W Hsi

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%.

Dosimetric evaluation of a novel polymer gel dosimeter for proton therapy

PURPOSE The aim of this study is to evaluate the dosimetric performance of a newly developed proton-sensitive polymer gel formulation for proton therapy dosimetry. METHODS Using passive scattered modulated and nonmodulated proton beams, the dose response of the gel was assessed. A next-generation optical CT scanner is used as the readout mechanism of the radiation-induced absorbance in the gel medium. Comparison of relative dose profiles in the gel to ion chamber profiles in water is performed. A simple and easily reproducible calibration protocol is established for routine gel batch calibrations. Relative stopping power ratio measurement of the gel medium was performed to ensure accurate water-equivalent depth dose scaling. Measured dose distributions in the gel were compared to treatment planning system for benchmark irradiations and quality of agreement is assessed using clinically relevant gamma index criteria. RESULTS The dosimetric response of the gel was mapped up to 600 cGy using an electron-based calibration technique. Excellent dosimetric agreement is observed between ion chamber data and gel. The most notable result of this work is the fact that this gel has no observed dose quenching in the Bragg peak region. Quantitative dose distribution comparisons to treatment planning system calculations show that most (> 97%) of the gel dose maps pass the 3%/3 mm gamma criterion. CONCLUSIONS This study shows that the new proton-sensitive gel dosimeter is capable of reproducing ion chamber dose data for modulated and nonmodulated Bragg peak beams with different clinical beam energies. The findings suggest that the gel dosimeter can be used as QA tool for millimeter range verification of proton beam deliveries in the dosimeter medium.

Validation of dosimetric field matching accuracy from proton therapy using a robotic patient positioning system

Large area, shallow fields are well suited to proton therapy. However, due to beam production limitations, such volumes typically require multiple matched fields. This is problematic due to the relatively narrow beam penumbra at shallow depths compared to electron and photon beams. Therefore, highly accurate dose planning and delivery is required. As the dose delivery includes shifting the patient for matched fields, accuracy at the 1–2 millimeter level in patient positioning is also required. This study investigates the dosimetric accuracy of such proton field matching by an innovative robotic patient positioner system (RPPS). The dosimetric comparisons were made between treatment planning system calculations, radiographic film and ionization chamber measurements. The results indicated good agreement amongst the methods and suggest that proton field matching by a RPPS is accurate and efficient. PACS number: 87.55.km

SU‐GG‐T‐270: Evaluation of a Novel 3‐D Polymer Gel Dosimetry System for Proton Radiotherapy

Purpose: Preliminary evaluation of a novel, large‐volume (2.2 liter), 3D polymergeldosimeter (BANG®‐3‐PRO) developed for protonradiotherapydosimetry is presented. The characteristics of dose response and its linear energy transfer (LET) dependence are addressed. The performance of the dosimeter is analyzed under the conditions of pristine and clinically relevant beam configurations. The ability to visualize and analyze protondose distributions in 3D is demonstrated. Method and Materials: The dosimeters were read out using a characterized optical computed tomographyscanner. The calibration was performed by correlating the change in optical density along the beam axis to the depth‐dose curve from a pristine proton beam delivery (28.4 cm range, 4.5 cm circular aperture) obtained using ion‐chamber measurements. To exclude the potential LET dependence, only the plateau region was used. Several pristine‐beam and spread‐out Bragg peak (SOBP) dose distributions were recorded and compared to ion‐chamber measurements, including the assessment of dose response in high‐LET regions. A volumetric dose distribution delivered using a pristine beam and an irregularly shaped aperture was recorded and reconstructed in three dimensions. Results: The dose response of the gel was linear in the studied range (25–150 cGy). Dose profiles from the gel showed good general agreement with ion chamber measurements, with excellent positional reproducibility of the distal and proximal edges along and perpendicular to the beam direction. In the case of SOBP delivery (140 cGy peak dose), no response quenching in the peak region (characteristic of other dosimeter types) was observed. For a pristine beam delivery (170 cGy peak dose), the geldosimeter demonstrated a ∼10% under‐response at the Bragg peak (BP). Conclusion: Preliminary investigations suggest that this 3D dosimeter is capable of reproducing proton beam dose distributions with high spatial accuracy, with little or no quenching at the BP or SOBP.

TH‐C‐230A‐05: Neutron Scattered Dose Equivalent to a Fetus From Proton Radiotherapy of the Mother

Scatteredneutrondose equivalent to a representative point for a fetus is evaluated in an anthropomorphic phantom of the mother undergoing protonradiotherapy. The effect on scatteredneutrondose equivalent to the fetus of changing the incident proton beam energy, aperture size, beam location, and air gap between the beam delivery snout and skin was studied for both a small field snout and a large field snout. Measurements of the fetus scatteredneutrondose equivalent were made by placing a neutron bubble detector 10 cm below the umbilicus of an anthropomorphic Rando® phantom enhanced by a wax bolus to simulate a second trimester pregnancy. The neutrondose equivalent in milliSieverts per protontreatment Gray increased with incident proton energy and decreased with aperture size, distance of the fetus representative point from the field edge, and increasing air gap. Neutrondose equivalent to the fetus varied from 0.025 to 0.450 mSv per proton Gray for the small field snout and from 0.097 to 0.871 mSv per proton Gray for the large field snout. There is likely to be no excess risk to the fetus of severe mental retardation for a typical protontreatment of 80 Gray to the mother since the scatteredneutrondose to the fetus of 69.7 mSv is well below the estimated radiation absorbed dose threshold of 600 mGy observed for the occurrence of severe mental retardation in prenatally exposed Japanese atomic bomb survivors. However based on the linear no threshold hypothesis and this same typical treatment for the mother, the excess risk to the fetus of radiation induced cancer death in the first 10 years of life is 17.4 per 10,000 children.

TU-D-BRB-09: Dosimetric Validation of the Proton-Sensitive BANG® 3 PRO-2 Polymer Gel Dosimeter Irradiated by Single Field Therapeutic Proton Beams

Purpose: A dosimetric validation of the next‐generation 3D polymergeldosimeter (BANG®‐3‐PRO‐2) developed for protonradiotherapydosimetry is presented. Dose response curve is determined using both proton and electron beams. Lateral and depth profiles are compared to ion chamber measurements. 2D dose distributions are evaluated against those generated by a treatment planning system (TPS). Methods and Materials: The dosimeters were read out using a characterized optical computed tomography scanner. The calibration was performed by correlating the change in optical density along the beam axis to the depth‐dose curve from a pristine proton beam delivery (28.4 cm range, 4.5 cm circular aperture) obtained using ion‐chamber measurements. The dose range of 150–650 cGy was covered. Depth and lateral pristine‐beam and spread out Bragg peak (SOBP) dose profiles were compared to ionchamber measurements. Results: The dose response exhibited a sigmoid behavior in the studied dose range. The dose distributions read out from the dosimeters showed good agreement with ion chamber scanned profiles and were able to accurately reproduce the key features of the expected dose distributions. Planar dose along the beam axis were compared to the Eclipse TPS dose maps using the gamma metric with Dd=3 mm and DD=3% or 15 cGy, whichever is greater. Over 95% of dose points pass these criteria. For SOBP delivery (315 cGy peak dose), no response quenching in the peak region (characteristic of other dosimeter types) was observed. For a pristine beam delivery (300 cGy peak dose), the geldosimeter demonstrated a ∼5% under‐response at the Bragg peak. Conclusion: This new polymer formulation is capable of reproducing single proton beam dose distributions with high spatial accuracy. Under dosing response in the BP or SOBP due to LET dependencies was found to be insignificant and superior to many other currently used proton‐sensitive dosimeters.

TH‐E‐M100F‐04: Dosimetric Evaluation of MIP‐Based Patient Aperture and Compensator Designs to Treat Lung Cancer Using Proton Therapy Under Free Breathing Conditions

Purpose: Most proton therapy beam delivery systems still use passive scattering and design patient apertures and compensators under the assumption of static anatomy. Treatment of moving tumors is typically avoided. We investigated the potential of proton therapy to treat lungtumors under free breathing conditions via a MIP‐based design of the aperture and compensator. Dosimetric effect evaluation of this approach was compared to two other strategies based on patient‐specific Internal Target Volumes (ITVs). Method and Materials: A ten phase 4DCT treatment planning study was performed for a 3‐field treatment of a lung patient. GTVs and normal tissue structures were delineated on 4DCT images. MIP images were generated by reassigning each pixel value to the maximum pixel value encountered in all 10 phases. Three sets of treatment plans were generated: Plan ITVEOI, Plan ITVMOE, and Plan ICTVMIP using the correspondingly designed patient aperture and compensator. Patient aperture and compensator were respectively optimized to ITVs derived from end‐of‐inhale or mid‐of‐exhale with 3D motion margins, or internal clinical tumor volume (ICTV) derived from MIP images. DVHs were calculated on ten phases using the same beam, aperture and compensator parameters to verify target dose coverage and dose to normal tissue following a prescribed dose 72Gy to the tumor.Results: Plan ICTVMIP assured the dose to 99% of the CTV (D99) through ten phases (AVG=97.40%, MIN=96.40%, SD=0.5), compared to the results from Plan ITVEOI (AVG= 71.00%, MIN =37.60%, SD=18.9) and Plan ITVMOE (AVG=94.70%, MIN=83.50%, SD=4.0). The average mean lung dose for each strategy was 14.60Gy (Plan ICTVMIP), 14.70Gy (Plan ITVMOE), and 15.20Gy (Plan ITVEOI). Conclusion: The MIP‐based patient aperture and compensator design provides superior tumor coverage and similar dose or lower dose to normal lungtissue compared to the designs based on ITVs derived from end‐of‐inhale or mid‐of‐exhale with 3D motion margins calculated from 4DCT.

SU‐E‐T‐14: Modeling of 3D Positron Emission Activity Distributions Induced by Proton Irradiation: A Semi‐Empirical Method

PURPOSE to present and validate a method for modeling three-dimensional positron emission (PE) activity distributions induced by proton beam irradiation for PET/CT delivery verification studies in homogeneous media. METHODS the method relies on modeling the 3D proton flux distribution by combining the analytical expression for the depth reduction of proton flux with the empirically obtained lateral distribution. The latter is extracted from the corresponding dose distribution under the assumption that the projectile energy is nearly constant in the lateral plane. The same assumption allows calculating the 3D induced activity distributions from proton flux distributions by parameterizing the energy-dependent activation cross-sections in terms of depth via the energy-range relation. Results of this modeling approach were validated against experimental PET/CT data from three phantom deliveries: unmodulated (pristine) beam, spread-out Bragg peak (SOBP) delivery without a range compensator, and SOBP with a range compensator. BANG3-Pro2 polymer gel was used as a phantom material because of its elemental soft-tissue equivalence. RESULTS the agreement between modeled and measured activity distributions was evaluated using 3D gamma index analysis method, which, despite being traditionally reserved for dose distribution comparisons, is sufficiently general to be applied to other quantities. The evaluation criteria were dictated by limitations of PET imaging and were chosen to correspond to count rate uncertainty (6% value difference) and spatial resolution (4 mm distance to agreement). With these criteria and the threshold of 6%, the fraction of evaluated voxels passing the gamma evaluation was 97.9% for the pristine beam, 98.9% for the SOBP without compensator, and 98.5% for SOBP with compensator. CONCLUSIONS results of gamma evaluation indicate that the activity distributions produced by the model are consistent with experimental data within the uncertainties of PET imaging for clinical proton beams deliveries. This work was supported by the Bankhead-Coley Florida Biomedical Research Program under Grant No. 1BD10-34212.

TU‐G‐BRB‐07: Evaluation of Tissue‐Equivalent 3D Polymer Gel Dosimeters as Phantoms for PET/CT Verification of Proton Beam Deliveries

Purpose: to investigate the potential of polymergeldosimeters for concurrent measurements of three‐dimensional positron emission activity and dose distributions; to evaluate the ability of this technique to identify dosimetric errors due to delivery uncertainties, including those due to range modification and target motion.Methods: three BANG3‐Pro2 geldosimeters irradiated by protonbeams were imaged in a PET/CT scanner, starting within 3 minutes after irradiation. The radiation was delivered as a pristine beam under static conditions, as well as an SOBP, with and without phantom motion. The motion trace was defined by a sinusoidal curve with 2 cm peak‐to‐peak amplitude and the frequency of 0.25 Hz. The dose was read out using an established optical CT scanning procedure. PET/CT images of activated gels were validated against analytical calculations of activity and correlated to measured dose. The effects of target motion on activity and dose distributions were evaluated by volumetric gamma analysis against the treatment plan. Results: The profiles of positron emission activity along the central beam axis were found to be consistent with analytical calculations. Temporal dependence of activity decay suggests that the observed PET signal is due mainly to decay of 15O and 11C. Lateral profiles were found to exhibit good spatial correlation throughout the beam range. This allowed using a modified gamma analysis method to compare the signatures of target motion in PET and doseimages. Mean gamma for static PET and dose datasets was 0.07 and 0.11, respectively. For the motion delivery, the mean gamma value increased to 0.63 for both datasets. The spatial distributions of the gamma criterion for PET and dose datasets were qualitatively similar. Conclusions: Polymergels can accurately capture both dosimetric and activation information. Dosimetric errors due to target motion can be quantified by PET/CT using a novel method of analysis. This work was supported by the Bankhead‐Coley Florida Biomedical Research Program under Grant No. 1BD10‐34212.

WE‐D‐204B‐03: Evaluating Intra‐Fractional Prostate Motions by Post‐Treatment PET/CT Imaging in In‐Vivo Verification

Purpose: To evaluate intra‐fractional prostate motion by post‐treatment PET/CT imaging in in‐vivo verification. Methods: A total of 50 PET/CT imaging studies were performed immediately after daily proton therapy treatment through a single lateral portal. The PET/CT and planning CT were registered by matching the pelvic bones, and the beam path of delivered protons was defined in‐vivo by the positron emission distribution seen only within the pelvic bones. At each fraction treatment, a beam path defined by the fiducial markers seen in the post‐treatment CT is used as a surrogate for the intended beam; it can be different from its planned location at initial treatment planning due to a translational shift during the patient positioning. The discordance between the PET‐defined path and the intended path were derived. The PET‐defined path was compared to the positron‐emission distribution in lipomatous tissues around prostate to determine whether the prostate motion occurred before or after beam delivery. Results: For 30 of the 50 studies with small discordance between the intended and PET‐defined paths, average displacements are 0.6 mm and 1.3 mm along anterior‐posterior (DAP) and superior‐inferior (DSI) directions, respectively. For the remaining 20 studies demonstrating a large discordance, 13 studies also show large misalignment while 7 studies show no mismatch between the field edge and the positron emission distribution in lipomatous tissues around the prostate. The standard deviations for DAP and DSI are 5.0 mm and 3.0 mm are for these 13 studies, and 4.6 mm and 3.6 mm for last 7 studies. Conclusion: Systematic analyses of proton‐activated positron emission distributions provide patient‐specific information on prostate motion and patient position variability during daily protonbeam delivery. Small fraction of PET/CT studies (approximately 14%) with ∼4‐mm displacement variations may require different margins. Such data are useful in establishing patient‐specific planning target volume (PTV) margins