Proton Dose Research Articles

Water or realistic compositions in proton radiotherapy? An analytical study

Purpose: Pre-clinical tests and simulation studies for radiotherapy are generally carried out using water or simplified materials. Investigating the effects of defining compositionally realistic media in proton transport studies was the objective of this work. Accurate modeling of the Bragg curve is a fundamental requirement for such a study.Methods and materials: An equation previously validated by experiments provides an appropriate analytical method for proton dose calculation in depth of the target. Owing to the dependency on protons ranges and the probability of undergoing non-elastic nuclear interactions (NNI), this formula comprises three parameters with values specified for initial proton energy and for the target material. As a result, knowledge of the depth-dose distribution using this analytical model is limited to the materials for which the data has been provided in nuclear data tables. In this study, we used our general formulas for calculating the protons ranges and the probability of undergoing NNI in desired compounds and mixtures with an arbitrary number of constituent elements. Furthermore, the protons dose distribution in the depth of these targets was leading off with determining the parameters appeared in the employed model using our mathematically easy to handle relations. For a number of tissues which may be of interest in proton radiotherapy studies but are missing in reference data tables, the mentioned parameters were calculated. Moreover, the resultant values for the protons ranges and the probability of undergoing NNIs were compared with those in water.Results: The results showed that the differences between the position of Bragg peaks in water and realistic media considered in this study were energy dependent, and ranged between a few millimeters. For proton beams of arbitrary chosen initial energies, the maximum dose delivered to the realistic media varied between about −0.02–4.42% in comparison with that to water.Conclusions: The effects observed (both in penetration and in the magnitude of the Bragg peaks) may be overshadowed by the different dose prescriptions depending on the quality of the treatment planning system, and dosimetry protocols used at the various therapy centers.

The visible signal responsible for proton therapy dosimetry using bare optical fibers is not Čerenkov radiation.

Proton beam dosimetry using bare plastic optical fibers has emerged as a simple approach to proton beam dosimetry. The source of the signal in this method has been attributed to Čerenkov radiation. The aim of this work was a phenomenological study of the nature of the visible light responsible for the signal in bare fiber optic dosimetry of proton therapy beams. Plastic fiber optic probes embedded in solid water phantoms were irradiated with proton beams of energies 100, 180, and 225 MeV produced by a proton therapy cyclotron. Luminescence spectroscopy was performed by a CCD-coupled spectrometer. The spectra were acquired at various depths in phantom to measure the percentage depth dose (PDD) for each beam energy. For comparison, the PDD curves were acquired using a standard multilayer ion chamber device. In order to further analyze the contribution of the Čerenkov radiation in the spectra, Monte Carlo simulation was performed using fluka Monte Carlo code to stochastically simulate radiation transport, ionizing radiation dose deposition, and optical emission of Čerenkov radiation. The measured depth doses using the bare fiber are in agreement with measurements performed by the multilayer ion chamber device, indicating the feasibility of using bare fiber probes for proton beam dosimetry. The spectroscopic study of proton-irradiated fibers showed a continuous spectrum with a shape different from that of Čerenkov radiation. The Monte Carlo simulations confirmed that the amount of the generated Čerenkov light does not follow the radiation absorbed dose in a medium. The source of the optical signal responsible for the proton dose measurement using bare optical fibers is not Čerenkov radiation. It is fluorescence of the plastic material of the fiber.

A correction scheme for a simplified analytical random walk model algorithm of proton dose calculation in distal Bragg peak regions

The lateral homogeneity assumption is used in most analytical algorithms for proton dose, such as the pencil-beam algorithms and our simplified analytical random walk model. To improve the dose calculation in the distal fall-off region in heterogeneous media, we analyzed primary proton fluence near heterogeneous media and propose to calculate the lateral fluence with voxel-specific Gaussian distributions. The lateral fluence from a beamlet is no longer expressed by a single Gaussian for all the lateral voxels, but by a specific Gaussian for each lateral voxel. The voxel-specific Gaussian for the beamlet of interest is calculated by re-initializing the fluence deviation on an effective surface where the proton energies of the beamlet of interest and the beamlet passing the voxel are the same. The dose improvement from the correction scheme was demonstrated by the dose distributions in two sets of heterogeneous phantoms consisting of cortical bone, lung, and water and by evaluating distributions in example patients with a head-and-neck tumor and metal spinal implants. The dose distributions from Monte Carlo simulations were used as the reference. The correction scheme effectively improved the dose calculation accuracy in the distal fall-off region and increased the gamma test pass rate. The extra computation for the correction was about 20% of that for the original algorithm but is dependent upon patient geometry.

A simplified analytical random walk model for proton dose calculation

We propose an analytical random walk model for proton dose calculation in a laterally homogeneous medium. A formula for the spatial fluence distribution of primary protons is derived. The variance of the spatial distribution is in the form of a distance-squared law of the angular distribution. To improve the accuracy of dose calculation in the Bragg peak region, the energy spectrum of the protons is used. The accuracy is validated against Monte Carlo simulation in water phantoms with either air gaps or a slab of bone inserted. The algorithm accurately reflects the dose dependence on the depth of the bone and can deal with small-field dosimetry. We further applied the algorithm to patients’ cases in the highly heterogeneous head and pelvis sites and used a gamma test to show the reasonable accuracy of the algorithm in these sites. Our algorithm is fast for clinical use.

Investigating deformable image registration and scatter correction for CBCT‐based dose calculation in adaptive IMPT

This work aims at investigating intensity corrected cone-beam x-ray computed tomography (CBCT) images for accurate dose calculation in adaptive intensity modulated proton therapy (IMPT) for prostate and head and neck (H&N) cancer. A deformable image registration (DIR)-based method and a scatter correction approach using the image data obtained from DIR as prior are characterized and compared on the basis of the same clinical patient cohort for the first time. Planning CT (pCT) and daily CBCT data (reconstructed images and measured projections) of four H&N and four prostate cancer patients have been considered in this study. A previously validated Morphons algorithm was used for DIR of the planning CT to the current CBCT image, yielding a so-called virtual CT (vCT). For the first time, this approach was translated from H&N to prostate cancer cases in the scope of proton therapy. The warped pCT images were also used as prior for scatter correction of the CBCT projections for both tumor sites. Single field uniform dose and IMPT (only for H&N cases) treatment plans have been generated with a research version of a commercial planning system. Dose calculations on vCT and scatter corrected CBCT (CBCTcor) were compared by means of the proton range and a gamma-index analysis. For the H&N cases, an additional diagnostic replanning CT (rpCT) acquired within three days of the CBCT served as additional reference. For the prostate patients, a comprehensive contour comparison of CBCT and vCT, using a trained physician's delineation, was performed. A high agreement of vCT and CBCTcor was found in terms of the proton range and gamma-index analysis. For all patients and indications between 95% and 100% of the proton dose profiles in beam's eye view showed a range agreement of better than 3 mm. The pass rate in a (2%,2 mm) gamma-comparison was between 96% and 100%. For H&N patients, an equivalent agreement of vCT and CBCTcor to the reference rpCT was observed. However, for the prostate cases, an insufficient accuracy of the vCT contours retrieved from DIR was found, while the CBCTcor contours showed very high agreement to the contours delineated on the raw CBCT. For H&N patients, no considerable differences of vCT and CBCTcor were found. For prostate cases, despite the high dosimetric agreement, the DIR yields incorrect contours, probably due to the more pronounced anatomical changes in the abdomen and the reduced soft-tissue contrast in the CBCT. Using the vCT as prior, these inaccuracies can be overcome and images suitable for accurate delineation and dose calculation in CBCT-based adaptive IMPT can be retrieved from scatter correction of the CBCT projections.

Magnetoresistance of proton irradiated Si0.97Ge0.03 whiskers

Whiskers are a new material that is characterized by high structural perfection, chemical resistance and strength which reaches the theoretically possible limit for crystals of small transverse dimensions. The test whiskers were synthesized by the method of chemical transport reactions in a closed bromide system using gold as the initiator of growth. The crystals were irradiated by protons with an energy of 6MeV and doses of 5×1013, 1015 and 1×1017 p+/cm2 at 40°C in a U-120 cyclotron.The effects of proton irradiation and high magnetic fields on the magnetoresistance of Si1-xGex (x = 0,03) whiskers in the 4.2–300K temperature range has been studied. A slight decrease in the electrical resistance of the crystals in the 4.2–40K temperature range during irradiation with small proton doses and a significant increase in their resistance in the entire investigated temperature range for a dose of 1×1017 p+/cm2 have been found. The ionization energy of the impurity atoms in different magnetic fields has been calculated. It has been revealed that the energy level of the impurity depends on the magnetic field but slightly which in turn indicates a independence of the concentration of holes on the magnetic field. It has been shown that a significant magnetoresistance at all studied temperatures was due to the magnetic field-caused decrease in the mobility of free carriers (holes). It has been found that the concentration of holes depends on magnetic field but a little. Conclusion has been made about a negligible expansion of the band gap in magnetic fields of up to 8T.

Clinical Implementation of a Proton Dose Verification System Utilizing a GPU Accelerated Monte Carlo Engine.

To develop a clinical infrastructure that allows for routine Monte Carlo dose calculation verification of spot scanning proton treatment plans and includes a simple biological model to aid in normal tissue protection. A graphical processing unit accelerated Monte Carlo dose engine was used as the calculation engine for dose verification on spot scanning proton plans. An infrastructure was built around this engine that allows for seamless exporting of treatment plans from the treatment planning system and importing of dose distribution from the Monte Carlo calculation via DICOM (digital imaging and communications in medicine). An easy-to-use Web-based interface was developed so that the application could be run from any computer. In addition to the standard relative biological effectiveness = 1.1 for proton therapy, a simple linear equation dependent on dose-weighted linear energy transfer was included. This was used to help detect possible high biological dose in critical structures. More than 270 patients were treated at our proton center in the first year of operation. Because most plans underwent multiple iterations before final approval, more than 1000 plans have been run through the system from multiple users with minimal downtime. The average time from plan export to importing of the Monte Carlo doses was less than 15 minutes. Treatment plans have been modified based on the nominal Monte Carlo dose or the biological dose. Monte Carlo dose calculation verification of spot scanning proton treatment plans is feasible in a clinical environment. The 3-dimensional dose verification, particularly near heterogeneities, has resulted in plan modifications. The biological dose data provides actionable feedback for end of range effects, especially in pediatric patients.

Ultra-short laser-accelerated proton pulses have similar DNA-damaging effectiveness but produce less immediate nitroxidative stress than conventional proton beams.

Ultra-short proton pulses originating from laser-plasma accelerators can provide instantaneous dose rates at least 107-fold in excess of conventional, continuous proton beams. The impact of such extremely high proton dose rates on A549 human lung cancer cells was compared with conventionally accelerated protons and 90 keV X-rays. Between 0.2 and 2 Gy, the yield of DNA double strand breaks (foci of phosphorylated histone H2AX) was not significantly different between the two proton sources or proton irradiation and X-rays. Protein nitroxidation after 1 h judged by 3-nitrotyrosine generation was 2.5 and 5-fold higher in response to conventionally accelerated protons compared to laser-driven protons and X-rays, respectively. This difference was significant (p < 0.01) between 0.25 and 1 Gy. In conclusion, ultra-short proton pulses originating from laser-plasma accelerators have a similar DNA damaging potential as conventional proton beams, while inducing less immediate nitroxidative stress, which probably entails a distinct therapeutic potential.

Nowadays proton therapy: Double scattering versus pencil beam scanning mode

Introduction Pencil beam scanning mode is becoming widely used in proton therapy. The biological consequences of the use of pencil beam scanning instead of double scattering mode have not been studied in detail yet. Most of the biological studies of relative biological effectiveness of protons were carried out using double scattering mode. The dose rates in pencil beam scanning are locally essentially higher than those in double scattering mode, which could affect the cell response. Purpose The main purpose of this study was to study the in vitro biological consequences of the use of pencil beam scanning in comparison to double scattering mode. Materials and methods Cell culture samples (normal human fibroblasts, medulloblastoma cell line DAOY) were irradiated by both modes using the same proton energies and doses. The initial energy of protons used for the irradiations was selected according to the clinical options in double scattering. The same energy was then adjusted for the pencil beam scanning mode. Several parameters (cell survival, apoptosis level, double strand break induction) of the cells were followed in various times post irradiation. Results The cell survival was found to be lower for the pencil beam scanning mode in both studied positions in the Bragg curve (plateau, peak). The apoptosis level and the induction of double strand breaks were found to be higher for pencil beam scanning mode. Conclusion We suggest that pencil beam scanning mode is slightly more efficient than double scattering mode. Disclosure No conflict.

Acoustic time-of-flight for proton range verification in water.

Measurement of the arrival times of thermoacoustic waves induced by pulsed proton dose depositions (protoacoustics) may provide a proton range verification method. The goal of this study is to characterize the required dose and protoacoustic proton range (distance) verification accuracy in a homogeneous water medium at a hospital-based clinical cyclotron. Gaussian-like proton pulses with 17 μs widths and instantaneous currents of 480 nA (5.6 × 10(7) protons/pulse, 3.4 cGy/pulse at the Bragg peak) were generated by modulating the cyclotron proton source with a function generator. After energy degradation, the 190 MeV proton pulses irradiated a water phantom, and the generated protoacoustic emissions were measured by a hydrophone. The detector position and proton pulse characteristics were varied. The experimental results were compared to simulations. Different arrival time metrics derived from acoustic waveforms were compared, and the accuracy of protoacoustic time-of-flight distance calculations was assessed. A 27 mPa noise level was observed in the treatment room during irradiation. At 5 cm from the proton beam, an average maximum pressure of 5.2 mPa/1 × 10(7) protons (6.1 mGy at the Bragg peak) was measured after irradiation with a proton pulse with 10%-90% rise time of 11 μs. Simulation and experiment arrival times agreed well, and the observed 2.4 μs delay between simulation and experiment is attributed to the difference between the hydrophone's acoustic and geometric centers. Based on protoacoustic arrival times, the beam axis position was measured to within (x, y) = (-2.0, 0.5) ± 1 mm. After deconvolution of the exciting proton pulse, the protoacoustic compression peak provided the most consistent measure of the distance to the Bragg peak, with an error distribution with mean = - 4.5 mm and standard deviation = 2.0 mm. Based on water tank measurements at a clinical hospital-based cyclotron, protoacoustics is a potential method for measuring the beam's position (x and y within 2.0 mm) and Bragg peak range (2.0 mm standard deviation), although range verification will require simulation or experimental calibration to remove systematic error. Based on extrapolation, a protoacoustic arrival time reproducibility of 1.5 μs (2.2 mm) is achievable with 2 Gy of total deposited dose. Of the compared methods, deconvolution of the excitation proton pulse is the best technique for extracting protoacoustic arrival times, particularly if there is variation in the proton pulse shape.

Radiation Hardness and Self-Healing of Perovskite Solar Cells.

The radiation hardness of CH3 NH3 PbI3 -based solar cells is evaluated from in situ measurements during high-energy proton irradiation. These organic-inorganic perovskites exhibit radiation hardness and withstand proton doses that exceed the damage threshold of crystalline silicon by almost 3 orders of magnitude. Moreover, after termination of the proton irradiation, a self-healing process of the solar cells commences.

Poster ‐ 40: Treatment Verification of a 3D‐printed Eye Phantom for Proton Therapy

Purpose:Ocular melanoma is a form of eye cancer which is often treated using proton therapy. The benefit of the steep proton dose gradient can only be leveraged for accurate patient eye alignment. A treatment‐planning program was written to plan on a 3D‐printed anatomical eye‐phantom, which was then irradiated to demonstrate the feasibility of verifying in vivo dosimetry for proton therapy using PET imaging.Methods:A 3D CAD eye model with critical organs was designed and voxelized into the Monte‐Carlo transport code FLUKA. Proton dose and PET isotope production were simulated for a treatment plan of a test tumour, generated by a 2D treatment‐planning program developed using NumPy and proton range tables. Next, a plastic eye‐phantom was 3D‐printed from the CAD model, irradiated at the TRIUMF Proton Therapy facility, and imaged using a PET scanner.Results:The treatment‐planning program prediction of the range setting and modulator wheel was verified in FLUKA to treat the tumour with at least 90% dose coverage for both tissue and plastic. An axial isotope distribution of the PET isotopes was simulated in FLUKA and converted to PET scan counts. Meanwhile, the 3D‐printed eye‐phantom successfully yielded a PET signal.Conclusions:The 2D treatment‐planning program can predict required parameters to sufficiently treat an eye tumour, which was experimentally verified using commercial 3D‐printing hardware to manufacture eye‐phantoms. Comparison between the simulated and measured PET isotope distribution could provide a more realistic test of eye alignment, and a variation of the method using radiographic film is being developed.

Quantum well intermixing technique using proton implantation for carrier confinement of vertical-cavity surface-emitting lasers

We investigated quantum well intermixing (QWI) using proton implantation to form the carrier confinement structure in the active layer of a vertical-cavity surface-emitting laser (VCSEL). The required potential barrier height is discussed referring to the result of numerical analysis. The bandgap change due to the QWI was investigated experimentally for various quantum well structures, proton dose densities, and thermal annealing conditions. A potential barrier height of 30 meV was observed using a high-indium and thin-well structure. High crystalline quality was confirmed by photoluminescence intensity measurement, even after the QWI process, and the lasing of the fabricated QWI-VCSEL was observed without any deterioration. The proposed technique would be effective in improving the device performance in a simple fabrication process.

SU‐F‐J‐194: Development of Dose‐Based Image Guided Proton Therapy Workflow

Purpose:To implement image‐guided proton therapy (IGPT) based on daily proton dose distribution.Methods:Unlike x‐ray therapy, simple alignment based on anatomy cannot ensure proper dose coverage in proton therapy. Anatomy changes along the beam path may lead to underdosing the target, or overdosing the organ‐at‐risk (OAR). With an in‐room mobile computed tomography (CT) system, we are developing a dose‐based IGPT software tool that allows patient positioning and treatment adaption based on daily dose distributions. During an IGPT treatment, daily CT images are acquired in treatment position. After initial positioning based on rigid image registration, proton dose distribution is calculated on daily CT images. The target and OARs are automatically delineated via deformable image registration. Dose distributions are evaluated to decide if repositioning or plan adaptation is necessary in order to achieve proper coverage of the target and sparing of OARs. Besides online dose‐based image guidance, the software tool can also map daily treatment doses to the treatment planning CT images for offline adaptive treatment.Results:An in‐room helical CT system is commissioned for IGPT purposes. It produces accurate CT numbers that allow proton dose calculation. GPU‐based deformable image registration algorithms are developed and evaluated for automatic ROI‐delineation and dose mapping. The online and offline IGPT functionalities are evaluated with daily CT images of the proton patients.Conclusion:The online and offline IGPT software tool may improve the safety and quality of proton treatment by allowing dose‐based IGPT and adaptive proton treatments.Research is partially supported by Mevion Medical Systems.

SU‐F‐J‐198: A Cross‐Platform Adaptation of An a Priori Scatter Correction Algorithm for Cone‐Beam Projections to Enable Image‐ and Dose‐Guided Proton Therapy

Purpose:Cone‐beam CT (CBCT) imaging may enable image‐ and dose‐guided proton therapy, but is challenged by image artefacts. The aim of this study was to demonstrate the general applicability of a previously developed a priori scatter correction algorithm to allow CBCT‐based proton dose calculations.Methods:The a priori scatter correction algorithm used a plan CT (pCT) and raw cone‐beam projections acquired with the Varian On‐Board Imager. The projections were initially corrected for bow‐tie filtering and beam hardening and subsequently reconstructed using the Feldkamp‐Davis‐Kress algorithm (rawCBCT). The rawCBCTs were intensity normalised before a rigid and deformable registration were applied on the pCTs to the rawCBCTs. The resulting images were forward projected onto the same angles as the raw CB projections. The two projections were subtracted from each other, Gaussian and median filtered, and then subtracted from the raw projections and finally reconstructed to the scatter‐corrected CBCTs. For evaluation, water equivalent path length (WEPL) maps (from anterior to posterior) were calculated on different reconstructions of three data sets (CB projections and pCT) of three parts of an Alderson phantom. Finally, single beam spot scanning proton plans (0–360 deg gantry angle in steps of 5 deg; using PyTRiP) treating a 5 cm central spherical target in the pCT were re‐calculated on scatter‐corrected CBCTs with identical targets.Results:The scatter‐corrected CBCTs resulted in sub‐mm mean WEPL differences relative to the rigid registration of the pCT for all three data sets. These differences were considerably smaller than what was achieved with the regular Varian CBCT reconstruction algorithm (1–9 mm mean WEPL differences). Target coverage in the re‐calculated plans was generally improved using the scatter‐corrected CBCTs compared to the Varian CBCT reconstruction.Conclusion:We have demonstrated the general applicability of a priori CBCT scatter correction, potentially opening for CBCT‐based image/dose‐guided proton therapy, including adaptive strategies.Research agreement with Varian Medical Systems, not connected to the present project.

MO‐A‐201‐00: A Cliff's Notes Version of Proton Therapy

Proton therapy is a rapidly growing modality in the fight against cancer. From a high‐level perspective the process of proton therapy is identical to x‐ray based external beam radiotherapy. However, this course is meant to illustrate for x‐ray physicists the many differences between x‐ray and proton based practices.Unlike in x‐ray therapy, proton dose calculations use CT Hounsfield Units (HU) to determine proton stopping power and calculate the range of a beam in a patient. Errors in stopping power dominate the dosimetric uncertainty in the beam direction, while variations in patient position determine uncertainties orthogonal to the beam path. Mismatches between geometric and range errors lead to asymmetric uncertainties, and so while geometric uncertainties in x‐ray therapy are mitigated through the use of a Planning Target Volume (PTV), this approach is not suitable for proton therapy. Robust treatment planning and evaluation are critical in proton therapy, and will be discussed in this course.Predicting the biological effect of a proton dose distribution within a patient is also a complex undertaking. The proton therapy community has generally regarded the Radiobiological Effectiveness (RBE) of a proton beam to be 1.1 everywhere in the patient, but there are increasing data to suggest that the RBE probably climbs higher than 1.1 near the end of a proton beam when the energy deposition density increases. This lecture will discuss the evidence for variable RBE in proton therapy and describe how this is incorporated into current proton treatment planning strategies.Finally, there are unique challenges presented by the delivery process of proton therapy. Many modern systems use a spot scanning technique which has several advantages over earlier scattered beam designs. However, the time dependence of the dose deposition leads to greater concern with organ motion than with scattered protons or x‐rays. Image guidance techniques in proton therapy may also differ from standard x‐ray approaches, due to equipment design or the desire to maximize efficiency within a high‐cost proton therapy treatment room. Differences between x‐ray and proton therapy delivery will be described. Learning Objectives: Understand how CT HU are calibrated to provide proton stopping power, and the sources of uncertainty in this process. Understand why a PTV is not suitable for proton therapy, and how robust treatment planning and evaluation are used to mitigate uncertainties. Understand the source and implications of variable RBE in proton therapy Learn about proton specific challenges and approaches in beam delivery and image guidanceJon Kruse has a research grant from Varian Medical Systems related to proton therapy treatment plannning.; J. Kruse, Jon Kruse has a research grant with Varian Medical Systems related to proton therapy planning.

MO‐A‐201‐01: A Cliff's Notes Version of Proton Therapy

Proton therapy is a rapidly growing modality in the fight against cancer. From a high‐level perspective the process of proton therapy is identical to x‐ray based external beam radiotherapy. However, this course is meant to illustrate for x‐ray physicists the many differences between x‐ray and proton based practices.Unlike in x‐ray therapy, proton dose calculations use CT Hounsfield Units (HU) to determine proton stopping power and calculate the range of a beam in a patient. Errors in stopping power dominate the dosimetric uncertainty in the beam direction, while variations in patient position determine uncertainties orthogonal to the beam path. Mismatches between geometric and range errors lead to asymmetric uncertainties, and so while geometric uncertainties in x‐ray therapy are mitigated through the use of a Planning Target Volume (PTV), this approach is not suitable for proton therapy. Robust treatment planning and evaluation are critical in proton therapy, and will be discussed in this course.Predicting the biological effect of a proton dose distribution within a patient is also a complex undertaking. The proton therapy community has generally regarded the Radiobiological Effectiveness (RBE) of a proton beam to be 1.1 everywhere in the patient, but there are increasing data to suggest that the RBE probably climbs higher than 1.1 near the end of a proton beam when the energy deposition density increases. This lecture will discuss the evidence for variable RBE in proton therapy and describe how this is incorporated into current proton treatment planning strategies.Finally, there are unique challenges presented by the delivery process of proton therapy. Many modern systems use a spot scanning technique which has several advantages over earlier scattered beam designs. However, the time dependence of the dose deposition leads to greater concern with organ motion than with scattered protons or x‐rays. Image guidance techniques in proton therapy may also differ from standard x‐ray approaches, due to equipment design or the desire to maximize efficiency within a high‐cost proton therapy treatment room. Differences between x‐ray and proton therapy delivery will be described. Learning Objectives: Understand how CT HU are calibrated to provide proton stopping power, and the sources of uncertainty in this process. Understand why a PTV is not suitable for proton therapy, and how robust treatment planning and evaluation are used to mitigate uncertainties. Understand the source and implications of variable RBE in proton therapy Learn about proton specific challenges and approaches in beam delivery and image guidanceJon Kruse has a research grant from Varian Medical Systems related to proton therapy treatment plannning.; J. Kruse, Jon Kruse has a research grant with Varian Medical Systems related to proton therapy planning.

SU‐D‐204‐02: BED Consistent Extrapolation of Mean Dose Tolerances

Purpose:The safe use of radiotherapy requires the knowledge of tolerable organ doses. For experimental fractionation schemes (e.g. hypofractionation) these are typically extrapolated from traditional fractionation schedules using the Biologically Effective Dose (BED) model. This work demonstrates that using the mean dose in the standard BED equation may overestimate tolerances, potentially leading to unsafe treatments. Instead, extrapolation of mean dose tolerances should take the spatial dose distribution into account.Methods:A formula has been derived to extrapolate mean physical dose constraints such that they are mean BED equivalent. This formula constitutes a modified BED equation where the influence of the spatial dose distribution is summarized in a single parameter, the dose shape factor. To quantify effects we analyzed 14 liver cancer patients previously treated with proton therapy in 5 or 15 fractions, for whom also photon IMRT plans were available.Results:Our work has two main implications. First, in typical clinical plans the dose distribution can have significant effects. When mean dose tolerances are extrapolated from standard fractionation towards hypofractionation they can be overestimated by 10–15%. Second, the shape difference between photon and proton dose distributions can cause 30–40% differences in mean physical dose for plans having the same mean BED. The combined effect when extrapolating proton doses to mean BED equivalent photon doses in traditional 35 fraction regimens resulted in up to 7–8 Gy higher doses than when applying the standard BED formula. This can potentially lead to unsafe treatments (in 1 of the 14 analyzed plans the liver mean dose was above its 32 Gy tolerance).Conclusion:The shape effect should be accounted for to avoid unsafe overestimation of mean dose tolerances, particularly when estimating constraints for hypofractionated regimens. In addition, tolerances established for a given treatment modality cannot necessarily be applied to other modalities with drastically different dose distributions.

TH‐CD‐209‐06: LET‐Based Adjustment of IMPT Plans Using Prioritized Optimization

Purpose:In‐vitro experiments suggest an increase in proton relative biological effectiveness (RBE) towards the end of range. However, proton treatment planning and dose reporting for clinical outcome assessment has been based on physical dose and constant RBE. Therefore, treatment planning for intensity‐modulated proton therapy (IMPT) is unlikely to transition radically to pure RBE‐based planning. We suggest a hybrid approach where treatment plans are initially created based on physical dose constraints and prescriptions, and are subsequently altered to avoid high linear energy transfer (LET) in critical structures while limiting the degradation of the physical dose distribution.Methods:To allow fast optimization based on dose and LET we extended a GPU‐based Monte‐Carlo code towards providing dose‐averaged LET in addition to dose for all pencil beams. After optimizing an initial IMPT plan based on physical dose, a prioritized optimization scheme is used to modify the LET distribution while constraining the physical dose objectives to values close to the initial plan. The LET optimization step is performed based on objective functions evaluated for the product of physical dose and LET (LETxD). To first approximation, LETxD represents a measure of the additional biological dose that is caused by high LET. Regarding optimization techniques, LETxD has the advantage of being a linear function of the pencil beam intensities.Results:The method is applicable to treatments where serial critical structures with maximum dose constraint are located in or near the target. We studied intra‐cranial tumors (high‐grade meningiomas, base‐of‐skull chordomas) where the target (CTV) overlaps with the brainstem and optic structures. Often, high LETxD in critical structures can be avoided while minimally compromising physical dose planning objectives.Conclusion:LET‐based re‐optimization of IMPT plans represents a pragmatic approach to bridge the gap between purely physical dose‐based and RBE‐based planning. It can be realized with fast GPU‐based Monte‐Carlo dose calculation.The project was in part supported by NIH grants U19 CA 021239‐36 and C06 CA 059267.

SU‐F‐T‐159: Monte Carlo Simulation Studies of Three‐Dimensional Dose Distribution for Polymer Gel Dosimeter and Radiochromic Gel Dosimeter in a Proton Beam

Purpose:The purpose of this simulation study is to evaluate the proton detectability of gel dosimeters, and estimate the three‐dimensional dose distribution of protons in the radiochromic gel and polymer gel dosimeter compared with the dose distribution in water.Methods:The commercial composition ratios of normoxic polymer gel and LCV micelle radiochromic gel were included in this simulation study. The densities of polymer and radiochromic gel were 1.024 and 1.005 g/cm3, respectively. The 50, 80 and 140 MeV proton beam energies were selected. The dose distributions of protons in the polymer and radiochromic gel were simulated using Monte Carlo radiation transport code (MCNPX 2.7.0, Los Alamos Laboratory). The water equivalent depth profiles and the dose distributions of two gel dosimeters were compared for the water.Results:In case of irradiating 50, 80 and 140 MeV proton beam to water phantom, the reference Bragg‐peak depths are represented at 2.22, 5.18 and 13.98 cm, respectively. The difference in the water equivalent depth is represented to about 0.17 and 0.37 cm in the radiochromic gel and polymer gel dosimeter, respectively. The proton absorbed doses in the radiochromic gel dosimeter are calculated to 2.41, 3.92 and 6.90 Gy with increment of incident proton energies. In the polymer gel dosimeter, the absorbed doses are calculated to 2.37, 3.85 and 6.78 Gy with increment of incident proton energies. The relative absorbed dose in radiochromic gel (about 0.47 %) is similar to that of water than the relative absorbed dose of polymer gel (about 2.26 %). In evaluating the proton dose distribution, we found that the dose distribution of both gel dosimeters matched that of water in most cases.Conclusion:As the dosimetry device, the radiochromic gel dosimeter has the potential particle detectability and is feasible to use for quality assurance of proton beam therapy beam.