A Kassaee

SU-E-T-610: Phosphor-Based Fiber Optic Probes for Proton Beam Characterization

Purpose: To investigate feasibility of using fiber optics probes with rare-earth-based phosphor tips for proton beam radiation dosimetry. We designed and fabricated a fiber probe with submillimeter resolution (<0.5 mm3) based on TbF3 phosphors and evaluated its performance for measurement of proton beam including profiles and range. Methods: The fiber optic probe with TbF3 phosphor tip, embedded in tissue-mimicking phantoms was irradiated with double scattering proton beam with energy of 180 MeV. Luminescence spectroscopy was performed by a CCD-coupled spectrograph to analyze the emission spectra of the fiber tip. In order to measure the spatial beam profile and percentage depth dose, we used singular value decomposition method to spectrally separate the phosphors ionoluminescence signal from the background Cerenkov radiation signal. Results: The spectra of the TbF3 fiber probe showed characteristic ionoluminescence emission peaks at 489, 542, 586, and 620 nm. By using singular value decomposition we found the contribution of the ionoluminescence signal to measure the percentage depth dose in phantoms and compared that with measurements performed with ion chamber. We observed quenching effect at the spread out Bragg peak region, manifested as under-responding of the signal, due to the high LET of the beam. However, the beam profiles were not dramatically affected by the quenching effect. Conclusion: We have evaluated the performance of a fiber optic probe with submillimeter resolution for proton beam dosimetry. We demonstrated feasibility of spectral separation of the Cerenkov radiation from the collected signal. Such fiber probes can be used for measurements of proton beams profile and range. The experimental apparatus and spectroscopy method developed in this work provide a robust platform for characterization of proton-irradiated nanophosphor particles for ultralow fluence photodynamic therapy or molecular imaging applications.

Adjuvant radiation therapy for bladder cancer: A dosimetric comparison of techniques

Trials of adjuvant radiation after cystectomy are under development. There are no studies comparing radiation techniques to inform trial design. This study assesses the effect on bowel and rectal dose of 3 different modalities treating 2 proposed alternative clinical target volumes (CTVs). Contours of the bowel, rectum, CTV-pelvic sidewall (common/internal/external iliac and obturator nodes), and CTV-comprehensive (CTV-pelvic sidewall plus cystectomy bed and presacral regions) were drawn on simulation images of 7 post-cystectomy patients. We optimized 3-dimensional conformal radiation (3-D), intensity-modulated radiation (IMRT), and single-field uniform dose (SFUD) scanning proton plans for each CTV. Mixed models regression was used to compare plans for bowel and rectal volumes exposed to 35% (V35%), 65% (V65%), and 95% (V95%) of the prescribed dose. For any given treatment modality, treating the larger CTV-comprehensive volume compared with treating only the CTV-pelvic sidewall nodes significantly increased rectal dose (V35% rectum, V65% rectum, and V95% rectum; p < 0.001 for all comparisons), but it did not produce significant differences in bowel dose (V95% bowel, V65% bowel, or V35% bowel). The 3-D plans, compared with both the IMRT and the SFUD plans, had a significantly greater V65% bowel and V95% bowel for each proposed CTV (p < 0.001 for all comparisons). The effect of treatment modality on rectal dosimetry differed by CTV, but it generally favored the IMRT and the SFUD plans over the 3-D plans. Comparison of the IMRT plan vs the SFUD plan yielded mixed results with no consistent advantage for the SFUD plan over the IMRT plan. Targeting a CTV that spares the cystectomy bed and presacral region may marginally improve rectal toxicity but would not be expected to improve the bowel toxicity associated with any given modality of adjuvant radiation. Using the IMRT or the SFUD plans instead of the 3-D conformal plan may improve both bowel and rectal toxicity.

SU-F-J-56: The Connection Between Cherenkov Light Emission and Radiation Absorbed Dose in Proton Irradiated Phantoms

PURPOSE Range verification in proton therapy is of great importance. Cherenkov light follows the photon and electron energy deposition in water phantom. The purpose of this study is to investigate the connection between Cherenkov light generation and radiation absorbed dose in a water phantom irradiated with proton beams. METHODS Monte Carlo simulation was performed by employing FLUKA Monte Carlo code to stochastically simulate radiation transport, ionizing radiation dose deposition, and Cherenkov radiation in water phantoms. The simulations were performed for proton beams with energies in the range 50-600 MeV to cover a wide range of proton energies. RESULTS The mechanism of Cherenkov light production depends on the initial energy of protons. For proton energy with 50-400 MeV energy that is below the threshold (∼483 MeV in water) for Cherenkov light production directly from incident protons, Cherenkov light is produced mainly from the secondary electrons liberated as a result of columbic interactions with the incident protons. For proton beams with energy above 500 MeV, in the initial depth that incident protons have higher energy than the Cherenkov light production threshold, the light has higher intensity. As the slowing down process results in lower energy protons in larger depths in the water phantom, there is a knee point in the Cherenkov light curve vs. depth due to switching the Cherenkov light production mechanism from primary protons to secondary electrons. At the end of the depth dose curve the Cherenkov light intensity does not follow the dose peak because of the lack of high energy protons to produce Cherenkov light either directly or through secondary electrons. CONCLUSION In contrast to photon and electron beams, Cherenkov light generation induced by proton beams does not follow the proton energy deposition specially close to the end of the proton range near the Bragg peak.

SU‐E‐T‐167: Characterization of In‐House Plastic Scintillator Detectors Array for Radiation Therapy

Purpose: To characterize basic performance of plastic scintillator detectors (PSD) array designed for dosimetry of radiation therapy. Methods: An in-house PSD array has been developed by placing single point PSD into customized 2D holder. Each point PSD is a plastic scintillating fiber-based detector designed for highly accurate measurement of small radiotherapy fields used in patient plan verification and machine commissioning and QA procedures. A parallel fiber without PSD is used for Cerenkov separation by subtracting from PSD readings. Cerenkov separation was confirmed by optical spectroscopy. Alternative Cerenkov separation approaches are also investigated. The optical signal was converted to electronic signal with a photodiode and then subsequently amplified. We measured its dosimetry performance, including percentage depth dose and output factor, and compared with reference ion chamber measurements. The PSD array is then placed along the radiation beam for multiple point dose measurement, representing subsets of PDD measurements, or perpendicular to the beam for profile measurements. Results: The dosimetry results of PSD point measurements agree well with reference ion chamber measurements. For percentage depth dose, the maximal differences between PSD and ion chamber results are 3.5% and 2.7% for 6MV and 15MV beams, respectively. For the output factors, PSD measurements are within 3% from ion chamber results. PDD and profile measurement with PSD array are also performed. Conclusions: The current design of multichannel PSD array is feasible for the dosimetry measurement in radiation therapy. Dose distribution along or perpendicular to the beam path could be measured. It might as well be used as range verification in proton therapy.A PS hollow fiber detector will be investigated to eliminate the Cerenkov radiation effect so that all 32 channels can be used.

SU-F-T-166: On the Nature of the Background Visible Light Observed in Fiber Optic Dosimetry of Proton Beams.

PURPOSE The nature of the background visible light observed during fiber optic dosimetry of proton beams, whether it is due to Cherenkov radiation or not, has been debated in the literature recently. In this work, experimentally and by means of Monte Carlo simulations, we shed light on this problem and investigated the nature of the background visible light observed in fiber optics irradiated with proton beams. METHODS A bare silica fiber optics was embedded in tissue-mimicking phantoms and irradiated with clinical proton beams with energies of 100-225 MeV at Roberts Proton Therapy Center. Luminescence spectroscopy was performed by a CCD-coupled spectrograph to analyze in detail the emission spectrum of the fiber tip across the visible range of 400-700 nm. Monte Carlo simulation was performed by using FLUKA Monte Carlo code to simulate Cherenkov light and ionizing radiation dose deposition in the fiber. RESULTS The experimental spectra of the irradiated silica fiber shows two distinct peaks at 450 and 650 nm, whose spectral shape is different from that of Cherenkov radiation. We believe that the nature of these peaks are connected to the point defects of silica including oxygen-deficiency center (ODC) and non-bridging oxygen hole center (NBOHC). Monte Carlo simulations confirmed the experimental observations that Cherenkov radiation cannot be solely responsible for such a signal. CONCLUSION We showed that Cherenkov radiation is not the dominant visible signal observed in bare fiber optics irradiated with proton beams. We observed two distinct peaks at 450 and 650 nm whose nature is connected with the point defects of silica fiber including oxygen-deficiency center and non-bridging oxygen hole center.

MO-F-CAMPUS-T-02: Dosimetric Accuracy of the CrystalBallâ„¢: New Reusable Radiochromic Polymer Gel Dosimeter for Patient QA in Proton Therapy

Purpose: To evaluate the accuracy of monoexponential normalization in a new class of commercial, reusable, human-soft-tissue-equivalent, radiochromic polymer gel dosimeters for patient-specific QA in proton therapy. Methods: Eight formulations of the dosimeter (sealed in glass spheres of 166 mm OD), were exposed to a 150 MeV proton beam (5 cm x 5 cm square field, range 15 cm, modulation10 cm), with max dose ranging from 2.5 Gy to 20 Gy, depending on formulation. Exposed dosimeters were promptly placed in the commercial OCTOPUS™ laser CT scanner which was programmed to scan the central slice every 5 minutes for 20 hours (15 seconds per slice scan). This procedure was repeated several times. Reconstructed data were analyzed using the log-lin scale to determine the time range over which a monoexponential relaxation model could be applied. Next, a simple test plan was devised and delivered to each dosimeter. The OCTOPUS™ was programmed to rescan the central slice at the end of each volume scan, for signal relaxation reference. Monoexponential normalization was applied to sinograms before FBP reconstruction. Dose calibration was based on a volume-lookup table built within the central spherical volume of 12 cm diameter. 3D gamma and sigma passing rates were measured at 3%/3mm criteria down to 50% isodose. Results: Approximately monoexponential signal relaxation time ranges from 25 minutes to 3.5 hours, depending on formulation, followed by a slower-relaxation component. Noise in reconstructed OD/cm images is less than 0.5%. Dose calibration accuracy is better than 99%. Measured proton PDDs demonstrate absence of Bragg-peak quenching. Estimated number of useful cycles is at least 20, with a theoretical limit above 100. 3D gamma and sigma passing rates exceed 95%. Conclusion: Monoexponential normalization was found to yield adequate dosimetric accuracy in the new class of commercial radiochromic polymer gel dosimeters for patient QA in proton therapy.

WE‐F‐16A‐03: 3D Printer Application in Proton Therapy: A Novel Method to Deliver Passive‐Scattering Proton Beams with a Fixed Range and Modulation for SRS and SRT

PURPOSE To present a novel technique to deliver passive-scattering proton beam with fixed range and modulation using a 3D printed patient-specific bolus for proton stereotactic radiosurgery and radiotherapy. METHODS A CIRS head phantom was used to simulate a patient with a small brain lesion. A custom bolus was created in the Eclipse Treatment Planning System (TPS) to compensate for the different water equivalent depths from the patient surface to the target from multiple beam directions. To simulate arc therapy, a plan was created on the initial CT using three passive-scattering proton beams with a fixed range and modulations irradiating from different angles. The DICOM-RT structure file of the bolus was exported from the TPS and converted to STL format for 3D printing. The phantom was rescanned with the printed custom bolus and head cup to verify the dose distribution comparing to the initial plan. EBT3 films were placed in the sagital plane of the target to verify the delivered dose distribution. The relative stopping power of the printing material(ABSplus-P430) was measured using the Zebra multi-plate ion chamber. RESULTS The relative stopping power of the 3D printing material, ABSplus-P430 was 1.05 which is almost water equivalent. The dose difference between verification CT and Initial CT is almost negligible. Film measurement also confirmed the accuracy for this new proton delivery technique. CONCLUSION Our method using 3D printed range modifiers simplify the treatment delivery of multiple passive-scattering beams in treatment of small lesion in brain. This technique makes delivery of multiple beam more efficient and can be extended to allow arc therapy with proton beams. The ability to create and construct complex patient specific bolus structures provides a new dimension in creating optimized quality treatment plans not only for proton therapy but also for electron and photon therapy.

Study of the Angular Dependence of a Prompt Gamma Detector Response during Proton Radiation Therapy

Abstract Purpose: Several studies have recently shown that the characteristics of prompt gamma (PG) rays emitted during proton radiation therapy are beneficial for verifying proton beam range during treatment delivery. Since PG rays are produced instantaneously upon the proton beam delivery, the viability of in vivo beam range verification using PG rays depends greatly on the design optimization of not only intrinsically highly efficient detectors, but also detector location around the beam to maximize detection efficiency. The purpose of this study is to characterize angular dependence of the PG detection rates as a function of proton beam energy to help develop the design of clinically feasible detectors. Materials and Methods: In this study as a part of the long-term goal of developing a clinically feasible multistage Compton camera, we performed a Monte Carlo–based study of the detector response in a water phantom over the clinical range of beam energies, 50 to 200 MeV, and characterized PG emission s...

SU-FF-T-473: Time Response Study of Calypso Localization and Tracking System for Moving Tumors

Purpose: To study the time response of the Calypso 4D localization and tracking system for prostate and pancreatic tumorsMethod and Materials: We have implanted Calypso beacon transponders into pancreas of three patients for tumor tracking during the radiation delivery. The system is originally designed for localization and tracking of prostate during radiotherapy. We have observed delay times for localization and delay of start time for tracking longer for pancreatic cases as compared to prostate cases. The system has successfully tracked our pancreatic patients for each treatment. To investigate the delay time as a function of tumor motion, we assembled a moving platform and placed a phantom with imbedded transponders on it. The phantom was moved sinusoidally with various frequencies. The ranges of frequencies included the motion correlated with prostate and pancreas. In the case of pancreas, there is direct correlation with the breathing cycle. Results: The usual delay time for prostate localization and delay time for start of tracking is about 10 to 12 seconds. For pancreatic cases, the time of localization and tracking increases by a factor of 9 to 10. The system delay does not affect its ability to accurately track tumor motion. Conclusion: The Calypso is a robust system that is capable of tracking tumors covering a wide spectrum of motion including respiratory motion. The delay time for localization and tracking modules are well accepted clinically.

SU‐FF‐T‐348: Beam Attenuation and Beam Spoiling Properties of An Electromagnetic Array Used for Patient Localization and Tumor Tracking

Purpose: To investigate the beam attenuation and beam spoiling properties of the Calypso electromagnetic array used for patient localization and tumor tracking during the radiation delivery Method and Materials: One of the main components of the Calypso 4D system is an electromagnetic array placed above patients who are implanted with transponders. We measured both the narrow and board beam attenuationproperties of the array including the effect of beam angle (0 to 90 degree). A photon diode placed in cylindrical graphite buildup cap was used for narrow geometry, field size of 1 cm ×1 cm , and a 0.6 cc cylindrical farmer chamber placed in a polystyrene buildup cap for broad geometry, field size of 10 cm × 10 cm. For broad beam geometry, distances of 2, 5, and 10 cm between the chamber and the array were used. Measurements were performed using a Varian Clinac IX linear accelerator with 6 MV and 15 MV photon beams. Beam spoiling properties of the array was studied by placing the array above a water equivalent phantom with an imbedded Markus parallel plate chamber and measuring depth doses. Depth doses were measured for both 6 MV and 15 MV photon beams with 10 cm × 10 cm field size for distances of 2, 5, and 10 cm between array and phantom surface. Results: Narrow beam geometry attenuation is 1.5 % and 1.3% for 6 MV and 15 MV beam respectively. Beam angle dependence is more pronounced at angles greater than 70 degrees with attenuation greater than 5%. Broad beam attenuation, in clinical cases, is nonexistent due to scatter contribution. Spoiling properties could be appreciable. Conclusion:Attenuationproperties of the array may be ignored for treatment delivery for most clinical cases. Spoiling effect could be appreciable depending on array distance to patient surface.