A Schreuder

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%.

Commissioning of output factors for uniform scanning proton beams

PURPOSE Current commercial treatment planning systems are not able to accurately predict output factors and calculate monitor units for proton fields. Patient-specific field output factors are thus determined by either measurements or empirical modeling based on commissioning data. The objective of this study is to commission output factors for uniform scanning beams utilized at the ProCure proton therapy centers. METHODS Using water phantoms and a plane parallel ionization chamber, the authors first measured output factors with a fixed 10 cm diameter aperture as a function of proton range and modulation width for clinically available proton beams with ranges between 4 and 31.5 cm and modulation widths between 2 and 15 cm. The authors then measured the output factor as a function of collimated field size at various calibration depths for proton beams of various ranges and modulation widths. The authors further examined the dependence of the output factor on the scanning area (i.e., uncollimated proton field), snout position, and phantom material. An empirical model was developed to calculate the output factor for patient-specific fields and the model-predicted output factors were compared to measurements. RESULTS The output factor increased with proton range and field size, and decreased with modulation width. The scanning area and snout position have a small but non-negligible effect on the output factors. The predicted output factors based on the empirical modeling agreed within 2% of measurements for all prostate treatment fields and within 3% for 98.5% of all treatment fields. CONCLUSIONS Comprehensive measurements at a large subset of available beam conditions are needed to commission output factors for proton therapy beams. The empirical modeling agrees well with the measured output factor data. This investigation indicates that it is possible to accurately predict output factors and thus eliminate or reduce time-consuming patient-specific output measurements for proton treatments.

Measurements of neutron dose equivalent for a proton therapy center using uniform scanning proton beams

PURPOSE Neutron exposure is of concern in proton therapy, and varies with beam delivery technique, nozzle design, and treatment conditions. Uniform scanning is an emerging treatment technique in proton therapy, but neutron exposure for this technique has not been fully studied. The purpose of this study is to investigate the neutron dose equivalent per therapeutic dose, H/D, under various treatment conditions for uniform scanning beams employed at our proton therapy center. METHODS Using a wide energy neutron dose equivalent detector (SWENDI-II, ThermoScientific, MA), the authors measured H/D at 50 cm lateral to the isocenter as a function of proton range, modulation width, beam scanning area, collimated field size, and snout position. They also studied the influence of other factors on neutron dose equivalent, such as aperture material, the presence of a compensator, and measurement locations. They measured H/D for various treatment sites using patient-specific treatment parameters. Finally, they compared H/D values for various beam delivery techniques at various facilities under similar conditions. RESULTS H/D increased rapidly with proton range and modulation width, varying from about 0.2 mSv/Gy for a 5 cm range and 2 cm modulation width beam to 2.7 mSv/Gy for a 30 cm range and 30 cm modulation width beam when 18 × 18 cm(2) uniform scanning beams were used. H/D increased linearly with the beam scanning area, and decreased slowly with aperture size and snout retraction. The presence of a compensator reduced the H/D slightly compared with that without a compensator present. Aperture material and compensator material also have an influence on neutron dose equivalent, but the influence is relatively small. H/D varied from about 0.5 mSv/Gy for a brain tumor treatment to about 3.5 mSv/Gy for a pelvic case. CONCLUSIONS This study presents H/D as a function of various treatment parameters for uniform scanning proton beams. For similar treatment conditions, the H/D value per uncollimated beam size for uniform scanning beams was slightly lower than that from a passive scattering beam and higher than that from a pencil beam scanning beam, within a factor of 2. Minimizing beam scanning area could effectively reduce neutron dose equivalent for uniform scanning beams, down to the level close to pencil beam scanning.

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.

Range and modulation dependencies for proton beam dose per monitor unit calculations

Calculations of dose per monitor unit (D/MU) are required in addition to measurements to increase patient safety in the clinical practice of proton radiotherapy. As in conventional photon and electron therapy, the D/MU depends on several factors. This study focused on obtaining range and modulation dependence factors used in D/MU calculations for the double scattered proton beam line at the Midwest Proton Radiotherapy Institute. Three dependencies on range and one dependency on modulation were found. A carefully selected set of measurements was performed to discern these individual dependencies. Dependencies on range were due to: (1) the stopping power of the protons passing through the monitor chamber; (2) the reduction of proton fluence due to nuclear interactions within the patient; and (3) the variation of proton fluence passing through the monitor chamber due to different source-to-axis distances (SADs) for different beam ranges. Different SADs are produced by reconfigurations of beamline elements to provide different field sizes and ranges. The SAD effect on the D/MU varies smoothly as the beam range is varied, except at the beam range for which the first scatterers are exchanged and relocated to accommodate low and high beam ranges. A geometry factor was devised to model the SAD variation effect on the D/MU. The measured D/MU variation as a function of range can be predicted within 1% using the three modeled dependencies on range. Investigation of modulated beams showed that an analytical formula can predict the D/MU dependency as a function of modulation to within 1.5%. Special attention must be applied when measuring the D/MU dependence on modulation to avoid interplay between range and SAD effects.

Utilization of optical tracking to assess efficacy of intracranial immobilization techniques in proton therapy.

We present a quantitative methodology to measure head interfraction movements within intracranial masks of commercial immobilization devices used for proton radiotherapy. A three‐points tracking (3PtTrack) method was developed to measure the mask location for each treatment field over an average of 10 fractions for seven patients. Five patients were treated in supine with the Qfix Base‐of‐Skull (BoS) headframe, and two patients were treated in prone with the CIVCO Uni‐frame baseplate. Patients were first localized by an in‐room, image‐guidance (IG) system, and then the mask location was measured using the 3PtTrack method. Measured mask displacements from initial location at the first fraction are considered equivalent to the head interfraction movement within the mask. The trends of head movements and couch displacements and rotation were analyzed in three major directions. The accuracy of 3PtTrack method was shown to be within 1.0 mm based on daily measurements of a QA device after localization by the IG system for a period of three months. For seven patients, mean values of standard deviation (SD) in anterior–posterior, lateral, and superior–inferior directions were 1.1 mm, 1.4 mm, and 1.6 mm for head movements, and were 1.4 mm, 1.8 mm, and 3.4 mm for couch displacements. The mean SD values of couch rotations were 1.1°, 0.9°, and 1.1° for yaw, pitch, and roll, respectively. The overall patterns of head movements and couch displacements were similar for patients treated in either supine or prone, with larger deviations in the superior–inferior (SI) direction. A suboptimal mask fixation to the frame of the mask to the H&N frame is likely the cause for the observed larger head movements and couch displacements in the SI direction compared to other directions. The optical‐tracking methodology provided a quantitative assessment of the magnitude of head motion. PACS number: 87.55.kmWe present a quantitative methodology to measure head interfraction movements within intracranial masks of commercial immobilization devices used for proton radiotherapy. A three-points tracking (3PtTrack) method was developed to measure the mask location for each treatment field over an average of 10 fractions for seven patients. Five patients were treated in supine with the Qfix Base-of-Skull (BoS) headframe, and two patients were treated in prone with the CIVCO Uni-frame baseplate. Patients were first localized by an in-room, image-guidance (IG) system, and then the mask location was measured using the 3PtTrack method. Measured mask displacements from initial location at the first fraction are considered equivalent to the head interfraction movement within the mask. The trends of head movements and couch displacements and rotation were analyzed in three major directions. The accuracy of 3PtTrack method was shown to be within 1.0 mm based on daily measurements of a QA device after localization by the IG system for a period of three months. For seven patients, mean values of standard deviation (SD) in anterior-posterior, lateral, and superior-inferior directions were 1.1 mm, 1.4 mm, and 1.6 mm for head movements, and were 1.4 mm, 1.8 mm, and 3.4 mm for couch displacements. The mean SD values of couch rotations were 1.1°, 0.9°, and 1.1° for yaw, pitch, and roll, respectively. The overall patterns of head movements and couch displacements were similar for patients treated in either supine or prone, with larger deviations in the superior-inferior (SI) direction. A suboptimal mask fixation to the frame of the mask to the H&N frame is likely the cause for the observed larger head movements and couch displacements in the SI direction compared to other directions. The optical-tracking methodology provided a quantitative assessment of the magnitude of head motion. PACS number: 87.55.km.

MO‐F‐BRA‐02: Measurements of Neutron Dose Equivalent for Uniform Scanning Proton Beams

Purpose: Neutron exposure is of concern in proton therapy, and varies with beam delivery technique and treatment conditions. The purpose of this study is to investigate the neutron dose equivalent per therapeutic dose, H/D, under various treatment conditions for uniform scanning beams employed at our proton therapy center. Methods: Using a wide energy neutron dose equivalent detector (SWENDI‐ II, ThermoScientific, MA), we measured the neutron dose equivalent as a function of proton range, modulation width, beam scanning area, collimation aperture size and snout position. The influence of other factors on neutron dose equivalent, such as aperture material and the presence of a compensator were also evaluated. Neutron dose equivalents for various treatment sites were estimated. Comparison among different proton facilities and modalities (passive scattering, uniform scanning and pencil beam scanning) was performed. Results: The H/D values for uniform scanning beams varied from about 0.2 mSv/Gy for a 5 cm range and 2 cm SOBP width beam to 2.3 mSv/Gy for 30 cm range and 30 cm modulation width beam. H/D increased rapidly with proton range and modulation width, and decreased slowly with aperture size and snout retraction. H/D increased almost proportionally to the beam scanning area. The presence of a compensator actually reduced the H/D compared to that without a compensator present. Aperture material and compensator material also has an influence on neutron dose equivalent, but the influence is relatively small. Conclusions: This study presents H/D as a function of treatment parameters for uniform scanning proton beams. For similar treatment conditions, the H/D value was slightly lower than that from the passive scattering beam but higher than that from a spot scanning beam. Minimizing the beam scanning area could effectively reduce neutron dose equivalent for uniform scanning beams, down to the level of that generated in spot scanning.

SU‐E‐T‐722: Investigation of Range and Dosimetric Calculation Uncertainty for a Proton Treatment Planning System Using an Animal Tissue Phantom

Purpose: Accurate determination of proton dose and penetration range is critical in proton therapy. The purpose of this study is to investigate the range and dosimetric calculation accuracy for a commercial proton therapytreatment planning system (TPS). Methods: A lamb leg and solid water plates were used as phantoms in this study. Radiochromic films were inserted in the phantom plates, at depths roughly at the center of spread out Bragg peak (SOBP), the 80%, 50% and 20% distal falloff planes according to TPS calculations. The phantom was scanned by a CT scanner with 1.25 mm slice thickness. A treatment plan was created, using the XiO TPS (CMS, St. Louis, MO). Two beams were used in the plan, one with a compensator and the other without it. Image guidance was used to align the phantom before proton beams were delivered according to the treatment plan. The film‐measured doses were compared to TPS calculations. Results: At the center of SOBP plane, the measured dose agrees well with TPS calculation. The difference for field size is within 2 mm, and the penumbra difference is about 1 mm. At the distal falloff planes, the dose difference became large, up to 40% or more. The measured doses at various distal planes were used to reconstruct the proton ranges, which were about 2–3% deeper than TPS calculations. The detailed causes of the range uncertainty and their relative contributions to the uncertainty are being investigated. Conclusions: The XiO TPS calculates dose relatively accurately at the center of SOBP but with a large uncertainty in distal falloff region. The degree of accuracy seems to depend on tissue heterogeneity and compensator complexity. Using an animal tissue phantom, the study provides quantitative and relevant data to determine planning parameters, such as the range uncertainty, in protontreatment planning.

SU‐GG‐T‐451: Scattering Factor of Energy‐Stacking Layer on Outputs of Modulated Protons Using Uniform Scanning Technique

Purpose: To evaluate scattering factor of energy‐stacking‐layers (ESL) on outputs of modulated protons using uniform scanning technique. Methods: Using scanning magnet instead of scatterer requires minimal material to spread protons laterally. However, scattering induced by ESL becomes one of factors, such as beam range (R w ), modulation width (M w ) and field‐size (FS) that affect outputs of modulated protons. Outputs as functions of R w and M w were measured on gantries at Midwest ProtonRadiotherapy Institute (MPRI) and Procure Oklahoma City (OKC) center. Using similar beam‐line configurations, the energy spread for same R w and M w between the two gantries were almost identical at the phantom. In addition, MU chamber and scanning magnet were placed at similar location. However, ELS was placed upstream at OKC versus downstream to the scanning magnet at MPRI. Distances from ESL to MU chamber varied largely between two gantries at different facilities. As a result, scatterings of ELS impacted outputs differently between the two gantries. Observed trends of measured outputs as a function of r, defined as (M w ‐ R w )/ M w , were further utilized to examine the effect of ELS scattering factor. Results: After outputs for each individual M w were normalized to the output of 17cm R w with same M w , normalized outputs of 3.5 M w varied more than 10% to outputs of 12.5 M w at 13 and 27 cm R w for OKC outputs while only 5% variations were seen for MPRI outputs. Trend of output as a function of r also varied larger for OKC than MPRI. Conclusion: Larger varied trend as a function of r indicates that the scattering factor of ELS placed upstream, farther away from MU chamber, presents a larger effect on outputs in terms of R w and M w . The effect of ELS scattering on outputs for field‐size dependency need be further studied.

SU‐GG‐T‐472: Output Modeling for a Contemporary Proton Therapy Center

Purpose: Currently, no commercial protontreatment planning systems have been FDA‐approved for Monitor Unit (MU) prediction. Patient and field specific MU numbers are therefore determined by either measurements or in‐house modeling. The purpose of this project is to develop a straightforward and accurate analytical model to predict output factors and MU numbers for the first ProCure Proton Therapy center at Oklahoma City. Method and Materials: Our proton therapy center consists of four treatment rooms: one fixed beam room, one gantry room and two inclined beam rooms. All treatment rooms employ a uniform scanning technique, with proton beams coming out of an IBA Cyclotron. An analytical model was developed to predict the output factor based on beam commissioning data, taking into account the effect of proton range, modulation width, field size as well as inverse square correction for each field on the dose output. Field sizes were estimated based on the aperture printout from CMS treatment planning system. The analytical model was retrospectively applied to 101 fields for patients treated at our center and compared to the measured data. Results: A straightforward analytical model was developed to predict the output factor and determine the MU for patients treated at out proton therapy center. The model predicted output factors within 2% for most fields, with only 4 fields between 2% and 3%, and 2 fields out of 3%.Conclusion: The study demonstrated that the analytical model was adequate for output factor predication. The model has been used to determine MU numbers for prostate patients and will be extended to other patients in the near future. More studies are planned to further validate the model and improve the modeling accuracy. Criteria will be developed to determine cases where the analytical model may be unreliable and measurements are deemed necessary.