Experimental realization of dynamic fluence field optimization for proton computed tomography.

Proton computed tomography (pCT) has high accuracy and dose efficiency in producing spatial maps of the relative stopping power (RSP) required for treatment planning in proton therapy. With fluence-modulated pCT (FMpCT), prescribed noise distributions can be achieved, which allows to decrease imaging dose by employing object-specific dynamically modulated fluence during the acquisition. For FMpCT acquisitions we divide the image into region-of-interest (ROI) and non-ROI volumes. In proton therapy, the ROI volume would encompass all treatment beams. An optimization algorithm then calculates dynamically modulated fluence that achieves low prescribed noise inside the ROI and high prescribed noise elsewhere. It also produces a planned noise distribution, which is the expected noise map for that fluence, as calculated with a Monte Carlo simulation. The optimized fluence can be achieved by acquiring pCT images with grids of intensity modulated pencil beams. In this work, we interfaced the control system of a clinical proton beam line to deliver the optimized fluence. Using three phantoms we acquired images with uniform fluence, with a constant noise prescription, and with an FMpCT task. Image noise distributions as well as fluence maps were compared to the corresponding planned distributions as well as to the prescription. Furthermore, we propose a correction method that removes image artifacts stemming from the acquisition with pencil beams having a spatially varying energy distribution that is not seen in clinical operation. RSP accuracy of FMpCT scans was compared to uniform scans and was found to be comparable to standard pCT scans. While we identified technical improvements for future experimental acquisitions, in particular related to an unexpected pencil beam size reduction and a misalignment of the fluence pattern, agreement with the planned noise was satisfactory and we conclude that FMpCT optimized for specific image noise prescriptions is experimentally feasible.

An optimization algorithm for dose reduction with fluence-modulated proton CT.

PURPOSE: Fluence-modulated proton computed tomography (FMpCT) using pencil beam scanning aims at achieving task-specific image noise distributions by modulating the imaging proton fluence spot-by-spot based on an object-specific noise model. In this work, we present a method for fluence field optimization and investigate its performance in dose reduction for various phantoms and image variance targets. METHODS: The proposed method uses Monte Carlo simulations of a proton CT (pCT) prototype scanner to estimate expected variance levels at uniform fluence. Using an iterative approach, we calculate a stack of target variance projections that are required to achieve the prescribed image variance, assuming a reconstruction using filtered backprojection. By fitting a pencil beam model to the ratio of uniform fluence variance and target variance, relative weights for each pencil beam can be calculated. The quality of the resulting fluence modulations is evaluated by scoring imaging doses and comparing them to those at uniform fluence, as well as evaluating conformity of the achieved variance with the prescription. For three different phantoms, we prescribed constant image variance as well as two regions-of-interest (ROI) imaging tasks with inhomogeneous image variance. The shape of the ROIs followed typical beam profiles for proton therapy. RESULTS: Prescription of constant image variance resulted in a dose reduction of 8.9% for a homogeneous water phantom compared to a uniform fluence scan at equal peak variance level. For a more heterogeneous head phantom, dose reduction increased to 16.0% for the same task. Prescribing two different ROIs resulted in dose reductions between 25.7% and 40.5% outside of the ROI at equal peak variance levels inside the ROI. Imaging doses inside the ROI were increased by 9.2% to 19.2% compared to the uniform fluence scan, but can be neglected assuming that the ROI agrees with the therapeutic dose region. Agreement of resulting variance maps with the prescriptions was satisfactory. CONCLUSIONS: We developed a method for fluence field optimization based on a noise model for a real scanner used in pCT. We demonstrated that it can achieve prescribed image variance targets. A uniform fluence field was shown not to be dose optimal and dose reductions achievable with the proposed method for FMpCT were considerable, opening an interesting perspective for image guidance and adaptive therapy.

Prediction of image noise contributions in proton computed tomography and comparison to measurements.

We present a method to accurately predict image noise in proton computed tomography (pCT) using data generated from a Monte Carlo simulation and a patient or object model that may be generated from a prior x-ray CT image. This enables noise prediction for arbitrary beam fluence settings and, therefore, the application of fluence-modulated pCT (FMpCT), which can achieve prescribed noise targets and may significantly reduce the integral patient dose. We extended an existing Monte Carlo simulation of a prototype pCT scanner to include effects of quenching in the energy detector scintillators and constructed a beam model from experimental tracking data. Simulated noise predictions were compared to experimental data both in the projection domain and in the reconstructed image. Noise prediction agreement between simulated and experimental data in terms of the root-mean-square (RMS) error was better than 7% for a homogeneous water phantom and a sensitometry phantom with tubular inserts. For an anthropomorphic head phantom, modeling the anatomy of a five-year-old child, the RMS error was better than 9% in three evaluated slices. We were able to reproduce subtle noise features near heterogeneities. To demonstrate the feasibility of Monte Carlo simulated noise maps for fluence modulation, we calculated a fluence profile that yields a homogeneous noise level in the image. Unlike for bow-tie filters in x-ray CT this does not require constant fluence at the detector and the shape of the fluence profile is fundamentally different. Using an improved Monte Carlo simulation, we demonstrated the feasibility of using simulated data for accurate image noise prediction for pCT. We believe that the agreement with experimental data is sufficient to enable the future optimization of FMpCT fluence plans to achieve prescribed noise targets in a fluence-modulated acquisition.

Spatially fractionated (GRID) radiation therapy using proton pencil beam scanning (PBS): Feasibility study and clinical implementation.

PURPOSE: GRID therapy is an effective treatment for bulky tumors. Linear accelerator (Linac)-produced photon beams collimated through blocks or multileaf collimators (MLCs) are the most common methods used to deliver this therapy. Utilizing the newest proton delivery method of pencil beam scanning (PBS) can further improve the efficacy of GRID therapy. In this study, we developed a method of delivering GRID therapy using proton PBS, evaluated the dosimetry of this novel technique and applied this method in two clinical cases. MATERIALS/METHODS: In the feasibility study phase, a single PBS proton beam was optimized to heterogeneously irradiate a shallow 20 × 20 × 12 cm3 target volume centered at a 6 cm depth in a water phantom. The beam was constrained to have an identical spot pattern in all layers, creating a "beamlet" at each spot position. Another GRID treatment using PBS was also performed on a deep 15 × 15 × 8 cm3 target volume centered at a 14 cm depth in a water phantom. Dosimetric parameters of both PBS dose distributions were compared with typical photon GRID dose distributions. In the next phase, four patients have been treated at our center with this proton GRID technique. The planning, dosimetry, and measurements for two representative patients are reported. RESULTS: For the shallow phantom target, the depth-dose curve of the PBS plan was uniform within the target (variation < 5%) and dropped quickly beyond the target (50% at 12.9 cm and 0.5% at 14 cm). The lateral profiles of the PBS plan were comparable to those of photon GRID in terms of valley-to-peak ratios. For the deep phantom target, the PBS plan provided smaller valley-to-peak ratios than the photon GRID technique. Pretreatment dose verification QA showed close agreement between the measurements and the plan (pass rate > 95% with a gamma index criterion of 3%/3 mm). Patients tolerated the treatment well without significant skin toxicity (radiation dermatitis grade ≤ 1). CONCLUSIONS: Proton GRID therapy using a PBS delivery method was successfully developed and implemented clinically. Proton GRID therapy offers many advantages over photon GRID techniques. The use of protons provides a more uniform beamlet dose within the tumor and spares normal tissues located beyond the tumor. This new PBS method will also reduce the dose to proximal organs when treating a deep-seated tumor.

A gas scintillator detector for 2D dose profile monitoring in pencil beam scanning and pulsed beam proton radiotherapy treatments.

In order to address dosimetry demands during proton therapy treatments utilizing pencil beam scanning and/or pulsed beam accelerators, we have developed a xenon-filled gas scintillation detector (GSD) that can monitor delivered dose and 2D beam centroid position pulse-by-pulse in real time, with high response linearity up to high instantaneous dose rates. We present design considerations for the GSD and results of beam tests carried out at operating proton therapy clinics. In addition to demonstrating spatial resolution with σ of a few hundred microns in each transverse dimension and relative dose precision better than 1% over large treatment areas, the test beam results also reveal the dependence of the GSD dose normalization on dose rate, beam energy, and gas impurities. The results demonstrate the promise of the GSD technology to provide an important addition to dosimetry approaches for next-generation ion beam therapy.

SU-E-T-301: A Novel Daily QA Device for Proton Therapy.

PURPOSE: We describe the design and use of a daily QA device for proton therapy. The device is designed for therapists to check the readiness of the IBA Proton Therapy System (IBA, Louvain-la-Neuve, Belgium) during morning QA. The checks include connectivity, positioning, mechanical, imaging and dosimetric parameters of the proton therapy system. METHODS: The device consists of a commercial QA device, (rf-DailyQA3 -Sun Nuclear Corporation, Melbourne, FL), in conjunction with a home-made acrylic phantom and mechanical indexing jig. The indexing jig indexes the rf-DailyQA3 to treatment couch. Fiducial markers embedded in the phantom are used for checking the x-ray image and alignment accuracy of the imaging system (VeriSuite, MedCom, Darmstadt- Germany). The rf- DailyQA3 is used to check the proton beam output, range and symmetry, which are acquired during one single beam delivery of 100 monitor units. We developed in-house software to calculate the variation of beam range and symmetry, based on readings from the various ion chambers inside the rf-DailyQA3. RESULTS: The device has been employed to perform daily QA since June 2010 at two operational proton treatment centers and will soon be implemented at ProCure's New Jersey center. All QA tests are performed by radiation therapists and reviewed by the medical physicist on duty. Due to the simplicity of the device and the associated processing software, the QA time is less than 20 minutes per room. The measurement data collected by the device during daily QA are recorded in the OIS. The integrity of the data is validated by comparing against other independent measurements. CONCLUSIONS: The daily QA device has been proven to be robust, reliable and user-friendly. The performance of this system has been proven to be stable and accurate using trend analyses. Key words:proton therapy, daily QA, output, range, symmetry.

Determination of penumbral widths from ion chamber measurements.

We have investigated methods of reconstructing beam profiles in the penumbral region using a set of axially symmetric chambers, differing only in the detector radius. In principal, the transfer functions, or kernels, of such chambers should be functions of radius only. Three chambers of radii 0.297, 0.556, and 0.714 cm have been used. The transfer functions of the chambers can be determined by deconvolving the profiles measured with each detector with the PPMC profile. The results indicate that the transfer functions can be parametrized accurately as a Gaussian cutoff at 1.75(r), with (r) the average radius of the chamber. Deconvolution of the measured profiles with the transfer functions yields a profile that agrees with the PPMC profile to +/- 0.5 mm over the 20-80% penumbra.