Proton Therapy Dose Research Articles

Neutron dose and its measurement in proton therapy-current State of Knowledge.

Proton therapy has shown dosimetric advantages over conventional radiation therapy using photons. Although the integral dose for patients treated with proton therapy is low, concerns were raised about late effects like secondary cancer caused by dose depositions far away from the treated area. This is especially true for neutrons and therefore the stray dose contribution from neutrons in proton therapy is still being investigated. The higher biological effectiveness of neutrons compared to photons is the main cause of these concerns. The gold-standard in neutron dosimetry is measurements, but performing neutron measurements is challenging. Different approaches have been taken to overcome these difficulties, for instance with newly developed neutron detectors. Monte Carlo simulations is another common technique to assess the dose from secondary neutrons. Measurements and simulations are used to develop analytical models for fast neutron dose estimations. This article tries to summarize the developments in the different aspects of neutron dose in proton therapy since 2017. In general, low neutron doses have been reported, especially in active proton therapy. Although the published biological effectiveness of neutrons relative to photons regarding cancer induction is higher, it is unlikely that the neutron dose has a large impact on the second cancer risk of proton therapy patients.

Modified fast adaptive scatter kernel superposition (mfASKS) correction and its dosimetric impact on CBCT-based proton therapy dose calculation.

While cone beam computed tomography (CBCT) is able to provide patient anatomical information, its image quality is severely degraded due to scatter contamination, which degrades the accuracy of CBCT-based dose distribution estimation in proton therapy. In this work, we combined two existing scatter kernel correction methods: the point-spread function (PSF)-based scatter kernel derivation method and the fast adaptive scatter kernel superposition (fASKS) model, and evaluated the impact of the modified fASKS (mfASKS) correction on the accuracy of proton dose distribution estimation. To evaluate feasibility of the mfASKS approach using accurate scatter distributions, both Monte Carlo simulations and experiments were performed for an on-board CBCT machine integrated with a proton therapy machine. We developed a strategy to modify central intensity, constant intensity, and amplitude of the scatter kernels derived from PSFs for the fASKS model. A parameter required for the fASKS model was derived by optimizing uniformity in the mfASKS-corrected reconstructed images. Subsequently, the mfASKS model was used to remove scatter in CBCT imaging. We quantitatively compared the Hounsfield Unit (HU) and proton stopping power ratio (SPR) images for five different phantoms. To assess improvement of dose calculation accuracy, a series of proton treatment plans were produced using the CBCT images with and without the mfASKS correction. The accuracies of both HU and SPR intensity quantifications are improved as a result of the mfASKS correction. Mean absolute water-equivalent path length difference to the true value decreases from 10.3 to 0.934mm for the Gammex phantom (simulation). At the same time, mfASKS is able to offer more accurate dose distributions, especially at the distal fall-off region where noticeable dose overestimation is observed in the uncorrected scenario. Mean absolute relative error of proton range in the pelvic phantom improves from 5.03% to 2.57% (experiment). mfASKS enables more accurate CBCT-based proton dose calculation. This technique has significant implications in image-guided radiotherapy and dose verifications in adaptive proton therapy.

Proton therapy for newly diagnosed pediatric diffuse intrinsic pontine glioma.

Diffuse intrinsic pontine glioma (DIPG) is a type of brain malignancy with a very poor prognosis. Although various radiation and chemotherapy protocols have been attempted, only conventional radiotherapy has yielded improvements in survival. In this study, we aimed to compare proton therapy versus conventional photon radiotherapy in terms of the outcomes of pediatric patients with DIPG. This retrospective review included 12 pediatric patients with newly diagnosed DIPG who received a total proton therapy dose of 54 Gy (relative biological effectiveness) in 30 fractions at the University of Tsukuba Hospital between 2011 and 2017 (proton group). We additionally reviewed the medical records of 10 patients with DIPG who previously underwent conventional photon radiotherapy at our institute (historical cohort). The median progression-free survival (PFS) duration was 5 months (range 1-11 months), and the 6-, 12-, and 18-month PFS rates were 50%, 33%, and 25%, respectively. The median overall survival (OS) duration was 9 months (range 4-48 months), and the 6-, 12-, 18-, and 24-month OS rates were 66.8%, 50%, 41%, and 20%, respectively. There were no significant differences in survival between the proton and historical groups (PFS, p = 0.169 and OS, p = 0.16). Proton therapy was well tolerated by the majority of patients. No severe adverse events, including radiation necrosis, were recorded. Proton therapy did not yield superior survival outcomes vs. conventional photon radiotherapy in patients with DIPG at our institution. Further research is needed to identify the factors associated with better survival in this population.

Spatial fractionation of the dose in proton therapy: Proton minibeam radiation therapy

In radiation therapy, a renewed interest is emerging for the study of spatially fractionated irradiation. In this article, a few applications using spatial fractionation of the dose will be discussed with a focus on proton minibeam radiation therapy. Examples of calculated dose (1D profiles and 2D dose distributions) and biological evidence obtained so far will be presented for various spatially fractionated techniques GRID, micro- and minibeam radiation therapy. Recent results demonstrating that proton minibeam radiation therapy leads to an increase in normal tissues sparing will be discussed, which opens the door to a dose escalation in the tumour and a possibly efficient treatment of very radioresistant tumours.

New Hybrid 3D Analytical Linear Energy Transfer (LET) Calculation Algorithm Based on the Pre-Calculated Data from Monte Carlo (MC) Simulations

The traditional 1D analytical LET model of the spot scanned proton therapy (SSPT) assumes a uniform LET distribution in the lateral direction for each depth, which will under-estimate the LET value in the low dose penumbra region and over-estimate the LET value in the high dose region compared with Monte Carlo (MC) simulations. We propose a fast and accurate hybrid 3D analytical LET calculation method based on the pre-calculated data from MC simulations. The method is analogous to the pencil beam algorithm to calculate the physical dose in proton therapy with the 3D LET calculation kernel generated by MC as follows: Step-1: Using a well-benchmarked MC code (Geant4) to generate LET distributions of single energy proton beams in water for all 97 energies and derive the dose-averaged LET (LETd) lateral profiles at various depths starting from the surface with 1mm depth interval increase until the depth with the 1% of the peak dose after the Bragg peak. Step-2: Using a customized “error function” to fit the aforementioned LET lateral profiles for every depth of every energy, and store the fitted coefficients as a lookup table. Step-3: During the dose/LET calculation, the stored fitted coefficients and the fitting function will be used to calculate the LETd values based on the spot energies and the water equivalence thickness (WET). The inhomogeneity is handled by taking WET into account in both beam and lateral directions. We then validated the improvement of our new method by comparing the calculated LET distributions with the results calculated from the MC and the previous 1D analytical LET model in twelve patients with different disease sites. Compared with the MC simulation results, the LETd calculation of our new method is systematically better than the traditional 1D model, with the average 3D gamma analysis results of all 12 patients with the threshold of 3% and 2mm increased from 94% to 98% in the region with dose >10% of the maixumum dose. The majority improvement comes from the more accurate LETd calculation in the low dose penumbra region (5%-20% of the prescription dose). In some cases, our new method only has less than 3% relatively difference compared with the MC simulation in the penumbra region, while the previous 1D model under-estimates the LETd value by about 20% to 50%. The calculation time of our new method is comparable to our previous 1D analytical LET calculation algorithm. Our new hybrid 3D analytical method (1) can calculate LETd in SSPT accurately and efficiently, (2) will be used for biological effects evaluation and LET-guided robust optimization in SSPT to improve the OAR protection, (3) is easily to be integrated by other analytical dose engine.

Fitting the Generalized Target Model to Cell Survival Data of Proton Radiation Reveals Dose-Dependent RBE and Inspires an Alternative Method to Estimate RBE in High-Dose Regions.

The imprecise estimation of the relative biological effectiveness (RBE) of proton radiation has been one of the main challenges for further calculating the biologically effective dose in proton therapy. Since dose levels can greatly influence the proton RBE, the relationship between the two should be clarified first. In addition, since the dose-response curves are usually too complex to readily assess RBE in high-dose regions, a reliable and simple method is needed to predict the RBE of proton radiation accurately in clinically relevant doses. The standard linear-quadratic (LQ) model is widely used to determine the RBE of particles for clinical applications. However, there has been some debate over its use when modeling the cell survival curves in high-dose regions, since those survival curves usually show linear behavior in the semilogarithmic plot. By considering both cellular repair effects and indirect effects of radiation, we have proposed a generalized target model with linear-quadratic linear (LQL) characteristics. For the more accurate evaluation of proton RBE in radiotherapy, here we used this generalized target model to fit the cell survival data in V79 and C3H 10T1/2 cells exposed to proton radiation with different LETs. The fitting results show that the generalized target model works as well as the LQ model in general. Based on the fitting parameters of the generalized target model, the RBE of six given doses DT (RBET) could be calculated in the corresponding cell lines with different LETs. The results show that the RBET gradually decreases with increased dose in both cell types. In addition, inspired by the calculation method of the maximum values of RBE (RBEM) in the low-dose region, a novel method was proposed for estimating the RBE in the high-dose region (RBEH) based on the slope ratio of the dose-response curves in this region. Linear regression analysis indicated a significant linear correlation between the proposed RBEH and the RBET in high-dose regions, which suggests that the current method can be used as an alternative tool, which is both simple and robust, to estimate RBE in high-dose regions.

Calculate Primary and Secondary Dose in Proton Therapy Using 200 and 250 MeV Proton Beam Energy

Proton therapy is an advanced radiotherapy technique that delivers a high dose to the tumor, while at the same time sparing the surrounding healthy tissue. This benefit lies in the physical and radiological properties of the protons, such as excellent ballistic accuracy, the combination of low lateral beam dispersion and localized depth-of-depth deposition, which gives proton therapy a high degree of accuracy. The order of a millimeter in the delivery of the dose. However, secondary radiation, mainly neutrons, and gammas, is created by the nuclear interactions that the protons produced in the patient. These secondary neutrons lead to undesirable doses deposited to healthy tissues located at a distance from the target volume, the consequence of which could be an increase in the risk of developing second cancers in treated patients and in particular in children. The objective of this work is to determine the depth doses and the distribution of secondary radiation produced during the penetration of a proton beam of energy of 200 MeV and 250 MeV by the Monte Carlo method into a phantom of water.

Influence of intravenous contrast agent on dose calculation in proton therapy using dual energy CT

The purpose of this study is to evaluate the effect of an intravenous (IV) contrast agent on proton therapy dose calculation using dual-energy computed tomography (DECT). Two DECT methods are considered. The first one, , attempts to accurately predict the proton stopping powers relative to water (SPR) of contrast enhanced (CE) DECT images, while the second generates a virtual non-contrast (VNC) volume that can be processed as a native non-contrast (NC) one. Both methods are compared against single-energy computed tomography (SECT). The accuracy of SPR predicted for different concentrations of IV contrast diluted in water is first evaluated using simulated data. Results then are validated in an experimental set-up comparing SPR predictions for both NC and CE images to measurements made with a multi-layer ionisation chamber (MLIC). Finally, the impact of IV contrast on dose calculation using both SECT and DECT is evaluated for one liver and one head and neck patient. Using simulated data, DECT is shown to be less sensitive to the presence of IV contrast than SECT, although the performance of the method is sensitive to the level of beam hardening considered. For different concentrations of IV contrast diluted in water, experimental MLIC measurement of SPR agrees with DECT predictions within 3% while SECT introduce errors above 20%. This error in the SPR value results in a range error of up to 3.2 mm (2.6%) for proton beams calculated on SECT CE patient images. The error is reduced below 1 mm using DECT with the and VNC methods. Globally, it is observed that the influence of IV contrast on proton therapy dose calculation is mitigated using DECT over SECT. In patient anatomies, the VNC approach provides the best agreement with the reference dose distribution.

PO-0908 Determination of surface dose in pencil beam scanning proton therapy

PO-0908 Determination of surface dose in pencil beam scanning proton therapy

Impact of TPS calculation algorithms on dose delivered to the patient in proton therapy treatments

To estimate the impact of dose calculation approaches adopted in different treatment planning systems (TPSs) on proton therapy dose delivered with pencil beam scanning (PBS).Treatment plans for six regular volumes in water and 15 clinical cases were optimized with Syngo-VC13 and exported for forward recalculation with Raystation-V7.0 pencil beam (RS-PBA) and Monte Carlo (RS-MC) algorithms and with the independent Fluka-MC engine. To verify clinical consistency between the two TPS dosimetric outcomes, the average percentage variations of clinical target volume (CTV) D98%, D50% and D2%, adopted for plan prescription and evaluation, were considered. Ionization chamber measurements served as a further reference for comparison in homogeneous conditions. CTV dose volume histogram (DVH) analysis and gamma evaluation with 3 mm—3% agreement criteria quantified the dose deviation of TPS calculation algorithms, in heterogeneous conditions, against the Fluka-MC code.CTV D50%, representing the plan dose prescription goal, was higher on average over H&N cases of (3.9 ± 0.9)% and (2.3 ± 0.6)% as calculated with RS-PBA and RS-MC, respectively, compared to Syngo. For tumors located in the pelvis district, average D50% variations of (1.6 ± 0.7)% and (1.2 ± 0.7)% were found. Syngo underestimated target near maximum doses with respect to all computation systems. Calculation accuracy in heterogeneous conditions of RS-PBA H&N plans resulted poor when a range shifter was required. Target DVH and γ-analysis showed excellent agreement between RS-MC and Fluka-MC, with γ-pass rates >98% for all patient groups.Different TPS dose calculation approaches mainly affected dose delivered in H&N proton treatments, while minor deviations were found for pelvic tumors. RS-MC proved to be the most accurate TPS dose calculation algorithm when compared to an independent MC simulation code.

Clinical Monte Carlo versus Pencil Beam Treatment Planning in Nasopharyngeal Patients Receiving IMPT.

Pencil beam (PB) analytical algorithms have been the standard of care for proton therapy dose calculations. The introduction of Monte Carlo (MC) algorithms may provide more robust and accurate planning and can improve therapeutic benefit. We conducted a dosimetric analysis to quantify the differences between MC and PB algorithms in the clinical setting of dose-painted nasopharyngeal cancer intensity-modulated proton radiotherapy. Plans of 14 patients treated with PB analytical algorithm optimized and calculated (PBPB) were retrospectively analyzed. The PBPB plans were recalculated using MC to generate PBMC plans and, finally, reoptimized and recalculated with MC to generate MCMC plans. The plans were compared across several dosimetric endpoints and correlated with documented toxicity. Robustness of the planning scenarios (PBPB, PBMC, MCMC) in the presence of setup and range uncertainties was compared. A median decrease of up to 5 Gy (P < .05) was observed in coverage of planning target volume high-risk, intermediate-risk, and low-risk volumes when PB plans were recalculated using the MC algorithm. This loss in coverage was regained by reoptimizing with MC, albeit with a slightly higher dose to normal tissues but within the standard tolerance limits. The robustness of both PB and MC plans remained similar in the presence of setup and range uncertainties. The MC-calculated mean dose to the oral avoidance structure, along with changes in global maximum dose between PB and MC dosimetry, may be associated with acute toxicity-related events. Retrospective analyses of plan dosimetry quantified a loss of coverage with PB that could be recovered under MC optimization. MC optimization should be performed for the complex dosimetry in patients with nasopharyngeal carcinoma before plan acceptance and should also be used in correlative studies of proton dosimetry with clinical endpoints.

First application of a novel SRAM-based neutron detector for proton therapy

In proton therapy, neutrons produced in collimators or in the patient body will contribute to unwanted additional radiation dose to the patient. This neutron dose is primarily associated with an increased risk of secondary cancer after treatment. Neutron detection in proton therapy has previously, to a large extent, been based on the use of passive detectors or detectors of large physical size which are not applicable for measurements inside phantoms. In this study, we present the first application of a neutron detector based on registration of radiation induced effects in Static Random Access Memories (SRAMs) which enables in-phantom fast neutron measurements. Measurements were performed in a water phantom irradiated by a 178 MeV proton pencil beam in a setup mimicking internally produced neutrons in pencil beam scanning (PBS) proton therapy. A neutron energy response model was developed for the SRAM detector to increase the accuracy of measurements in radiation fields with a broad range of neutron energies, and thereby make the detector applicable in the proton therapy setting. FLUKA Monte Carlo (MC) simulations and thermal neutron measurements with thermoluminescence detectors (TLDs) were conducted to evaluate the SRAM detector performance.Both experimental results (SRAM detector and TLDs) and MC simulations indicated a steep decrease in the neutron fluence with increasing lateral distance from the beam axis, while less variation was observed with depth. At Bragg peak depth, experimental values of fast neutron dose (H*(10)) were reduced from 1.4 mSv/Gy at 5.2 cm lateral distance to the beam axis to 0.22 mSv/Gy at 13.7 cm lateral distance. The MC simulations also showed that the SRAM detection threshold of 3 MeV was sufficiently low to capture approximately 90% of the neutron dose in PBS proton therapy. The differences in detector response due to variations in the neutron energy spectrum were small (<10%), and may be corrected using position-specific calibration factors. The results demonstrate the potential for estimating spatial out-of-field neutron dose based on the SRAM detector measurements in combination with the neutron energy response model.

Power-law relationship in the long-tailed sections of proton dose distributions

The halo portion of a proton therapy dose creates a long tail in proton dose distributions, but so far study of this phenomenon has been limited. We used statistical methods and mathematical models to confirm that the long-tailed portion of proton dose distributions exhibits a power-law relationship. By analyzing 299 measured dose profiles, we found that all proton lateral dose distributions had a significant power-law scaling correlation with a high correlation coefficient in the tail. We set up a dual-mechanism model, containing both direct and indirect impact mechanisms. In the direct impact mechanism, the proximal dose deposition is mainly due to the direct impact of a proton on a particle. In the indirect mechanism, the impact of a proton on a given particle is considered in terms of the proton’s impact on a neighboring particle that then impacts the given particle. We found that the indirect impact mechanism led to a tail in the distribution exhibiting a power-law relationship because the probability of the indirect impacts was proportional to the distance; i.e., the longer the distance, the larger the indirect impact probability. Upon analyzing the experimental data, we observed that the power-law exponent increased with proton energy.

Improvements in pencil beam scanning proton therapy dose calculation accuracy in brain tumor cases with a commercial Monte Carlo algorithm

A commercial Monte Carlo (MC) algorithm (RayStation version 6.0.024) for the treatment of brain tumors with pencil beam scanning (PBS) proton therapy is validated and compared via measurements and analytical calculations in clinically realistic scenarios. For the measurements a 2D ion chamber array detector (MatriXX PT) was placed underneath the following targets: (1) an anthropomorphic head phantom (with two different thicknesses) and (2) a biological sample (i.e. half a lamb’s head). In addition, we compared the MC dose engine versus the RayStation pencil beam (PB) algorithm clinically implemented so far, in critical conditions such as superficial targets (i.e. in need of a range shifter (RS)), different air gaps, and gantry angles to simulate both orthogonal and tangential beam arrangements. For every plan the PB and MC dose calculations were compared to measurements using a gamma analysis metrics (3%, 3 mm). For the head phantom the gamma passing rate (GPR) was always >96% and on average >99% for the MC algorithm; the PB algorithm had a GPR of ⩽90% for all the delivery configurations with a single slab (apart 95% GPR from the gantry of 0° and small air gap) and in the case of two slabs of the head phantom the GPR was >95% only in the case of small air gaps for all three (0°, 45°, and 70°) simulated beam gantry angles. Overall the PB algorithm tends to overestimate the dose to the target (up to 25%) and underestimate the dose to the organ at risk (up to 30%). We found similar results (but a bit worse for the PB algorithm) for the two targets of the lamb’s head where only two beam gantry angles were simulated. Our results suggest that in PBS proton therapy a range shifter (RS) needs to be used with caution when planning a treatment with an analytical algorithm due to potentially great discrepancies between the planned dose and the dose delivered to the patient, including in the case of brain tumors where this issue could be underestimated. Our results also suggest that a MC evaluation of the dose has to be performed every time the RS is used and, mostly, when it is used with large air gaps and beam directions tangential to the patient surface.

Assessing Novel Drugs and Radiation Technology in the Chemoradiation of Oropharyngeal Cancer.

Integrating immunotherapy, proton therapy and biological dose escalation into the definitive chemoradiation of oropharyngeal cancer poses several challenges. Reliable and reproducible data must be obtained in a timely fashion. However, despite recent international radiotherapy contouring guidelines, controversy persists as to the applicability of such guidelines to all cases. Similarly, a lack of consensus exists concerning both the definition of the organ at risk for oral mucositis and the most appropriate endpoint to measure for this critical toxicity. Finally, the correlation between early markers of efficacy such as complete response on PET CT following treatment and subsequent survival needs elucidation for biological subsets of oropharyngeal cancer.

Knowledge of endoscopic ultrasound-delivered fiducial composition and dimension necessary when planning proton beam radiotherapy.

Background and study aims Little consideration has been given to selection of endoscopic ultrasound-guided fiducials for proton radiotherapy and the resulting perturbations in the therapy dose and pattern. Our aim was to assess the impact of perturbations caused by six fiducials of different composition and dimensions in a phantom gel model. Materials and methods The phantom was submerged in a water bath and irradiated with a uniform 10 cm × 10 cm field of 119.7 MeV monoenergetic spot scanning protons delivered through a 45 mm range shifter. The proton “Bragg Peak” was evaluated. Results Dose perturbations manifesting as dose reductions up to 30 % were observed. A carbon composite (1 × 5 mm) and gold (0.4 × 10 mm) fiducial with backload potential rather than dedicated EUS pre-loaded gold fiducial needles had the best performance in terms of minimizing the dose perturbation. Conclusions Our data demonstrate that a carbon composite fiducial has a less untoward effect on proton therapy dose distribution than dedicated EUS pre-loaded gold fiducial needles. Such information is important to consider when selecting fiducials specifically for proton therapy.

Exradin W1 plastic scintillation detector for in vivo skin dosimetry in passive scattering proton therapy

In vivo skin dosimetry is desirable in passive scattering proton therapy because of the possibility of high entrance dose with a small number of fields. However, suitable detectors are needed to determine skin dose in proton therapy. Plastic scintillation detectors (PSDs) are particularly well suited for applications in proton therapy because of their water equivalence, small size, and ease of use. We investigated the utility of the Exradin W1, a commercially available PSD, for in vivo skin dosimetry during passive scattering proton therapy. We evaluated the accuracy of the Exradin W1 in six patients undergoing proton therapy for prostate cancer, as part of an Institutional Review Board-approved protocol. Over 22 weeks, we compared in vivo PSD measurements with in-phantom ionization chamber measurements and doses from the treatment planning system, resulting in 96 in vivo measurements. Temperature and ionization quenching correction factors were applied on the basis of the dose response of the PSD in a phantom. The calibrated PSD exhibited an average 7.8% under-response (±1% standard deviation) owing to ionization quenching. We observed 4% under-response at 37 °C relative to the calibration-temperature response. After temperature and quenching corrections were applied, the overall PSD dose response was within ±1% of the expected dose for all patients. The dose differences between the PSD and ionization chamber measurements for all treatment fields were within ±2% (standard deviation 0.67%). The PSD was highly accurate for in vivo skin dosimetry in passively scattered proton beams and could be useful in verifying proton therapy delivery.

Tutorials in radiotherapy physics: Advanced topics with problems and solution. Author: PatrickN. McDermott, CRC Press, Taylor &amp; Francis Group, Boca Raton, FL, 2016. Paperback 302 pp. Price: $99.95. ISBN 9781482251678.

This book is intended to be a textbook with systematic mathematical descriptions of five advanced topics in medical physics. These topics include Physics in Medical Linacs, Proton Therapy, Convolution and Superposition Dose Computation Algorithms, Dose Calculations with Boltzmann Transport Equation, Tumor Control and Normal Tissue Complication Probability Models. The book establishes fundamental understandings and introduces current advancements in these topics. It contains a great number of high quality images and mathematical equations to enhance the reading experience. There are also problems and questions at the end of each chapter. It should be of interests to the medical physics community. The advanced topics covered in this book are not covered well in traditional medical physics textbooks. This book is written as a synthesis from a large variety of sources so the readers can learn the topics from the book and have the capability to explore further readings. The topics in the book are developed with consistent mathematical notations from fundamental physics knowledge and equations. The topics covered in this book are self-contained study modules which can be used as textbook for graduate medical physics program, physics residency program, or self-study materials for practicing medical physicists. They are advanced topics for medical physicists to understand the current trend and developments in the field. The contents are well-written and illustrated for readers to develop quantitative understanding of the material. Due to its extensive content of the mathematical formulation in each topic, the prerequisite of the book is a bachelor's degree in physics and a graduate-level course in radiological physics. The book covers five topics in medical physics, which are independent of each other. The first chapter discusses the physics of electron acceleration in medical linacs. Maxwell's equation and boundary conditions are applied to develop quantitative understanding of various designs of waveguides. The content focuses on the derivations relevant to the electron acceleration mechanisms exploited by the medical linacs. The second chapter, proton therapy, introduces the history and treatment machine developments of several vendors. The focus is on the physics of the proton accelerators and transport subsystems. The pros and cons of the double scattering and the pencil beam scanning modalities are illustrated. It however does not provide in-depth details on proton treatment planning techniques. Chapter three discusses the popular photon convolution and superposition dose calculation algorithms, which are adopted by most of the commercial treatment planning systems. The major steps of these model-based algorithms are introduced in this chapter, including the calculations of incident energy fluence, kernels, medium heterogeneities and additional electron contaminations. The dose calculation algorithm by Boltzmann transport equation is gaining popularity in the medical physics community. Chapter four illustrates the deterministic transport approach to dose calculations for both photons and electrons. A particular helpful topic on Tumor Control and Normal Tissue Complication Probability Models is covered in chapter five. Not only the basic, but more advanced discussions on serial or parallel organ NTCP model and heterogeneity tumor control model formulations are addressed. The best aspect of this book is that it presents each topic from the perspective of fundamental physics principles and adheres to rigorous standards of mathematical presentation. It avoids superficial and qualitative presentation of core principles. Instead, the details are derived using consistent mathematical notation to facilitate the reading and understanding. This feature is unique from other available textbooks. The topics are all self-contained and up-to-date with the current state of technology development. The entire book provides a concise, easy to follow, bottom up approach to the topics. It is well-written and pleasant to read. The complete and systematic mathematical derivations in the book require the readers to possess a level of physics and mathematics background, including vector calculus, differential equations, advanced undergraduate electricity and magnetism, and matrix algebra to completely follow the materials. Nonetheless, it is a great textbook for individuals to acquire understanding of these advanced topics on a fundamental level. Wei Zou is an assistant professor at the University of Pennsylvania, Perelman School of Medicine, Department of Radiation Oncology. Her research interests are advanced techniques in radiation therapy and proton radiotherapy.

Feasibility of MR-only proton dose calculations for prostate cancer radiotherapy using a commercial pseudo-CT generation method

A magnetic resonance (MR)-only radiotherapy workflow can reduce cost, radiation exposure and uncertainties introduced by CT-MRI registration. A crucial prerequisite is generating the so called pseudo-CT (pCT) images for accurate dose calculation and planning. Many pCT generation methods have been proposed in the scope of photon radiotherapy. This work aims at verifying for the first time whether a commercially available photon-oriented pCT generation method can be employed for accurate intensity-modulated proton therapy (IMPT) dose calculation.A retrospective study was conducted on ten prostate cancer patients. For pCT generation from MR images, a commercial solution for creating bulk-assigned pCTs, called MR for Attenuation Correction (MRCAT), was employed. The assigned pseudo-Hounsfield Unit (HU) values were adapted to yield an increased agreement to the reference CT in terms of proton range. Internal air cavities were copied from the CT to minimise inter-scan differences. CT- and MRCAT-based dose calculations for opposing beam IMPT plans were compared by gamma analysis and evaluation of clinically relevant target and organ at risk dose volume histogram (DVH) parameters. The proton range in beam’s eye view (BEV) was compared using single field uniform dose (SFUD) plans.On average, a mm) gamma pass rate of 98.4% was obtained using a dose threshold after adaptation of the pseudo-HU values. Mean differences between CT- and MRCAT-based dose in the DVH parameters were below 1 Gy (). The median proton range difference was mm, with on average 96% of all BEV dose profiles showing a range agreement better than 3 mm.Results suggest that accurate MR-based proton dose calculation using an automatic commercial bulk-assignment pCT generation method, originally designed for photon radiotherapy, is feasible following adaptation of the assigned pseudo-HU values.

Abstract ID: 85 Investigating the physics of a CBCT projection shading correction based on a prior CT

The adoption of cone beam computed tomography (CBCT) image guidance in proton therapy has spurred research on CBCT image correction for dose calculation. Initially, methods based on deformable image registration gained attention [1] , however projection correction approaches based on prior CT information [2] have been shown to perform well for several body sites [3] . The shading correction algorithm used in [2] , [3] relies on image processing of the subtraction of digitally reconstructed radiographs (DRR) of a DIR-registered prior CT, and CBCT projections. In this study we have performed Monte Carlo (MC) simulation of CBCT imaging to disentangle the different sources of projection degradation, and to evaluate the physicality of the shading correction. The GATE MC toolkit’s fixed-forced-detection actor was employed, along with source and detector models optimized for an Elekta XVI CBCT system, to simulate CBCT imaging of an electron density phantom. The shading correction algorithm was applied to the measured projections and the so-called scatter component (SCA) was compared to the MC simulated scatter. Measured and simulated CBCT projections (scatter + primary) agreed well, with the largest discrepancy found for a bone insert (3% of log transformed projections). Important disagreement was observed with the MC scatter signal when the contribution from beam hardening was kept in the SCA. Undoing the beam hardening correction from the SCA using functions derived from MC primary projections and DRRs greatly improved agreement of scatter signals from MC and SCA (3% on average), with the largest discrepancy found for the bone insert (12%). Residual discrepancies were shown to stem from the intrinsic limitations of the SCA. Proton therapy dose calculations on corrected CBCT will be presented in addition to the results above.