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Background: Proton linear energy transfer (LET) is associated with the relative biological effectiveness of radiation on tissues. Monte Carlo (MC) simulations have been known to be the preferred method to calculate LET. Detectors have also been built to measure LET, but they need to be calibrated with MC simulations. Purpose: To propose and test a MC‐free method for determining LET from the measured integral depth dose (LFI) of the protons of interest. Method and materials: LFI consists of three steps: (1) IDD measurements, (2) extraction of energy spectrum (ES) from the IDD, and (3) LET determination from the extracted ES and the stopping power of each energy. To validate the accuracy of the extraction of ES, we use Gaussian ES to synthesize IDD, extract ES from the synthesized IDD, and then compare the original (ground truth) and extracted ES. LETs calculated from the original and extracted ES are also compared. To obtain the LET of protons of interest, we measure IDDs by a large‐area plane‐parallel ionization chamber in water. Finally, TOPAS MC is employed to simulate IDDs, ES, and LETs. From the simulated IDD, the extracted ES and LET are compared with the simulations from TOPAS MC. Results: From the synthesized IDDs, the LETs agreed excellently when the peak energies ≥10 and 1.25 MeV with depth resolutions 0.1 and 0.01 mm, respectively. For energy <1.25 MeV, even higher depth resolution than 0.01 mm is required. From the MC simulated IDDs, our track‐averaged LET excellently agreed with MC simulation, but not the LETd. Our LETd was smaller than MC simulated LETd in the shallow region but larger in the distal Bragg peak region. Conclusion: LET can be accurately determined from the IDD. This method can be used in the clinic to commission or validate LETs from other measurement methods or a treatment planning system. [ABSTRACT FROM AUTHOR]

Background: Pencil beam scanning (PBS) proton therapy for moving targets is known to be impacted by interplay effects between the scanning beam and organ motion. While respiratory motion in the thoracic region is the major cause for organ motion, interplay effects depend on the delivery characteristics of proton accelerators. Purpose: To evaluate the impact of different types of PBS proton accelerators and spot sizes on interplay effects, mitigations, and plan quality for Stereotactic Body Radiation Therapy (SBRT) treatment of non‐small cell lung cancer (NSCLC). Methods: Twenty NSCLC patients treated with photon SBRT were selected to represent varying tumor volumes and respiratory motion amplitudes (median: 0.6 cm with abdominal compression) for this retrospective study. For each patient, plans were created using: (1) cyclotron‐generated proton beams (CPB) with spot sizes of σ = 2.7–7.0 mm; (2) linear accelerator proton beams (LPB) (σ = 2.9–5.5 mm); and (3) linear accelerator proton minibeams (LPMB) (σ = 0.9–3.9 mm). The energy switching time is one second for CPB, and 0.005 s for LPMB and LPB. Plans were robustly optimized on the gross tumor volume (GTV) using each individual phase of four‐dimensional computed tomography (4DCT) scans. Initially, single‐field optimization (SFO) plans were evaluated; if the plan quality did not meet the dosimetric requirement, multi‐field optimization (MFO) was used. MFO plans were created for all patients for comparisons. For each patient, all plans were normalized to have the same dose received by 99% of the GTV. Interplay effects were evaluated by computing the dose on 10 breathing phases, based on the spot distribution. Volumetric repainting (VR) was performed 2–6 times for each plan. We compared volume receiving 100% of the prescribed dose (V100%RX) of the GTV, and normal lung V20Gy. Results: Twelve of 20 plans can be optimized sufficiently with SFO. SFO plans were less sensitive to the interplay effect compared to MFO plans in terms of target coverage for both LPB and LPMB. The following comparisons showed results utilizing the MFO technique. In the interplay evaluation without repainting, the mean V100%RX of the GTV were 99.42 ± 0.6%, 97.52 ± 3.9%, and 94.49 ± 7.3% for CPB, LPB, and LPMB plans, respectively. Following VR (2 × for CPB; 3 × for LPB; 5 × for LPMB), V100%RX of the GTV were improved (on average) by 0.13%, 1.84%, and 4.63%, respectively, achieving the acceptance criteria of V100%RX > 95%. Because of fast energy switch in linear accelerator proton machines, the delivery time for VR plans was the lowest for LPB plans, while delivery time for LPMB was on average 1 min longer than CPB plans. The advantage of small spot machines was better sparing in normal lung V20Gy, even when VR was applied. Conclusion: In the absence of repainting, proton machines with large spot sizes generated more robust plans against interplay effects. The number of VR increased with decreasing spot sizes to achieve the acceptance criteria. VR improved the plan robustness against interplay effects for modalities with small spot sizes and fast energy changes, preserving the low dose sparing aspect of the LPMB, even when motion is included. [ABSTRACT FROM AUTHOR]

Purpose: Proton pencil beam energy spectrum is an essential parameter for calculations of dose and linear energy transfer. We extract the energy spectrum from measured integral depth dose (IDD). Methods: A measured IDD (measIDD) in water is decomposed into many IDDs of mono‐energetic pencil beams (monoIDDs) in water. A simultaneous iterative technique is used to do the decomposition that extracts the energy spectrum of protons from the measIDD. The monoIDDs are generated by our analytic random walk model‐based proton dose calculation algorithm. The linear independence of the monoIDDs is considered and high spatial resolution monoIDDs are used to improve their linear independence. To validate the extraction, first we use synthesized IDDs (synIDD) with Gaussian energy spectrum and compare the extracted energy spectrum with the Gaussian; second, for the energy spectrum extracted from measIDDs, the accuracy of the extraction is validated by comparing the calculated IDD from the energy spectrum with the measIDD. The measIDDs are derived from commissioning of a cyclotron proton pencil beam system with a Bragg peak ionization chamber. The nominal energy of the pencil beams is from 70 to 245 MeV. The monoIDDs are generated for energies from 0.05 to 275 MeV in steps of 0.05 MeV with a spatial resolution of 1 mm. Results: The difference of the extracted and original Gaussian energy spectrum peaked at 75 and 80 MeV was <1%. As the energy decreased, the difference increased but was reduced by using 0.1‐mm monoIDDs. The difference was not sensitive to the energy interval of monoIDDs when the interval increased from 0.05 to 1 MeV. For the energy spectrum extraction from measIDDs, there was a main peak near the nominal energy but the spectrum was not in Gaussian distribution. In three example cases (70, 160, and 245 MeV), the relative differences of the measIDDs and calculated IDDs were within 3.4%, 2.9%, and 4.7% of the Bragg peak value, respectively. Fitting the spectrum by Gaussian distribution, we had σ = 0.87, 1.51, and 0.86 MeV, respectively, for these three examples, and the relative differences of the resultant calculated IDDs from the measIDDs were within 4.7%, 7.4%, and 4.5%, respectively. Conclusions: Our algorithm accurately extracted the energy spectrum from the synIDDs and measIDDs. High‐resolution monoIDDs are necessary to extract low‐energy spectrum. Energy spectrum extraction from measIDD reveals important information for beam modeling. [ABSTRACT FROM AUTHOR]

Proton therapy is an expanding radiotherapy modality in the United States and worldwide. With the number of proton therapy centers treating patients increasing, so does the need for consistent, high‐quality clinical commissioning practices. Clinical commissioning encompasses the entire proton therapy system's multiple components, including the treatment delivery system, the patient positioning system, and the image‐guided radiotherapy components. Also included in the commissioning process are the x‐ray computed tomography scanner calibration for proton stopping power, the radiotherapy treatment planning system, and corresponding portions of the treatment management system. This commissioning report focuses exclusively on intensity‐modulated scanning systems, presenting details of how to perform the commissioning of the proton therapy and ancillary systems, including the required proton beam measurements, treatment planning system dose modeling, and the equipment needed. [ABSTRACT FROM AUTHOR]

Purpose: The goal was to develop and test a large diameter parallel plate ionization chamber capable of intercepting at least 98% of the proton beamlets tested with the system. Methods: A commercial synchrotron proton therapy system was used for the study (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat‐V). The energies investigated were in the range of 100 to 192 MeV. Three beam spot options available from the system were used. A PTW Bragg peak IC of diameter 84 mm (BP84) (Model PTW34070) was employed for comparison in a scanning water phantom. A prototype of 150 mm diameter was produced (PTW, Freiburg, Germany; model: T34089) and used for the testing. Monte Carlo calculations were also performed with FLUKA to guide the BP150 design and for comparison to the radiological measurements. For comparison, a 40 cm diameter ideal virtual detector was included in the Monte Carlo model. Results: The measured proton range R90 agrees between the BP84 and BP150 ionization chambers within +0.06/−0.27 mm across the energies 100–192 MeV, which is less than the daily experimental setup uncertainty of 0.4 mm. The differences in the absolute integral depth dose curves (IDDs) between the BP84 and BP150 ranged from 0.3% to 1.0% for the spot sizes and beam energies tested. As predicted by the Monte Carlo modeling, the greatest differences were found in the plateau region of the IDDs. Also, the IDDs measured with the BP150 were very similar to those of the ideal 40 cm diameter detector Monte Carlo simulations. Conclusions: We conclude that the BP150 offers a small, but a useful reduction in uncertainty from the nuclear halo effect for the system under test. [ABSTRACT FROM AUTHOR]

Purpose: Task Group (TG) 224 was established by the American Association of Physicists in Medicine's Science Council under the Radiation Therapy Committee and Work Group on Particle Beams. The group was charged with developing comprehensive quality assurance (QA) guidelines and recommendations for the three commonly employed proton therapy techniques for beam delivery: scattering, uniform scanning, and pencil beam scanning. This report supplements established QA guidelines for therapy machine performance for other widely used modalities, such as photons and electrons (TG 142, TG 40, TG 24, TG 22, TG 179, and Medical Physics Practice Guideline 2a) and shares their aims of ensuring the safe, accurate, and consistent delivery of radiation therapy dose distributions to patients. Methods: To provide a basis from which machine‐specific QA procedures can be developed, the report first describes the different delivery techniques and highlights the salient components of the related machine hardware. Depending on the particular machine hardware, certain procedures may be more or less important, and each institution should investigate its own situation. Results: In lieu of such investigations, this report identifies common beam parameters that are typically checked, along with the typical frequencies of those checks (daily, weekly, monthly, or annually). The rationale for choosing these checks and their frequencies is briefly described. Short descriptions of suggested tools and procedures for completing some of the periodic QA checks are also presented. Conclusion: Recommended tolerance limits for each of the recommended QA checks are tabulated, and are based on the literature and on consensus data from the clinical proton experience of the task group members. We hope that this and other reports will serve as a reference for clinical physicists wishing either to establish a proton therapy QA program or to evaluate an existing one. [ABSTRACT FROM AUTHOR]

Particle therapy is rapidly expanding and claiming its position as the treatment modality of choice in teletherapy. However, the rate of expansion continues to be restricted by the size and cost of the associated particle therapy systems and their operation. Additional technical limitations such as dose delivery rate, treatment process efficiency, and achievement of superior dose conformity potentially hinder the complete fulfillment of the promise of particle therapy. These topics are explored in this review considering the current state of particle therapy systems and what improvements are required to overcome the current challenges. Beam production systems (accelerators), beam transport systems including gantries and beam delivery systems are addressed explicitly in these regards. [ABSTRACT FROM AUTHOR]

Purpose: Proton dose coverage is sensitive to proton beam range. The current practice of CT number–based registration for patient positioning focuses on matching the target and is not sufficient for proton therapy because the proton range depends on the medium traversed by the beam. Patient body deformations and anatomical changes result in range deviation in the target. We propose proton range–based registration to minimize the range deviation. Methods: The range was calculated from cone beam–computed tomography (CBCT) of the patient on couch, and the range deviation was the difference of the calculated range from that on the initial (day 1) CBCT. In the investigated prostate cases in which the main cause of range deviation was the rotation of femur bones, and in the investigated abdomen cases in which the main cause of range deviation was body growth and anatomic change, our range‐based registration was used to obtain the optimal beam angle by minimizing the range deviation. The new angle was limited to be ±5° from that planned to prevent potentially increased dose to the organs at risk. To demonstrate the benefit of range‐based registration, we investigated the range at the voxels on the surface of the target volume. The calculation error of range deviation due to CBCT scatter was investigated by using solid water phantoms with different thicknesses. Range‐based registration using both CBCTs and CTs was performed in cases of two patients with pelvic rhabdomyosarcoma and one patient with upper abdominal tumor. The range was represented by the water‐equivalent thickness to shorten the computation for online application purposes. Results: In the phantom study, the calculation error of range deviation due to CBCT scatter was within 2 mm for a 1‐cm thickness change (the mean range deviation was 0.8 mm). In the CT study of the prostate cases, the range deviation (mean ± root‐mean‐square deviation) on the contour in each slice was efficiently reduced from 3.6 ± 2.8 mm to 2.1 ± 1.4 mm, with most slices being within 3 mm; in the CT study of the abdomen cases, the range deviation of the whole set was reduced from 4.4 ± 1.9 mm to 3.5 ± 2.1 mm. Both the mean and root‐mean‐square deviation of the range deviation on each treatment day were decreased. The dose coverage on the target was improved and the dose on the OARs was only slightly changed. Conclusion: Range‐based registration can efficiently mitigate range deviation due to patient positioning and anatomical changes. It can shorten patient positioning time and reduce the patient's dose from CBCT. [ABSTRACT FROM AUTHOR]

Although most radiation therapy treatments are delivered using X-rays, few preclinical data generated using UHDR X-ray irradiation have been published. Next, the paper "Image Guidance for FLASH Radiotherapy" examines the needed developments, beyond state-of-the-art, in image-guidance techniques for better planning, delivery, and treatment response assessment of FLASH radiotherapy, including new opportunities enabled by the unique nature of UHDR pulsed beams. The rapid advancement of FLASH radiation research is changing our fundamental understanding of the in vivo interactions of high doses of radiation in both animals and humans. [Extracted from the article]

Purpose: In light-ion beam dose delivery with the scanning technique the spacing between adjacent spots is an important parameter during treatment planning. In order to study the effect of spot spacing on dose conformity and robustness for single field uniform dose configurations, fundamental geometrical properties of placement of Gaussian beamlets are explored. In particular, the dependence of penumbra width and flatness on spot width and spot spacing is investigated. Methods: Infinitesimal calculus and analytical methods are used to derive simple expressions for the lateral penumbra and the flatness of one-dimensional dose profiles in continuous scanning and uniform discrete spot scanning. In the same way expressions for the fundamental modes of perturbation of the spot sequence are developed. A numerical, matrix-based approach is followed to optimize weights spot-by-spot. Results: Generally the lateral penumbra widths lie between 1.13σb and 1.68σb with σb being the standard deviation of the beam spot profile. For regularly placed spots of equal weight with spot spacing λ the lateral penumbra is given by 1.68σ′ where σ′ results from quadratic subtraction of λ/12 from σb. The quantization error is identified as additional parameter describing the lateral dose conformity. It's variance is given by λ2/12 for a bunch of spots with uniform weights. The matrix-based optimization of weights for a one-dimensional dose box results in a lateral penumbra of typically 1.4σb. This value reduces to about 1.3σb if also the positions of the beam spots are optimized for the considered field size. The analytical formulas for uniform discrete scanning can be used as rough approximations of the best-case scenarios for weight-optimized dose profiles if the spot spacing is defined as effective spot spacing. Conclusions: The trade-off between flatness, quantization error, and robustness on the one side and penumbra width on the other side can be described analytically for equally weighted spots. Treatment planning systems often perform a least-squares optimization of the individual spot weights which results in smaller lateral penumbras and smaller quantization errors than for uniform discrete scanning. However, the benefit of this weight optimization decreases with increasing λ (in the regime λ>σb). The spot spacing, which is obtained from the scenario that the optimization objective is met by uniform discrete scanning, poses a sharp upper limit for the spot spacing λ in weight optimization methods. [ABSTRACT FROM AUTHOR]

In proton therapy delivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayer Faraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayer ionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to ±21% over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than ±3%. To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies. [ABSTRACT FROM AUTHOR]

Calculations of dose per monitor unit (D/MU) are required in addition to measurements to increase patient safety in the clinical practice of proton radiotherapy. As in conventional photon and electron therapy, the D/MU depends on several factors. This study focused on obtaining range and modulation dependence factors used in D/MU calculations for the double scattered proton beam line at the Midwest Proton Radiotherapy Institute. Three dependencies on range and one dependency on modulation were found. A carefully selected set of measurements was performed to discern these individual dependencies. Dependencies on range were due to: (1) the stopping power of the protons passing through the monitor chamber; (2) the reduction of proton fluence due to nuclear interactions within the patient; and (3) the variation of proton fluence passing through the monitor chamber due to different source-to-axis distances (SADs) for different beam ranges. Different SADs are produced by reconfigurations of beamline elements to provide different field sizes and ranges. The SAD effect on the D/MU varies smoothly as the beam range is varied, except at the beam range for which the first scatterers are exchanged and relocated to accommodate low and high beam ranges. A geometry factor was devised to model the SAD variation effect on the D/MU. The measured D/MU variation as a function of range can be predicted within 1% using the three modeled dependencies on range. Investigation of modulated beams showed that an analytical formula can predict the D/MU dependency as a function of modulation to within 1.5%. Special attention must be applied when measuring the D/MU dependence on modulation to avoid interplay between range and SAD effects. [ABSTRACT FROM AUTHOR]