Y Zheng

Quantitative megavoltage radiation therapy dosimetry using the storage phosphor KCl: Eu2+.

This work, for the first time, reports the use of europium doped potassium chloride (KCl:Eu2+) storage phosphor for quantitative megavoltage radiation therapy dosimetry. In principle, KCl:Eu2+ functions using the same photostimulatated luminescence (PSL) mechanism as commercially available BaFBr0.85I0.15:Eu2+ material that is used for computed radiography (CR) but features a significantly smaller effective atomic number-18 versus 49-making it a potentially useful material for nearly tissue-equivalent radiation dosimetry. Cylindrical KCl:Eu2+ dosimeters, 7mm in diameter and 1mm thick, were fabricated in-house. Dosimetric properties, including radiation hardness, response linearity, signal fading, dose rate sensitivity, and energy dependence, were studied with a laboratory optical reader after irradiation by a linear accelerator. The overall experimental uncertainty was estimated to be within ±2.5%. The findings were (1) KCl:Eu2+ showed satisfactory radiation hardness. There was no significant change in the stimulation spectra after irradiation up to 200Gy when compared to a fresh dosimeter, indicating that this material could be reused at least 100 times if 2Gy per use was assumed, e.g., for patient-specific IMRT QA. (2) KCl:Eu2+ exhibited supralinear response to dose after irradiation from 0to800cGy. (3) After x ray irradiation, the PSL signal faded with time and eventually reached a fading rate of about 0.1%∕h after 12h. (4) The sensitivity of the dosimeter was independent of the dose rate ranging from 15to1000cGy∕min. (5) The sensitivity showed no beam energy dependence for either open x ray or megavoltage electron fields. (6) Over-response to low-energy scattered photons was comparable to radiographic film, e.g., Kodak EDR2 film. By sandwiching dosimeters between low-energy photon filters (0.3mm thick lead foils) during irradiation, the over-response was reduced. The authors have demonstrated that KCl:Eu2+ dosimeters have many desirable dosimetric characteristics that make the material conducive to radiation therapy dosimetry. In the future, a large-area KCl:Eu2+-based CR plate with a thickness of the order of a few microns, created using modern thin film techniques, could provide a reusable, quantitative, high-resolution two-dimensional dosimeter with minimal energy dependence.

Theoretical and empirical investigations of KCl:Eu2+ for nearly water-equivalent radiotherapy dosimetry

PURPOSE The low effective atomic number, reusability, and other computed radiography-related advantages make europium doped potassium chloride (KCl : Eu2+) a promising dosimetry material. The purpose of this study is to model KCl : Eu2+ point dosimeters with a Monte Carlo (MC) method and, using this model, to investigate the dose responses of two-dimensional (2D) KCl : Eu2+ storage phosphor films (SPFs). METHODS KCl : Eu2+ point dosimeters were irradiated using a 6 MV beam at four depths (5-20 cm) for each of five square field sizes (5 x 5-25 x 25 cm2). The dose measured by KCl : Eu2+ was compared to that measured by an ionization chamber to obtain the magnitude of energy dependent dose measurement artifact. The measurements were simulated using DOSXYZnrc with phase space files generated by BEAMnrcMP. Simulations were also performed for KCl : Eu2+ films with thicknesses ranging from 1 microm to 1 mm. The work function of the prototype KCl : Eu2+ material was determined by comparing the sensitivity of a 150 microm thick KCl : Eu2+ film to a commercial BaFBr0.85 I0.15 : Eu(2+)-based SPF with a known work function. The work function was then used to estimate the sensitivity of a 1 microm thick KCl : Eu2+ film. RESULTS The simulated dose responses of prototype KCl : Eu2+ point dosimeters agree well with measurement data acquired by irradiating the dosimeters in the 6 MV beam with varying field size and depth. Furthermore, simulations with films demonstrate that an ultrathin KCl : Eu2+ film with thickness of the order of 1 microm would have nearly water-equivalent dose response. The simulation results can be understood using classic cavity theories. Finally, preliminary experiments and theoretical calculations show that ultrathin KCl : Eu2+ film could provide excellent signal in a 1 cGy dose-to-water irradiation. CONCLUSIONS In conclusion, the authors demonstrate that KCl : Eu(2+)-based dosimeters can be accurately modeled by a MC method and that 2D KCl : Eu2+ films of the order of 1 microm thick would have minimal energy dependence. The data support the future research and development of a KCl : Eu2+ storage phosphor-based system for quantitative, high-resolution multidimensional radiation therapy dosimetry.

SU‐GG‐AUD‐06: Stray Radiation Exposure During Proton Radiotherapy of the Prostate: The Influence of the Patient On Scatter and Production

Purpose: To characterize the scatter, production, and attenuation of secondary radiation in patients receiving passively‐scattered protonradiotherapy for prostate cancer.Methods and Materials: A proton therapytreatment was simulated using a Monte Carlo model of a double scattering treatment machine. Whole body effective dose (E) from secondary radiation was estimated from a weighted sum of doses to the major organs in an anthropomorphic phantom. The effect of the patient on secondary dose was quantified by comparing E with ambient dose equivalent, H*(10), which was based on free‐in‐air spectral fluence calculations at isocenter. Various treatment parameters (proton beam energy, range modulation width, field size, and snout position) were varied in order to study their influence on E and H*(10). Results: The calculated E for the simulated treatment was 7.8 mSv/Gy, while the calculated H*(10) at isocenter was 16 mSv/Gy. Both E and H*(10) approximately doubled over the range of modulation widths and energies studied. As field size increased from 0×0 to 15×15, E doubled, while H*(10) decreased by 30%. When the snout position was changed from 30 cm to 48 cm, E decreased by less than 20%, while H*(10) decreased by 44% over the same interval. Simulations revealed that, while E is predominated by neutrons generated in the nozzle, neutrons produced in the patient contributed significantly (up to 40%) to dose equivalent in near‐field organs. In most cases, H*(10) provided a conservative estimate of E. However — because H*(10) does not account for neutrons created in the patient — it did not conservatively estimate E for large field sizes, where neutron production in the patient becomes significant. Conclusions:Neutrons generated in the patient contribute significantly to exposures to organs near the irradiated volume. When evaluating stray radiation exposure, production, scatter, and attenuation in the patient should be taken into consideration.

TU‐C‐AUD‐02: Study of Neutron Exposure During Passively Scattered Proton Therapy

Purpose: Stray radiation exposures are of concern for patients receiving protonradiotherapy and vary strongly with several treatment factors such as proton energy, field size and modulation width. The purposes of this study were to conservatively estimate neutron exposures for a contemporary passive scatteringprotontreatment unit and to understand how they vary with treatment factors. Method and Materials: We simulated all 24 options (each range modulator and second scatterer combination is accounted for one option) for a passive scatteringproton therapy unit with MCNPX. Spectral neutron fluence from simulations was then converted to neutrondose equivalent using corresponding dose conversion factors. We studied the neutrondose equivalent per therapeutic absorbed dose (H/D) as a function of treatment factors including proton energy, location in the treatment room, treatmentfield size, and spread‐out Bragg peak (SOBP) width using Monte Carlo simulation.Results: The H/D value at isocenter for a 250‐MeV medium field size option was estimated to be 20 mSv Gy−1. H/D decreased to about 20% from 250 Mev to 160 MeV. H/D fell off sharply with distance from the treatment unit, approximately following a power law; H/D was about 10% higher for a large field option than a medium field option for the same energy. H//D almost doubled when SOBP width was increased from a pristine peak to 16 cm. An analytical model was developed, which predicted H/D values within 28% of those obtained in simulations; this value is within typical neutron measurement uncertainties. Conclusion: The results quantified how treatment factors influence H/D values. The in‐air method with a closed aperture presented here provides a simple and straightforward approach that could be adopted for facility inter‐comparisons. In addition, an analytical model was developed to quickly estimate H/D values.

SU-GG-T-380: Design and Shielding Considerations for the World's First Compact Synchrocyclotron Proton Therapy Unit

Purpose: Our institution will soon take delivery of the Monarch 250 Compact Proton Therapy Unit (Still River Systems). We performed facility shielding calculations that accounted for the produced neutron flux. Method and Materials: Workload assumptions (80Gy/day) yielded the number of protons produced per day. The vendor did not provide extraction efficiency, neutron yields from the treatment field shaping system, or neutron yield estimates from the synchrocyclotron. To provide a conservative neutron yield we assumed; the largest scatteredfield size and complete portal blocking, conservatively low cyclotron production efficiency (25%), and a high quality factor (20) for neutron dose equivalent calculations. Neutrons were assumed to be produced at the isocenter location, by stopping the SOBP protons in a Cu target. The room design includes a sub‐floor to allow rotation of the accelerator below the sub‐floor level. Classical analytical methods were used to calculate required barrier thicknesses based on neutron yields, accounting for angular dependences, and based on concrete (standard or high density (HD)), earth, and steel. Maze calculations used analytical approaches, heavily influenced by the number of bounce paths, and maze area. Conservative use and occupancy factors were used. Results: The shielding calculations resulted in barrier thicknesses similar to existing proton facilities; from 4ft to 6.25ft of HD concrete. Some design features included; reducing the maze ceiling height to reduce the source area of scatteredneutrons, adding an additional leg at the door entrance to allow for an unshielded door, and adding steel to surround unavoidable barrier voids. To reduce cost, the barriers that did not shield occupy‐able space, used standard concrete. Conclusion: The neutron yield from the Monarch 250 is unknown, so conservative values were used in the shielding calculations. Vault design strategies were developed to reduce overall construction cost. We allow for contingencies if we underestimated any barrier thickness.

SU‐GG‐T‐381: Monte Carlo Study of Neutron Dose Equivalent for a Compact Proton Therapy Unit

Purpose: A compact proton therapy unit (Still River Systems, Littleton, MA) is under development. This will be the first such unit in the world and neutron exposure for this machine is unknown. Our purpose is, using Monte Carlo methods, to model the detailed beam line, simulate neutron fluence, and estimate neutron dose equivalent for this proton therapy unit. Method and Materials: The compact proton therapy unit utilizes a synchrocyclotron, which will be aligned together with the beam shaping system (BSS) in a single room. We modeled the proton source coming out of the cyclotron as a Gaussian distribution with a mean energy of 250 MeV. The BSS uses passive scattering techniques to spread out the beam laterally and in depth. We modeled each BSS component in detail according to the design data provided by the manufacturer. The MCNPX code (Version 2.5.0) system was used. The neutron spectral fluence in air was simulated with a closed aperture and without range modulation. Neutron dose equivalents were then calculated using the neutron fluence to dose equivalent conversion factors based on ICRP publication 74. Up to 108proton histories were tracked per simulation. Results: Fourteen options (combinations of modulator wheel and scatterer) were modeled. Neutron spectral fluence was simulated and analyzed. The neutron dose equivalent per therapeuticproton dose (H/D) at isocenter ranged from 0.82 mSv/Gy for a shallow range and small field size option, and up to 17.4 mSv Gy−1 for a deep penetrating, and large field size option. Conclusion: The H/D values from this compact unit are comparable to those from conventional multi‐room proton therapy units employing passive scattering techniques. The H/D values decrease with decreasing maximum treatment depth (range), but are relatively higher for very shallow range options, compared with existing passive scattering units.

SU‐FF‐T‐457: Second Cancer Risk From Secondary Neutrons for a Boy Who Received Proton Craniospinal Irradiation

Purpose:Proton fields used in radiotherapy expose healthy tissue to secondary neutrons emanating from the treatment unit (or “external neutrons”) and produced within the patient (or “internal neutrons”), which provide no known benefit to the patient and may increase a patients risk of developing a second cancer. The objectives of this study were to calculate equivalent doses to organs and tissues and to estimate second cancer risk from secondary neutrons for a boy who received craniospinal irradiation (CSI) with proton beams. Method and Materials: Using Monte Carlo simulations, equivalent dose from secondary neutrons was calculated in organs and tissues for the 10‐year‐old boy. In order to maximize the accuracy and realism of the simulations, the geometric model comprised a detailed passive‐scattering proton treatment unit and a voxelized phantom that was created from the actual CT images of the patient. All treatment fields were included. The proton treatment included CSI at 30.6 Gy plus a boost of 23.4 Gy to the clinical target volume in the brain. Based on the equivalent dose values, effective dose and second cancer risk from secondary neutrons were predicted. Results: The effective dose from secondary neutrons was 418 mSv, of which 344 mSv was from external neutrons and 74 mSv was from internal neutrons. The lifetime attributable risks of second cancer incidence and mortality were 6.2% and 3.4%, respectively. The cancer sites that carried the highest risks were the lungs, colon, and thyroid. The risks were predominated by the two spinal fields; very little risk was associated with the boost fields. Conclusion: The results of this study provide an estimate of the secondary neutrondose and corresponding risk for a pediatric patient undergoing proton CSI and support the suitability of passively‐scattered proton beams for the treatment of tumors of the central nervous system in children.

MO‐FF‐A1‐05: Quantitative Radiation Therapy Dosimetry Using Reusable Storage Phosphor KCl:Eu2+

Purpose: To investigate the quantitative use of a novel KCl:Eu 2+ dosimeter for radiation therapydosimetry.Methods and Materials: Cylindrical KCl:Eu 2+ dosimeters 7 mm in diameter and 1 mm thick were fabricated.Dosimetricproperties were studied with an in‐house optical readout system after irradiation by a linear accelerator. The overall experimental uncertainty was estimated to be within ±2.5%. Results: 1) KCl:Eu 2+ showed satisfactory radiation hardness. There was no significant change in the stimulation spectra after irradiation up to 200 Gy when compared to a fresh sample, indicating that this material could be reused at least 100 times if 2 Gy per use was assumed, e.g., for a patient‐specific IMRT QA; 2) KCl:Eu 2+ exhibited supra‐linear response to dose after irradiation from zero cGy to 800 cGy; 3) After x‐ray irradiation the PSL signal decayed with time and eventually reached a plateau (0.1% per hour) after 12 hours, which was significantly better than a commercial CR(computed radiography) plate; 4) The sensitivity of the dosimeter was independent of the dose rate ranging from 15 cGy/min (underneath a MLC) to 1000 cGy/min; 5) The sensitivity showed no energy dependence for either open x‐ray or megavoltage electron fields; 6) Over‐response to low‐energy scattered photons was comparable to radiographic film, e.g., a Kodak EDR2 film. Reduction of dosimeter thickness to tens of microns would minimize the energy dependence. Conclusion: We have demonstrated that KCl:Eu 2+ dosimeters have many desirable dosimetriccharacteristics and that KCl:Eu 2+ has great potential to replace BaFBr 0.85 I 0.15 : Eu 2+ as the primary storage phosphormaterial for radiation therapydosimetry. In the future, a large‐area KCl:Eu 2+ based CR plate created using modern thin film techniques would provide a reusable, quantitative, high‐resolution two‐dimensional dosimeter with minimal energy dependence. This work is supported in part by an NIH grant CA131690.

SU‐FF‐T‐271: Measurement Validation of Attenuation Values of Proton Facility Shielding Materials

Purpose: Strategies for design and shielding methods vary considerably with proton facilities. Material chosen for shielding is balance of effectiveness and cost. For space‐restricted environments, high‐density materials are desirable. Shielding calculations, whether analytical or statistical rely, on the neutron attenuations properties of chosen materials. The purpose of this study was to measure and calculate attenuation of various materials to be used for shielding. Methods and Materials: We acquired samples of the actual materials to be used to shield neutrons for our new facility. The original calculations relied on analytical methods and classically accepted values for neutron attenuation. Planned materials include: standard concrete (SC)∼136lb/ft3, high‐density concrete (HDC)∼250lb/ft3, Hematite mix (HM), steel (S), and borated polyethylene (BP). For the measurements, we used a 5Ci plutonium‐beryllium (239PuBe) neutron source (1″ in diameter and 5″ in length) with an energy spectrum ranging up to 11MeV. A Bonner‐sphere spectrometer was used for neutron measurements. The material samples were configured in a manner to allow measurements through various paths. A Fortran code was used to reconstruct the neutron spectrum and calculate the neutron dose equivalent. Results: Transmission curves were generated to yield TVL values, which were compared to those used in our analytical calculations. Measured TVL (cm) for the SC, HDC, HM, S, and BP were 55.9, 55.1, 63.1, 35.6, 60.6, respectively. The TVL (cm) used for the calculations were 85.6, 52.4, 32.9 respectively for SC, HDC, and S. Conclusions: Measured TVL values matched analytical calculations very closely, with the exception of standard concrete. The measurements performed in this study assisted in better understanding of attenuation properties of each material and increased our comfort level pre‐construction. As expenditures in the range of millions of dollars were about to be spent, the question of material choices, and the validity of the actual material mix were alleviated.

WE‐E‐BRB‐07: Quality Assurance of Proton Range Compensators by CT Scanning

Purpose: Patient specific range compensators (RCs) are required in passively scattered proton therapy. Conventionally, quality assurance (QA) of RC is performed by either a visual inspection or spot check of thicknesses for some drill points (e.g. 4). A more comprehensive QA of RCs is needed for precise delivery of proton beams. The purpose of this study was to develop a QA procedure by use of computed tomography(CT).Method and Materials: For the procedure, one or more range compensators would be scanned with a CT scanner. Software was developed to automatically segment the range compensator on CTimages and calculate the thickness matrix for the RC. The CT‐scanned RC thickness matrix was then compared to the expected one, which was reconstructed from the DICOM ION Radiotherapy Plan exported from a proton treatment planning system. The actual milling tool size and shape were taken into account when the RC was reconstructed. The scanned RC thickness matrix was automatically aligned to the planned RC matrix. In addition to comparing the physical thickness of the RC, we also verified the water equivalence thickness of the range compensator based on the CT scan. This would allow us to detect possible impurities or other defects of the RC. As part of the validation, we tested not only an unaltered RC, but RC that was damaged or altered post‐construction. Results: We developed software to process the scanned CTimages of the RC and reconstruct the planned RC from the radiation therapy treatment plan. We compared both the physical thickness and water equivalent thickness between the scanned RC and planned RC to quality assurance the manufacturing of the RC. Our process was able to detect deviations of less than 2 mm. Conclusion: We have developed a procedure to efficiently and comprehensively QA the RC.