Dmitri Nichiporov

Clinical characterization of a proton beam continuous uniform scanning system with dose layer stacking

A proton beam delivery system on a gantry with continuous uniform scanning and dose layer stacking at the Midwest Proton Radiotherapy Institute has been commissioned and accepted for clinical use. This paper was motivated by a lack of guidance on the testing and characterization for clinical uniform scanning systems. As such, it describes how these tasks were performed with a uniform scanning beam delivery system. This paper reports the methods used and important dosimetric characteristics of radiation fields produced by the system. The commissioning data include the transverse and longitudinal dose distributions, penumbra, and absolute dose values. Using a 208 MeV cyclotrons proton beam, the system provides field sizes up to 20 and 30 cm in diameter for proton ranges in water up to 27 and 20 cm, respectively. The dose layer stacking method allows for the flexible construction of spread-out Bragg peaks with uniform modulation of up to 15 cm in water, at typical dose rates of 1-3 Gy/min. For measuring relative dose distributions, multielement ion chamber arrays, small-volume ion chambers, and radiographic films were employed. Measurements during the clinical commissioning of the system have shown that the lateral and longitudinal dose uniformity of 2.5% or better can be achieved for all clinically important field sizes and ranges. The measured transverse penumbra widths offer a slight improvement in comparison to those achieved with a double scattering beam spreading technique at the facility. Absolute dose measurements were done using calibrated ion chambers, thermoluminescent and alanine detectors. Dose intercomparisons conducted using various types of detectors traceable to a national standards laboratory indicate that the measured dosimetry data agree with each other within 5%.

Energy spectrum control for modulated proton beams

In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to +/-21% over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than +/-3%. To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.

Beam characteristics in two different proton uniform scanning systems: A side‐by‐side comparison

PURPOSE To compare clinically relevant dosimetric characteristics of proton therapy fields produced by two uniform scanning systems that have a number of similar hardware components but employ different techniques of beam spreading. METHODS This work compares two technologically distinct systems implementing a method of uniform scanning and layer stacking that has been developed independently at Indiana University (IU) and by Ion Beam Applications, S. A. (IBA). Clinically relevant dosimetric characteristics of fields produced by these systems are studied, such as beam range control, peak-to-entrance ratio (PER), lateral penumbra, field flatness, effective source position, precision of dose delivery at different gantry angles, etc. RESULTS Under comparable conditions, both systems controlled beam range with an accuracy of 0.5 mm and a precision of 0.1 mm. Compared to IBA, the IU system produced pristine peaks with a slightly higher PER (3.23 and 3.45, respectively) and smaller, symmetrical, lateral in-air penumbra of 1 mm compared to about 1.9/2.4 mm in the inplane/crossplane (IP/CP) directions for IBA. Large field flatness results in the IP/CP directions were similar: 3.0/2.4% for IU and 2.9/2.4% for IBA. The IU system featured a longer virtual source-to-isocenter position, which was the same for the IP and CP directions (237 cm), as opposed to 212/192 cm (IP/CP) for IBA. Dose delivery precision at different gantry angles was higher in the IBA system (0.5%) than in the IU system (1%). CONCLUSIONS Each of the two uniform scanning systems considered in this work shows some attractive performance characteristics while having other features that can be further improved. Overall, radiation field characteristics of both systems meet their clinical specifications and show comparable results. Most of the differences observed between the two systems are clinically insignificant.

Verification of absolute ionization chamber dosimetry in a proton beam using carbon activation measurements.

Reference ionization chamber dosimetry implemented in a clinical proton beam and based on the ICRU 59 recommendations has been verified with an independent carbon activation method. The 12C(p,pn)11C nuclear reaction was used to measure the beam fluence and entrance dose. A method to transfer from the entrance dose to the dose at the ion chamber calibration position has been developed. Measurements performed in a monochromatic 200 MeV beam show that the ratio of absolute doses measured using the carbon activation and the ion chamber methods is 1.017 +/- 0.03 (type A uncertainty). This result is within the uncertainties of both methods employed, which are estimated at +/- 4.3% (carbon activation) and +/- 2.7% (ion chamber calibration).

SU‐E‐T‐491: A FLUKA Monte Carlo Computational Model of a Scanning Proton Beam Therapy Nozzle at IU Proton Therapy Center

PURPOSE Charged particle therapy, especially proton therapy is a growing treatment modality worldwide. Monte Carlo (MC) simulation of the interactions of proton beam with equipment, devices and patient is a highly efficient tool that can substitute measurements for complex and unrealistic experiments. The purpose of this study is to design a MC model of a treatment nozzle to characterize the proton scanning beam and commissioning the model for the Indiana University Health Proton Therapy Center (IUHPTC. METHODS The general purpose Monte Carlo code FLUKA was used for simulation of the proton beam passage through the elements of the treatment nozzle design. The geometry of the nozzle was extracted from the design blueprints. The initial parameters for beam simulation were determined from calculations of beam optics design to derive a semi-empirical model to describe the initial parameters of the beam entering the nozzle. The lateral fluence and energy distribution of the beam entering the nozzle is defined as a function of the requested range. The uniform scanning model at the IUHPTC is implemented. The results of simulation with the beam and nozzle model are compared and verified with measurements. RESULTS The lateral particle distribution and energy spectra of the proton beam entering the nozzle were compared with measurements in the interval of energies from 70 MeV to 204.8 MeV. The accuracy of the description of the proton beam by MC simulation is better than 2% compared with measurements, providing confidence for complex simulation in phantom and patient dosimetry with the MC simulated nozzle and the uniform scanning proton beam. CONCLUSIONS The treatment nozzle and beam model was accurately implemented in the FLUKA Monte Carlo code and suitable for the research purpose to simulate the scanning beam at IUHPTC.

SU‐E‐T‐10: A Computational Model for the Estimation of Biological Dose in a Clinical Proton Beam

Purpose: The variability of relative biological effectiveness (RBE) of a proton beam with spatial distribution is well known. It is dependent on the proton beam characteristics, type of tissue and biological indices. A computational model based on a proton spectrum and LET is used for the estimation of the biological dose. Methods: The depth dose curve of the pristine beam is divided into two regions: the proximal region from the surface to the Bragg peak and the distal region beyond the peak. Knowing the beam energy at the surface in the proximal region, and using the published energy‐range tables, we estimated the effective proton energy at any depth proximal to the Bragg peak. Based on the energy, the LET and RBE is estimated. In the distal region, we assume that the effective beam energy changes monotonically from the value calculated at the peak to the final energy of about 3MeV at the 3% dose level on the distal edge. Applying the calculated RBE factor to the measured depth dose profile we obtain the biological dose in the pristine beam. The method can easily be applied to modulated beams. Results: Our computation model shows that it is possible to predict the biological dose for any combination of beam energy, and for SOBP size based on the available RBE data. Our results showed that the biological dose for a SOBP has finite slope rising to higher values close to distal edge of the peak. Conclusions: A computational model provides an unique tool to estimate the biological dose in any combination of beam energy and SOBP size. If the RBE vs LET data for a tissue is know our computational approach could provide the biological dose accurately in a clinical beam.

SU‐GG‐T‐336: Effect of Treatment and Beam Parameters on Surface Dose in Proton Beam Therapy

Purpose: The advantage of lower skin dose in protonbeam therapy may result in less radiation‐related side effects which are typically seen in hypo‐fractionated conventional external beam therapies. In this study, we evaluate the surface dose (SD) in proton therapy as a function of various beam parameters. Materials & Methods: SD is defined as the ratio of absorbed dose on CAX at surface to that at the middle of the Spread‐Out Bragg Peak (SOBP). SD in proton therapy is affected by several parameters: energy ( E ), SOBP , source‐to‐surface distance ( SSD ), air gap ( g ), field radius ( r ), material thickness upstream of surface besides air ( t ), atomic number of medium ( Z ), beam angle relative to surface (q), and nozzle type ( N ). The parameters, t, Z and q are not included in this study.. Results: Giving a proton range in water of 27 cm, SD rises from 30 to 90% with SOBP increasing from 0 to17cm. At high energy, SD is 5% higher in the uniform scanning (US) nozzle than in double scattering (DS) nozzle. This can be explained by the large difference in source‐to‐axis distances in US and DS nozzles (250 cm vs 320 cm). SSD and g have minimal impact on SD for r > 2.5cm. For small fields ( r < 2.5cm), SD increases significantly with field size decreases. Within 15mm of the surface there is a small but pronounced 2% buildup in protonbeam at high energies, which may be partially due to secondary electrons, secondary protons and heavy charged particles. Conclusions: In a clinical setting, SD is most significantly affected by SOBP extent, followed by field size, SAD, and beamenergy. In general, SD ranges from 50 to 95%. It is important to clearly understand and minimize SD in Proton therapy, especially in hypo‐fractioned treatments.

SU‐FF‐T‐373: Investigation of Humidity Effects On Beam Monitor Performance in a Proton Clinical Gantry

Purpose: To investigate the effect of ambient humidity on performance characteristics of ionization chambers used as beam monitors in two clinical proton gantry systems, and to choose optimal electrodematerials.Method and Materials: Dose calibration results for two identical, vented beam monitors, with electrodes constructed of Mylar film with graphitecoating, and each installed in a gantry nozzle, were analyzed for stability over a period of 1 year. Additionally, two ion chambers of similar design, but constructed using Kapton for base layer and either i) gold‐plated copper or ii) graphite paint for electrodes, were fabricated for testing purposes. The test chambers were placed in a controlled humidity environment and their performance was evaluated in a proton beam under various humidity conditions. Results: Mylar‐based electrodes of the gantry beam monitors provide a dose calibration that varied within ±2% (maximum deviation from mean), over a period of 1 year while rooms relative humidity (RH) varied from 20% to 70%, with brief excursions to 15% and 80%. Kapton‐based copper gold‐plated electrodes provided linear (±1%) response in dose rates up to 180 Gy/min, and were stable within ±1% in terms of measured charge per monitor unit, in the range of 10%–90% RH. The response of Kapton‐based carbon paint electrodes was linear (±1%) in dose rate up to 8 Gy/min and showed a slight rising trend (about 3% increase) in the 50% – 80% RH range. Conclusion: Treatment room humidity may affect the performance of Mylar‐based electrodes in a beam monitor with open‐to‐atmosphere design. When humidity of 70% RH or higher is possible, copper gold‐plated electrodes on Kapton show superior performance compared with graphite paint electrodes.

TH‐C‐M100F‐06: Therapeutic Uniform Scanning Proton Beam Development and Characterization: Longitudinal Results

Purpose: To utilize a new depth‐dose detector and to construct, develop, characterize and quantify longitudinal beam profiles with an active uniform scanning proton beam delivery system. Method and Materials: The active uniform scanning beam delivery system is composed of two primary devices—wobbler magnet and binary range modulator. Through transverse scanning a large proton beam spot at about 10–15Hz, the wobbler magnet creates a flat integral transverse field. The binary range modulator constructs spread out Bragg peaks (SOBP) by incrementally stepping a pristine Bragg peak by 3mm or 6mm in depth. A depth‐dose detector was constructed to acquire all the longitudinal beam data. This detector, the Multi Ion ChamberDetector (MLIC), is composed of 122 parallel‐plate ion chambers in a 1D array. Results: Upon proper calibration and commissioning, the MLIC detector performed well for the acquisition of longitudinal beam data, providing adequate discrete data with 1.8mm spatial resolution. The full width half maximum of various energy pristine Bragg peaks was measured. This data necessitated changing the range modulator step size from 6mm to 3mm for all SOBPs below a certain proton energy (i.e. equivalent to 12.0cm range in water). The pristine peak and SOBP exhibit a range deficit that decreases as proton energy decreases. This effect is also correlated to the FWHM of the pristine peak. A library of various SOBP extents, from 14.5cm to 2.2cm, was constructed and shown to pass clinical specification. For a given SOBP, the skewness, defined as the slope of the 100% isodose or SOBP “flattop”, was correlated to proton range in water. Conclusion: The MLIC proved useful, efficient and satisfactory in measuring depth‐dose profiles for active scanning proton beams. The active scanning beam delivery system delivered therapeutically useable fields with increased modularity and flexibility compared to passive scattering techniques.

TU-FF-A2-04: Preliminary Radiological Characterization of An Active Proton Beam Spreading System for Therapeutic Use

Purpose: At most proton therapy centers, a clinically useable radiation field is delivered by means of a passive beam spreading system. However, at this proton therapy facility, an active proton beam spreading system has been installed in an isocentric gantry treatment room. Because there exists no standard acceptance test guidelines for an active proton beam delivery system, this presentation serves to report methods and preliminary results from the acceptance testing and early commissioning phase. Method and Materials: Though passive spreading is the most common method of beam formation, the use of metal scatterers reduces the maximum beam range and results in a higher integral dose from concomitant scattered radiation. The active proton beam delivery methods, such as uniform scanning, have the potential of increased range and lower integral dose in comparison to passive spreading. Ionization chamber scans in a step‐by‐step mode in a scanning water phantom were used to acquire the bulk of the data. Results: In the transverse plane perpendicular to the beam axis, the active beam delivery system is capable of delivering a field to within the clinical specification of +/− 2.5%. In addition, the penumbra from the beam delivery system at 5 and 25 cm depth increase from 1.6mm to 10.0mm along the pristine Bragg peak. Along the longitudinal axis, the radiation field meets the range requirement. The SOBP causes a range deficit with the pristine peak of 0.25mm measured from 80–20% dose levels. Also, the SOBP extent flatness exceeds the clinical specification of +/− 2.5% and adjustable with a skewness parameter on the fly. Conclusion: The active beam delivery system at this proton therapy facility is capable of delivering a therapeutically acceptable radiation beam. Furthermore, the active system represents a significant step enroute to the goal of intensity modulated proton therapy.