circular phantom. The beam was arced 90 degrees (-45 degrees to +45 degrees), and 10-mm thick lead collimation was placed at +/- 30 degrees. For the fixed beam at 10 MeV, the PBRA- calculated dose agreed with measured dose to within 2.0-mm distance to agreement (DTA) in the regions of high-dose gradient and 2.0% in regions of low dose gradient. At 15 MeV, the PBRA agreed to within a 2.0-mm DTA in the regions of high-dose gradient; however, the PBRA underestimated the dose by as much as 5.3% over small regions at depths less than 2 cm because it did not model electrons scattered from the edge of the skin collimation. For arced beams at 10 MeV, the agreement was 1-mm DTA in the high-dose gradient regions, and 2% in the low-dose gradient regions. For arced beams at 15 MeV, the agreement was 1 mm in the high-dose gradient regions, and in the low-dose gradient region at depth less than 2 cm, as much as 5% dose difference was observed. This study demonstrated the ease with which skin collimation can be incorporated into the PBRA. The good agreement of PBRA calculated with measured dose shows that the PBRA is likely sufficiently accurate for clinical use in the presence of skin collimation for electron arc therapy. To further improve the accuracy of the PBRA in regions having significant electrons scattered from the edge of the skin collimation would require transporting the electrons through the lead skin collimation near its edges.

Variations of auroral hydrogen emission near substorm onset

Abstract. The results of coordinated optical ground-based observations of the auroral substorm on 26 March 2004 in the Kola Peninsula are described. Imaging spectrograph data with high spectral and temporal resolution recorded the Doppler profile of the Hα hydrogen emission; this allows us to estimate the average energy of precipitating protons and the emission intensity of the hydrogen Balmer line. Two different populations of precipitating protons were observed during an auroral substorm. The first of these is associated with a diffuse hydrogen emission that is usually observed in the evening sector of the auroral oval and located equatorward of the discrete electron arcs associated with substorm onset. The average energy of the protons during this precipitation was ~20–35 keV, and the energy flux was ~3x10–4Joule/m2s. The second proton population was observed 1–2min after the breakup during 4–5min of the expansion phase of substorm into the zone of bright, discrete auroral structures (N-S arcs). The average energy of the protons in this population was ~60 keV, and the energy flux was ~2.2x10–3Joule/m2s. The observed spatial structure of hydrogen emission is additional evidence of the higher energy of precipitated protons in the second population, relative to the protons in the diffuse aurora. We believe that the most probable mechanism of precipitation of the second population protons was pitch-angle scattering of particles due to non-adiabatic motion in the region of local dipolarization near the equatorial plane.Keywords. Auroral ionosphere; Particle precipitation; Storms and substorms

Treatment of extensive scalp lesions with segmental intensity-modulated photon therapy

To compare static electron therapy, electron arc therapy, and photon intensity-modulated radiation therapy (IMRT) for treatment of extensive scalp lesions and to examine the dosimetric accuracy of the techniques. A retrospective treatment-planning study was performed to evaluate the relative merits of static electron fields, arcing electron fields, and five-field photon IMRT. Thermoluminescent dosimeters (TLD) were used to verify the accuracy of the techniques. The required thickness of bolus was investigated, and an anthropomorphic phantom was also used to examine the effects of air gaps between the wax bolus used for the IMRT technique and the patient's scalp. Neither static nor arcing electron techniques were able to provide a reliable coverage of the planning target volume (PTV), owing to obliquity of the fields in relation to the scalp. The IMRT technique considerably improved PTV dose uniformity, though it irradiated a larger volume of brain. Either 0.5 cm or 1.0 cm of wax bolus was found to be suitable. Air gaps of up to 1 cm between the bolus and the patient's scalp were correctly handled by the treatment-planning system and had negligible influence on the dose to the scalp. Photon IMRT provides a feasible alternative to electron techniques for treatment of large scalp lesions, resulting in improved homogeneity of dose to the PTV but with a moderate increase in dose to the brain.

SU-FF-T-326: Application of the Pencil-Beam Redefinition Algorithm to Electron Arc Therapy

Purpose: A dose algorithm suitable for electron arc therapy is not available on existing commercial treatment planning systems. This project investigated the potential for simulating an electron arc therapy beam by summing (1° angular steps) fixed-beam dose distributions calculated using the pencil-beam redefinition algorithm (PBRA). Method and Materials: Similar to the Hogstrom pencil-beam algorithm (PBA), the PBRA was commissioned for fixed beam geometries typical of electron arc therapy. The PBRA was constrained to calculate dose output for fixed beams as a function of field width and SSD. For arced beams, a small width correction was incorporated to conserve integral dose. Resulting arced beam calculations of dose output and mid-arc depth dose were evaluated by comparing to data measured using cylindrical water phantoms (ρ=12, 15 cm). Calculated 2D relative dose distributions in the plane of rotation were evaluated using film dosimetry in a polystyrene cylindrical phantom (ρ=13.5 cm). A wide range of treatment parameter combinations was investigated — three field sizes, three radii of curvature, and multiple arc angles at 10 and 15 MeV. Results: For arced beams, the maximum difference between PBRA-calculated and measured dose output at mid-arc was 2%. Along mid arc, the maximum dose difference in low dose-gradient regions was 3.4%, and the maximum distance to agreement (DTA) of dose values in high dose-gradient regions was 2.2 mm. Away from mid-arc, the maximum dose difference was 2%, and the maximum DTA was 1.2 mm. Conclusion: Results showed that the PBRA summation method can calculate dose in homogeneous phantoms within 3.4%. Since PBRA calculations in patients have been shown to be within 4% by Boyd et al., this method should be suitably accurate for use with electron arc therapy patients. User experience with the PBA and ease of PBRA beam commissioning make the PBRA attractive for this application.

Local???Regional Radiation Therapy After Breast Reconstruction: What Is the Appropriate Target Volume?

The oncologic safety and cosmetic outcome of immediate breast reconstruction in breast cancer patients requiring radiation therapy remains ill-defined. Between 1980 and 1998, 18 patients were treated at the University of Utah Medical Center with mastectomy, immediate breast reconstruction, and adjuvant radiation therapy delivered via an electron arc technique. A case-control study was performed matching reconstructed patients in a 1:2 ratio with patients undergoing mastectomy without reconstruction, using number of lymph nodes and tumor size. Median follow-up was 61 months for the reconstructed group. Five-year local-regional control, disease-free survival, and overall survival rates were 87%, 58%, and 74% respectively in the reconstructed group, versus 88%, 57%, and 67% respectively in the matched control group. Cosmesis was good/excellent in 11 of 13 living patients (85%). Significant capsular contraction occurred in 18% of prosthetic reconstruction patients, and revisional surgery was required in 24% of prosthetic reconstruction patients. Utilizing the electron arc technique, the median radiation dose to the chest wall at the midlevel of the ribs was 20% of the prescribed dose, and no patient failed deep to the implant. These results suggest that in appropriately selected patients, structures deep to the reconstruction are not at high risk for local-regional recurrence, and immediate breast reconstruction yields comparable local-regional control, disease-free survival, and overall survival rates to nonreconstructed patients, with acceptable cosmetic results.

A Comparative Study of Seed Crystals For The Phosphorus Crystallization Process

This study was performed to select a preferred seed crystal material for the phosphorus crystallization process through a comparative study of four materials: electron arc furnace, blast furnace and converter slag, and phosphate rock. Leaching and phosphorus removal tests were conducted to evaluate the efficacy of the four materials as seeding agents. Converter slag demonstrated a much larger leaching capacity with respect to calcium and hydroxide ions than did either electron arc furnace or blast furnace slag. The average phosphorus removal efficiencies of phosphorus rock, and the electron arc furnace, blast furnace and converter slag were 35.9%, 74.2%, 99.1%, and 94.5% following 20, 17, 32, and 175 days of leach time, respectively. Consideration of both technical and cost factors suggests that converter slag may have competitive advantages over the other three seed materials. X-ray diffraction (XRD) analysis showed that peak patterns for hydroxyapatite increased slightly as the hydrous flow time was extended. Scanning electron micrograph (SEM) images revealed that finely distributed cubic crystals were deposited on the surfaces of converter slag. When analyzed by energy dispersive x-ray spectrometer (EDS) mapping, the crystals gave a composition mole ratio (Ca/P) of 1.97 and they were determined to be hydroxyapatite.