Proton Range Verification Research Articles

Feasibility of proton-activated implantable markers for proton range verification using PET

Proton beam range verification using positron emission tomography (PET) currently relies on proton activation of tissue, the products of which decay with a short half-life and necessitate an on-site PET scanner. Tissue activation is, however, negligible near the distal dose fall-off region of the proton beam range due to their high interaction energy thresholds. Therefore Monte Carlo simulation is often supplemented for comparison with measurement; however, this also may be associated with systematic and statistical uncertainties. Therefore, we sought to test the feasibility of using long-lived proton-activated external materials that are inserted or infused into the target volume for more accurate proton beam range verification that could be performed at an off-site PET scanner. We irradiated samples of ≥98% 18O-enriched water, natural Cu foils, and >97% 68Zn-enriched foils as candidate materials, along with samples of tissue-equivalent materials including 16O water, heptane (C7H16), and polycarbonate (C16H14O3)n, at four depths (ranging from 100% to 3% of center of modulation (COM) dose) along the distal fall-off of a modulated 160 MeV proton beam. Samples were irradiated either directly or after being embedded in Plastic Water® or balsa wood. We then measured the activity of the samples using PET imaging for 20 or 30 min after various delay times. Measured activities of candidate materials were up to 100 times greater than those of the tissue-equivalent materials at the four distal dose fall-off depths. The differences between candidate materials and tissue-equivalent materials became more apparent after longer delays between irradiation and PET imaging, due to the longer half-lives of the candidate materials. Furthermore, the activation of the candidate materials closely mimicked the distal dose fall-off with offsets of 1 to 2 mm. Also, signals from the foils were clearly visible compared to the background from the activated Plastic Water® and balsa wood phantoms. These results indicate that markers made from these candidate materials could be used for in vivo proton range verification using an off-site PET scanner.

Energy- and time-resolved detection of prompt gamma-rays for proton range verification

In this work, we present experimental results of a novel prompt gamma-ray detector for proton beam range verification. The detection system features an actively shielded cerium-doped lanthanum(III) bromide scintillator, coupled to a digital data acquisition system. The acquisition was synchronized to the cyclotron radio frequency to separate the prompt gamma-ray signals from the later-arriving neutron-induced background. We designed the detector to provide a high energy resolution and an effective reduction of background events, enabling discrete proton-induced prompt gamma lines to be resolved. Measuring discrete prompt gamma lines has several benefits for range verification. As the discrete energies correspond to specific nuclear transitions, the magnitudes of the different gamma lines have unique correlations with the proton energy and can be directly related to nuclear reaction cross sections. The quantification of discrete gamma lines also enables elemental analysis of tissue in the beam path, providing a better prediction of prompt gamma-ray yields. We present the results of experiments in which a water phantom was irradiated with proton pencil-beams in a clinical proton therapy gantry. A slit collimator was used to collimate the prompt gamma-rays, and measurements were performed at 27 positions along the path of proton beams with ranges of 9, 16 and 23 g cm−2 in water. The magnitudes of discrete gamma lines at 4.44, 5.2 and 6.13 MeV were quantified. The prompt gamma lines were found to be clearly resolved in dimensions of energy and time, and had a reproducible correlation with the proton depth-dose curve. We conclude that the measurement of discrete prompt gamma-rays for in vivo range verification of clinical proton beams is feasible, and plan to further study methods and detector designs for clinical use.

SU-D-WAB-05: Image-Based Proton Range Verification Using Intensity-Corrected CBCT

Purpose: Using CBCT to detect variations in proton beam path prior to treatment is an important step in the quality assurance of proton therapy. However, the Hounsfield Units (HUs) from a CBCT differs slightly from a simulation CT, thereby introducing additional uncertainties in the conversion from HUs to proton stopping power. The aim of this work is to demonstrate the feasibility of using deformable image registration to map simulation CT HUs onto a treatment CBCT for range verification during proton therapy. Methods: A simulation CT and two simulated proton CBCT of a head and neck patient were used in this study. We used a diffeomorphic deformable image registration algorithm with symmetric normalization in conjunction with cross correlation similarity metrics (ANTs: Advanced Normalization Tools) to deform the simulation CT onto the CBCTs. Polar plots of proton beam water equivalent thickness (WET) were generated using a ray tracing algorithm at different gantry angles in the unmodified CBCT, corrected CBCT and simulation CT for analysis. Results: With no anatomical variation, corrected CBCT and simulation CT image exhibited excellent agreement in WETs (mean difference of 1.26 mm, interquartile range 0.42∼1.79mm). In the unmodified CBCT image, larger difference in WETs (mean value 3.38 mm, interquartile range 1.90∼5.03mm) attributed to errors in HU to stopping power conversion were observed. With tumor shrinkage the differences in the WET polar plots between planning CT and corrected CBCT correlated with the location of anatomical change in the nasal/oral cavity, indicating potential deviations in proton dose deposition. Conclusion: It is feasible to use deformable image registration to correct CBCT HUs for image based proton range verification. Polar plots of deviations in WET between CBCT and simulation CT is potentially an efficient and practical quality assurance tool in the use of CBCT for proton therapy applications.

SU‐E‐CAMPUS‐T‐03: Patient‐Based Study On Characteristic Prompt Gamma‐Ray Spectra Using a Prompt Gamma Imaging System During Proton Therapy

Purpose: To demonstrate the adaptation of prompt gamma (PG) imaging for the spot scanning in a patient phantom as an independent tool for non‐invasive proton beam range verification. Methods: A cylindrical high‐purity germanium (HPGe) detector was modeled using GEANT4 Monte Carlo toolkit and implemented into a modulated proton beam scanning model. Simulations were run to calculate the spectrum of energy deposited by gamma ray interactions in the modeled HPGe detector from a water phantom and a patient CT phantom during irradiations with proton beam. Images for individual spots (spot patterns were determined from a real patient treatment plan) will be reconstructed using all detected gammas as well as the gamma characteristic of specific nuclear excitations. Results: Dose depth profiles along arbitrary lines in the beam direction showed the dependence of PG 50% falloff on the location of the dose falloff within the water phantom and the patient CT phantom. The PG energy spectra at the emission point (inside phantoms) and the detector outside phantoms were illustrated. Reconstructed gamma emission profile will be compared with dose depth profile. Prompt gamma images will be reconstructed for spot and/or treatment field. Conclusion: This work indicated the first study of clinical implementation of the PG imaging for proton range verification using the incorporated detector response to the patient CT phantom irradiated with proton beam. The results showed feasible correlation between the spot scanning does distribution and prompt gamma ray production. This study will be used in future investigation of detector design optimization following measurements that will be carried out using custom‐build prototype detector system using Compton reconstruction.

WE‐G‐500‐02: BEST IN PHYSICS (JOINT IMAGING‐THERAPY)‐Proton Beam Range Verification Using Proton Activated Fiducials and Off‐Site PET

Purpose: Proton range uncertainty places limits on the accuracy of proton therapy. PET imaging of activated endogenous tissue following proton treatment has been suggested as a tool for proton range verification. However, this approach is limited due to the weak activation of tissue near the proton distal fall off and the need for expensive in room PET scanners. To overcome these limitations, we investigated the use of patient implantable proton activated elements such as Zn68(p,n)Ga68 which is strongly activated by low energy protons and decays by positron emission with a 68min half life. This study compares PET signals from irradiated Zn68 enriched foil and a tissue substitute polycarbonate,(C16O3H14). Methods: Zn68 (>97%) enriched foils simulating implanted fiducials of 10 mm3 (10 × 10 × 0.1 mm thick) and polycarbonate sheets simulating tissue volumes of 305 mm3 (20 × 20 × 0.76 mm thick) were irradiated with a 160 MeV, 10‐cm SOBP proton beam. 2 foils and 2 sheets were placed at each of four depths over the distal fall‐off range (100% ∼ 40% of SOBP dose). A dose of 12.5 Gy was delivered, simulating thoracic SBRT. 47 min following activation a 2.5 hour PET scan was acquired and the corresponding images were reconstructed. The PET signal intensity of each foil and sheet was then measured and normalized to the results of the shallowest depth. Monte Carlo simulation was also performed for comparison. Results: Normalized mean activity concentration for Zn68 (polycarbonate) at the distal fall off region at 100∼99%, 99∼98%, 93∼81%, 62∼40% dose were 100±8% (2.6±0.4%), 83±11% (1.4±0.5%), 96±48% (1.4±0.8%), 64±2% (2.4±1.0%). Monte Carlo simulation agreed with measurements and were 82% (6%), 100% (2%), 84% (1%), 38% (0%). Conclusion: The higher activation of Zn68 over the distal fall‐off suggests the possibility of using Zn68 as proton activated fiducials for proton range verification.

TH‐C‐144‐02: Experimental Study of Discrete Prompt Gamma Lines for In‐Vivo Proton Range Verification

Purpose: A novel method for in vivo proton beam range verification is presented, using energy‐resolvable proton‐induced prompt gamma lines. Methods: Prompt gamma‐rays emitted near the end‐of‐range of a clinical proton beam originate from transitions between the lower states of excited nuclear reaction products. We developed a prompt gamma‐ray detection system with the aim of resolving the resulting discrete gamma lines. The ability to quantify the strength of these lines can provide valuable information for in vivo range verification: known cross sections may be used as prior knowledge, and uncertainties resulting from differences in tissue composition could be reduced. A LaBr3:Ce scintillation detector was used, which provides high energy resolution. The pulses from the photomultiplier tube were acquired with analog‐to‐digital converters at high sampling rates, synchronized to the cyclotron radiofrequency to separate the quasi‐continuous neutron‐induced background from the prompt gamma signals of interest. Results: Preliminary measurements were performed with a 150 MeV proton pencil‐beam targeted on the center of a water phantom with a 15 cm diameter. A 1 cm lead slit collimator was used perpendicular to the beam direction, centered at the end‐of‐range. The acquired gamma spectra show the 6.13 MeV gamma line from oxygen‐16, and its escape peak, can be clearly resolved with an energy resolution of ∼1% FWHM. Conclusion: Our results show the feasibility of measuring discrete prompt gamma lines using a practical detection system. These gamma lines can be attributed to specific nuclear reactions with established properties, potentially enabling accurate proton range verification using a small number of measurement points. Experiments with different collimators and proton energies are currently being performed to determine an optimal configuration for range verification.

Determination of elemental tissue composition following proton treatment using positron emission tomography

Positron emission tomography (PET) has been suggested as an imaging technique for in vivo proton dose and range verification after proton induced-tissue activation. During proton treatment, irradiated tissue is activated and decays while emitting positrons. In this paper, we assessed the feasibility of using PET imaging after proton treatment to determine tissue elemental composition by evaluating the resultant composite decay curve of activated tissue. A phantom consisting of sections composed of different combinations of 1H, 12C, 14N, and 16O was irradiated using a pristine Bragg peak and a 6 cm spread-out Bragg-peak (SOBP) proton beam. The beam ranges defined at 90% distal dose were 10 cm; the delivered dose was 1.6 Gy for the near monoenergetic beam and 2 Gy for the SOBP beam. After irradiation, activated phantom decay was measured using an in-room PET scanner for 30 min in list mode. Decay curves from the activated 12C and 16O sections were first decomposed into multiple simple exponential decay curves, each curve corresponding to a constituent radioisotope, using a least-squares method. The relative radioisotope fractions from each section were determined. These fractions were used to guide the decay curve decomposition from the section consisting mainly of 12C + 16O and calculate the relative elemental composition of 12C and 16O. A Monte Carlo simulation was also used to determine the elemental composition of the 12C + 16O section. The calculated compositions of the 12C + 16O section using both approaches (PET and Monte Carlo) were compared with the true known phantom composition. Finally, two patients were imaged using an in-room PET scanner after proton therapy of the head. Their PET data and the technique described above were used to construct elemental composition (12C and 16O) maps that corresponded to the proton-activated regions. We compared the 12C and 16O compositions of seven ROIs that corresponded to the vitreous humor, adipose/face mask, adipose tissue, and brain tissue with ICRU 46 elemental composition data. The 12C and 16O compositions of the 12C + 16O phantom section were estimated to within a maximum difference of 3.6% for the near monoenergetic and SOBP beams over an 8 cm depth range. On the other hand, the Monte Carlo simulation estimated the corresponding 12C and 16O compositions in the 12C + 16O section to within a maximum difference of 3.4%. For the patients, the 12C and 16O compositions in the seven ROIs agreed with the ICRU elemental composition data, with a mean (maximum) difference of 9.4% (15.2%). The 12C and 16O compositions of the phantom and patients were estimated with relatively small differences. PET imaging may be useful for determining the tissue elemental composition and thereby improving proton treatment planning and verification.

Time-of-flight neutron rejection to improve prompt gamma imaging for proton range verification: a simulation study

Therapeutic proton and heavier ion beams generate prompt gamma photons that may escape from the patient. In principle, this allows for real-time, in situ monitoring of the treatment delivery, in particular, the hadron range within the patient, by imaging the emitted prompt gamma rays. Unfortunately, the neutrons simultaneously created with the prompt photons create a background that may obscure the prompt gamma signal. To enhance the accuracy of proton dose verification by prompt gamma imaging, we therefore propose a time-of-flight (TOF) technique to reject this neutron background, involving a shifting time window to account for the propagation of the protons through the patient. Time-resolved Monte Carlo simulations of the generation and transport of prompt gamma photons and neutrons upon irradiation of a PMMA phantom with 100, 150 and 200 MeV protons were performed using Geant4 (version 9.2.p02) and MCNPX (version 2.7.D). The influence of angular collimation and TOF selection on the prompt gamma and neutron longitudinal profiles is studied. Furthermore, the implications of the proton beam microstructure (characterized by the proton bunch width and repetition period) are investigated. The application of a shifting TOF window having a width of ΔTOFz = 1.0 ns appears to reduce the neutron background by more than 99%. Subsequent application of an energy threshold does not appear to sharpen the distal falloff of the prompt gamma profile but reduces the tail that is observed beyond the proton range. Investigations of the influence of the beam time structure show that TOF rejection of the neutron background is expected to be effective for typical therapeutic proton cyclotrons.

MO-A-213AB-06: Validation of Nuclear Reaction Models to Simulate Proton Therapy Range Verification Using Prompt Gamma-Rays.

The impact of nuclear reaction model differences on simulation of prompt gamma-ray imaging for proton therapy range verification was assessed. Four nuclear reaction models were used to simulate gamma emission in proton beams, and were validated against experimental cross-sections. Proton-induced nuclear reactions on carbon, oxygen, nitrogen and calcium were investigated with the Monte Carlo toolkits GEANT4 9.5 and MCNPX 2.7, and the dedicated nuclear reaction codes TALYS 1.4 and EMPIRE 3.1. Absolute cross-sections of discrete prompt gamma lines and the total gamma production were obtained for the 1-200 MeV incident proton energy range. They were compared to 34 discrete line measurements reported in literature. Using these cross-sections, we analyzed the gamma production along the path of proton beams passing through various tissues. The differences in absolute discrete line cross-sections as predicted by the models ranged from almost zero to an order of magnitude, depending on the gamma line and incident proton energy. Overall, the dedicated nuclear reaction codes provided a better fit to most experimental excitation functions. For a 150 MeV proton beam stopping in soft tissue, these differences amount to a variation by a factor of 4 of the gamma emission around the Bragg peak location. The maximum of gamma production near the end of proton range differed by 7 mm, and the change of the 50% emission fall-off position was 4 mm. There is a clear need for improvement of nuclear reaction models to accurately simulate proton range verification using prompt gamma-rays. Current simulation codes show large uncertainties in both the total gamma yield and the correlation of gamma emission with the proton Bragg peak. GEANT4 and MCNPX in particular appear to have limited predictive power.

New concept of a range verification system for proton therapy using a photon counting detector

A range verification method plays an important role in the quality assurance of the proton therapy offering the high conformity and reduction in radiation dose. To localize the distal falloff of the dose distribution, secondary particles (C-11, O-15, and N-13) produced by the proton interaction within the patient body can be used as a measure of the beam range. We proposed a multi-modality imaging system for X-ray and gamma-ray coincidence imaging using CdZnTe detectors to measure proton range verification. The detector system consists of two parallel planes of detectors and an X-ray generator. An X-ray image is acquired using one detector for the verification of 2-dimensional anatomical structure of the patient, and the paired gamma rays from the annihilation are imaged with two modules to determine the maximum range of proton penetration. Image registration is intrinsic because the X-ray and gamma ray images are acquired in the same geometry. 110 and 140MeV proton beam, a cylindrical tissue phantom, and two rectangular CdZnTe detectors were modeled, and the imaging performance of this system was evaluated using GATE simulation. The results showed the potential benefits of an X-ray/gamma-ray imaging with photon counting detectors for range verification in proton therapy.

Yields of positron and positron emitting nuclei for proton and carbon ion radiation therapy: A simulation study with GEANT4

A Monte Carlo application is developed to investigate the yields of positron-emitting nuclei (PEN) used for proton and carbon ion range verification techniques using the GEANT4 Toolkit. A base physics list was constructed and used to simulate incident proton and carbon ions onto a PMMA or water phantom using pencil like beams. In each simulation the total yields of PEN are counted and both the PEN and their associated positron depth-distributions were recorded and compared to the incident radiation's Bragg Peak. Alterations to the physics lists are then performed to investigate the PEN yields dependence on the choice of physics list. In our study, we conclude that the yields of PEN can be estimated using the physics list presented here for range verification of incident proton and carbon ions.

Monitoring proton radiation therapy with in-room PET imaging

We used a mobile positron emission tomography (PET) scanner positioned within the proton therapy treatment room to study the feasibility of proton range verification with an in-room, stand-alone PET system, and compared with off-line equivalent studies. Two subjects with adenoid cystic carcinoma were enrolled into a pilot study in which in-room PET scans were acquired in list-mode after a routine fractionated treatment session. The list-mode PET data were reconstructed with different time schemes to generate in-room short, in-room long and off-line equivalent (by skipping coincidences from the first 15 min during the list-mode reconstruction) PET images for comparison in activity distribution patterns. A phantom study was followed to evaluate the accuracy of range verification for different reconstruction time schemes quantitatively. The in-room PET has a higher sensitivity compared to the off-line modality so that the PET acquisition time can be greatly reduced from 30 to <5 min. Features in deep-site, soft-tissue regions were better retained with in-room short PET acquisitions because of the collection of 15O component and lower biological washout. For soft tissue-equivalent material, the distal fall-off edge of an in-room short acquisition is deeper compared to an off-line equivalent scan, indicating a better coverage of the high-dose end of the beam. In-room PET is a promising low cost, high sensitivity modality for the in vivo verification of proton therapy. Better accuracy in Monte Carlo predictions, especially for biological decay modeling, is necessary.

Proton range verification using a range probe: definition of concept and initial analysis

Proton therapy can be particularly sensitive to changes or errors in range. Thus, methods for the in vivo measurement of range could be of great use to improve the quality and accuracy of proton-based radiotherapy. In this paper, we introduce the concept of the ‘range probe’. This is a low-dose, high-energy proton pencil beam that would pass through a patient, and whose integral Bragg peak would be measured on the exit side using a multi-layer detector. We propose that by comparing the measured integral Bragg peak with that calculated based on the patient's planning CT, such a range probe could provide useful information about the accuracy of range calculations in vivo. To study the feasibility of this approach, a Monte Carlo-based study has been performed. Using a patient's planning CT, MC simulations (VMCpro) have been made for single pencil beams laterally traversing the head and stopping in a simulated range telescope behind the patient. Range probes have been calculated for different locations, and the residual range from the Bragg peak ‘signal’ in the range telescope has been assessed for different assumed detector thicknesses. The sensitivity of this approach to changes in CT values, calibration curve and positional shifts of the CT have been investigated. From our analysis, range resolutions of 1 mm may be possible with a detector thickness of 4 mm for homogeneous regions. Additionally, for heterogeneous regions, changes of the Bragg peak shape due to spatial shifts of the CT could be a sensitive measure for detecting patient set-up errors directly in the treatment position. The concept of the proton ‘range probe’ appears to be feasible for high-resolution range verification. We now want to test this concept experimentally using different possible range telescope detectors.

In Vivo Proton Beam Range Verification Using Spine MRI Changes

In proton therapy, uncertainty in the location of the distal dose edge can lead to cautious treatment plans that reduce the dosimetric advantage of protons. After radiation exposure, vertebral bone marrow undergoes fatty replacement that is visible on magnetic resonance imaging (MRI). This presents an exciting opportunity to observe radiation dose distribution in vivo. We used quantitative spine MRI changes to precisely detect the distal dose edge in proton radiation patients. We registered follow-up T1-weighted MRI images to planning computed tomography scans from 10 patients who received proton spine irradiation. A radiation dose-MRI signal intensity curve was created using the lateral beam penumbra in the sacrum. This curve was then used to measure range errors in the lumbar spine. In the lateral penumbra, there was an increase in signal intensity with higher dose throughout the full range of 0-37.5 Gy (RBE). In the distal fall-off region, the beam sometimes appeared to penetrate farther than planned. The mean overshoot in 10 patients was 1.9 mm (95% confidence interval, 0.8-3.1 mm), on the order of the uncertainties inherent to our range verification method. We have demonstrated in vivo proton range verification using posttreatment spine MRI changes. Our analysis suggests the presence of a systematic overshoot of a few millimeters in some proton spine treatments, but the range error does not exceed the uncertainty incorporated into the treatment planning margin. It may be possible to extend our technique to MRI sequences that show early bone marrow changes, enabling adaptive treatment modification.

Systematic analysis of biological and physical limitations of proton beam range verification with offline PET/CT scans

The clinical use of offline positron emission tomography/computed tomography (PET/CT) scans for proton range verification is currently under investigation at the Massachusetts General Hospital (MGH). Validation is achieved by comparing measured activity distributions, acquired in patients after receiving one fraction of proton irradiation, with corresponding Monte Carlo (MC) simulated distributions. Deviations between measured and simulated activity distributions can either reflect errors during the treatment chain from planning to delivery or they can be caused by various inherent challenges of the offline PET/CT verification method. We performed a systematic analysis to assess the impact of the following aspects on the feasibility and accuracy of the offline PET/CT method: (1) biological washout processes, (2) patient motion, (3) Hounsfield unit (HU) based tissue classification for the simulation of the activity distributions and (4) tumor site specific aspects. It was found that the spatial reproducibility of the measured activity distributions is within 1 mm. However, the feasibility of range verification is restricted to a limited amount of positions and tumor sites. Washout effects introduce discrepancies between the measured and simulated ranges of about 4 mm at positions where the proton beam stops in soft tissue. Motion causes spatial deviations of up to 3 cm between measured and simulated activity distributions in abdominopelvic tumor cases. In these later cases, the MC simulated activity distributions were found to be limited to about 35% accuracy in absolute values and about 2 mm in spatial accuracy depending on the correlativity of HU into the physical and biological parameters of the irradiated tissue. Besides, for further specific tumor locations, the beam arrangement, the limited accuracy of rigid co-registration and organ movements can prevent the success of PET/CT range verification. All the addressed factors explain why the proton beam range can only be verified within an accuracy of 1–2 mm in low-perfused bony structures of head and neck patients for which an accurate co-registration of predominant bony anatomy is possible, as shown previously. However, most of the limitations of the current approach are conquerable. By implementing technological and methodological improvements like the use of in-room PET scanners, PET measurements could soon be used to provide proton range verification in clinical routine.

WE‐C‐BRC‐01: In Vivo Proton Beam Range Verification Using Spine MRI Changes

Purpose: Uncertainty in the location of the distal dose edge is one of the main concerns about proton therapy. It leads to cautious treatment plans that partially neutralize the dosimetric advantage of protons. Vertebral bone marrow responds to radiation with fatty replacement that is visible on post‐treatment MRI. This presents a unique opportunity to visualize radiation effects in vivo. We have developed a method that uses spine MRI changes to precisely localize the distal dose edge in spinal proton radiation patients. Method and Materials: We carefully registered treatment planning CT scans and follow‐up T1‐weighted MRI scans from 10 proton spinal radiation patients. A radiation dose‐MRI signal intensity curve was created using the lateral beam penumbra in the sacrum. This curve was then used to quantitatively examine possible systematic or spatially varying proton range errors. Results: In the lateral penumbra there was a gradual increase in signal intensity with higher dose throughout the full dose range of 0–37.5 Gy. In the distal dose fall‐off region, the beam appeared to penetrate farther than planned in the central part of the vertebral bodies. The mean overshoot in five patients was 2.71 mm (95% confidence interval 1.13–4.28 mm). These errors are probably not clinically significant with current treatment planning procedures. Conclusion: We have demonstrated in vivo proton range verification based on post‐treatment spine MRI changes. Our analysis indicates that if range errors occur in spine treatments, their magnitude is at most a few millimeters in the majority of patients. It may be possible to extend our technique to MRI sequences that show early bone marrow changes. It could then be used for adaptive modification of spine radiation plans in order to reduce radiation dose to bone marrow and other normal tissues.

SU-FF-J-129: In-Room Proton Range Verification Using Mobile NeuroPET - Feasibility Study

Purpose: At present a very promising method for an in vivo and non‐invasive monitoring of radiation treatments with protonbeams is positron emission tomography(PET). Offline (post treatment) PET/CT imaging is currently under investigation for proton range verification. However, the long (∼15min) time interval between irradiation and PETimaging and the fact that the patients need to move to a different room for the PET scan introduces various uncertainties. This study investigates the feasibility and the potential advantages of using a mobile NeuroPET scanner in the treatment room compared to the offline PET/CT scanner.Method and Materials: A circular SOBP proton field of 16 cm water equivalent range and 10 cm modulation was delivered to a PMMA phantom (9 × 9 × 20 cm3). A total dose of 8 GyE(RBE=1.1) was administered. PET data were acquired in listmode starting within 7 min after irradiation once at the NeuroPET scanner and once at a commercial PET/CT scanner. For image coregistration markers that are recognizable in the CT as well as in the PET were used. Results: The 50% distal fall‐off position along the activity depth profiles of the two scanners were found to agree within 2.5 mm. The NeuroPET scanner showed comparable performance over the scanning period of 30 minutes than the conventional PET/CT scanner.Conclusion: The promising results of the phantom study encourage first patient NeuroPET scans following protontreatment. The use of the movable NeuroPET scanner in the treatment room promises to enable high precision PET range verification with minimized disturbing influences of biological washout and reduced geometrical uncertainties due to patient repositioning.