The first probe of a FLASH proton beam by PET.

The recently observed FLASH effect related to high doses delivered with high rates has the potential to revolutionize radiation cancer therapy if promising results are confirmed and an underlying mechanism understood. Comprehensive measurements are essential to elucidate the phenomenon. We report the first-ever demonstration of measurements of successive in-spill and post-spill emissions of gammas arising from irradiations by a FLASH proton beam. A small positron emission tomography (PET) system was exposed in an ocular beam of the Proton Therapy Center at MD Anderson Cancer Center to view phantoms irradiated by 3.5 × 1010protons with a kinetic energy of 75.8 MeV delivered in 101.5 ms-long spills yielding a dose rate of 164 Gy s-1. Most in-spill events were due to prompt gammas. Reconstructed post-spill tomographic events, recorded for up to 20 min, yielded quantitative imaging and dosimetric information. These findings open a new and novel modality for imaging and monitoring of FLASH proton therapy exploiting in-spill prompt gamma imaging followed by post-spill PET imaging.

The first PET glimpse of a proton FLASH beam.

We demonstrate the first ever recorded positron-emission tomography (PET) imaging and dosimetry of a FLASH proton beam at the Proton Center of the MD Anderson Cancer Center. Two scintillating LYSO crystal arrays, read out by silicon photomultipliers, were configured with a partial field of view of a cylindrical poly-methyl methacrylate (PMMA) phantom irradiated by a FLASH proton beam. The proton beam had a kinetic energy of 75.8 MeV and an intensity of about 3.5 × 1010protons that were extracted over 101.5 ms-long spills. The radiation environment was characterized by cadmium-zinc-telluride and plastic scintillator counters. Preliminary results indicate that the PET technology used in our tests can efficiently record FLASH beam events. The instrument yielded informative and quantitative imaging and dosimetry of beam-activated isotopes in a PMMA phantom, as supported by Monte Carlo simulations. These studies open a new PET modality that can lead to improved imaging and monitoring of FLASH proton therapy.

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system.

PURPOSE: To present our method and experience in commissioning dose models in water for spot scanning proton therapy in a commercial treatment planning system (TPS). METHODS: The input data required by the TPS included in-air transverse profiles and integral depth doses (IDDs). All input data were obtained from Monte Carlo (MC) simulations that had been validated by measurements. MC-generated IDDs were converted to units of Gy mm(2)/MU using the measured IDDs at a depth of 2 cm employing the largest commercially available parallel-plate ionization chamber. The sensitive area of the chamber was insufficient to fully encompass the entire lateral dose deposited at depth by a pencil beam (spot). To correct for the detector size, correction factors as a function of proton energy were defined and determined using MC. The fluence of individual spots was initially modeled as a single Gaussian (SG) function and later as a double Gaussian (DG) function. The DG fluence model was introduced to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle, especially from the spot profile monitor. To validate the DG fluence model, we compared calculations and measurements, including doses at the center of spread out Bragg peaks (SOBPs) as a function of nominal field size, range, and SOBP width, lateral dose profiles, and depth doses for different widths of SOBP. Dose models were validated extensively with patient treatment field-specific measurements. RESULTS: We demonstrated that the DG fluence model is necessary for predicting the field size dependence of dose distributions. With this model, the calculated doses at the center of SOBPs as a function of nominal field size, range, and SOBP width, lateral dose profiles and depth doses for rectangular target volumes agreed well with respective measured values. With the DG fluence model for our scanning proton beam line, we successfully treated more than 500 patients from March 2010 through June 2012 with acceptable agreement between TPS calculated and measured dose distributions. However, the current dose model still has limitations in predicting field size dependence of doses at some intermediate depths of proton beams with high energies. CONCLUSIONS: We have commissioned a DG fluence model for clinical use. It is demonstrated that the DG fluence model is significantly more accurate than the SG fluence model. However, some deficiencies in modeling the low-dose envelope in the current dose algorithm still exist. Further improvements to the current dose algorithm are needed. The method presented here should be useful for commissioning pencil beam dose algorithms in new versions of TPS in the future.

Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system.

PURPOSE: Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time. METHODS: The detector system consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCD camera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame. RESULTS: The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU). CONCLUSIONS: Our LS detector system has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.

SU-E-T-395: Achievability and Optimization of Synchrotron-Based Respiratory Gated Spot Scanning Proton Beam Delivery.

PURPOSE: The purpose was to investigate the interplay of residual motion in realistically delivered respiratory gated spot scanning proton beam by a synchrotron. METHODS: A MatriXX 2D ion-chamber array detector was placed on a moving platform. The platform with the 2D ion-chamber array detector was moved based on sin4 motion with 3s and 5s cycle and 20 mm amplitude. Its motion was monitored by a laser displacement sensor (ZS-LDS2VT, omron, Japan). The respiration gate threshold level was set at 30% duty cycle and the residual motion within the gate window was approximately 6 mm. A 10×10 cm2 uniform field was delivered by a matrix of 13×13 spots with ∼ 8 mm spot size (s) and 8 mm spot spacing. Measurements were done for the field delivered with a single painting and multiple re-painting, from 2 to 12 times, for both orthogonal and parallel scan directions. The same field was also measured without moving the detector, defined as the static reference dose. Dose homogeneity was compared between with gated and the static dose distributions. RESULTS: The worst single painting result of the dose homogeneity ratio was 0.90 in 3s motion cycle and 0.93 in 5s motion cycle with the orthogonal scan pattern, and 0.97 in 3s and 0.98 in 5s motion with the parallel scan pattern, respectively . The homogeneity ratio improved to over 0.98 by 4∼6 times repainting in orthogonal and only 2 times re-painting with the parallel scan. CONCLUSIONS: The respiratory gated spot scanning proton beam delivery is sensitive to spot movement direction relative to the residual motion of the target. A proper selection of the number of repainting and the scan direction can improve beam delivery quality. The study offers a basic understanding when implementing respiratory gated spot scanning proton beam treatment.

SU-D-BRCD-01: Evaluation of Zebra Multi-Layer Ionization Chamber System for Patient Treatment Field and Machine QA for Spot Scanning and Passive Scattering Proton Beams.

PURPOSE: To evaluate Zebra multi-layer ionization chamber system for patient treatment field and machine QA for spot scanning proton beams (SSPB) and passive scattering proton beams (PSPB). METHODS: Zebra dose measurement system (IBA Dosimetry), consisting of 180 parallel platechambers with 2 mm detector spacing, was used for measuring proton beamdepth dose curves (DDC) for spread out Bragg peaks (SOBP) and single spot pristine Bragg peaks (PBP). The measurements were performed for 100 to 250 MeV PSPB and 89.2 to 221.8 MeV SSPB using the Hitachi ProBeat synchrotron based delivery system. An in-house Matlab based analysis software was used to compare the Zebra measured DDC with those measured by the Markus chamber in a PTW water tank (MC-WT). Several verification plans in the water phantom were created for patient treatment fields using the Eclipse treatment planning system (TPS). The DDC for individual verification fields were measured using the Zebra andcomparisons were made with the TPS calculations. RESULTS: The dosedifferences between the Zebra and MC-WT measurements in the plateau regions of the DDC are within 2% for various energies of PSPB, but are larger than 2% at the sharp dose distal gradient regions. The values for distal penumbra widths, range and SOBP widths from Zebra and MC-WT measurements agree within 0.5 mm, 1.5 mm, and 2 mm, respectively. The Zebra measured values of the range of the single spots also agreed within 1 mm with their established values from other measurements. The Zebra measured DDC of verification plan of patient treatment fields showed goodagreement with those from the TPS. CONCLUSIONS: Our investigation shows that Zebra can be useful for fast and reasonably accurate measurements of the DDC of pristine and spread-out Bragg peaks of both spot scanning and passive scattering proton beams.