Cyclotron and linear accelerator generated scanning proton beams for lung cancer SBRT: Interplay effects and mitigations.

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.

Technical note: Determination of proton linear energy transfer from the integral depth dose.

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.

Heat stress does not induce wasting symptoms in the giant California sea cucumber (Apostichopus californicus).

Oceanic heatwaves have significant impacts on disease dynamics in marine ecosystems. Following an extreme heatwave in Nanoose Bay, British Columbia, Canada, a severe sea cucumber wasting event occurred that resulted in the mass mortality of Apostichopus californicus. Here, we sought to determine if heat stress in isolation could trigger wasting symptoms in A. californicus. We exposed sea cucumbers to (i) a simulated marine heatwave (22 °C), (ii) an elevated temperature treatment (17 °C), or (iii) control conditions (12 °C). We measured the presence of skin lesions, mortality, posture maintenance, antipredator defences, spawning, and organ evisceration during the 79-hour thermal exposure, as well as 7-days post-exposure. Both the 22 °C and 17 °C treatments elicited stress responses where individuals exhibited a reduced ability to maintain posture and an increase in stress spawning. The 22 °C heatwave was particularly stressful, as it was the only treatment where mortality was observed. However, none of the treatments induced wasting symptoms as observed in the Nanoose Bay event. This study provides evidence that sea cucumber wasting may not be triggered by heat stress in isolation, leaving the cause of the mass mortality event observed in Nanoose unknown.

Ultra-high dose rate radiation production and delivery systems intended for FLASH.

Higher dose rates, a trend for radiotherapy machines, can be beneficial in shortening treatment times for radiosurgery and mitigating the effects of motion. Recently, even higher doses (e.g., 100 times greater) have become targeted because of their potential to generate the FLASH effect (FE). We refer to these physical dose rates as ultra-high (UHDR). The complete relationship between UHDR and the FE is unknown. But UHDR systems are needed to explore the relationship further and to deliver clinical UHDR treatments, where indicated. Despite the challenging set of unknowns, the authors seek to make reasonable assumptions to probe how existing and developing technology can address the UHDR conditions needed to provide beam generation capable of producing the FE in preclinical and clinical applications. As a preface, this paper discusses the known and unknown relationships between UHDR and the FE. Based on these, different accelerator and ionizing radiation types are then discussed regarding the relevant UHDR needs. The details of UHDR beam production are discussed for existing and potential future systems such as linacs, cyclotrons, synchrotrons, synchrocyclotrons, and laser accelerators. In addition, various UHDR delivery mechanisms are discussed, along with required developments in beam diagnostics and dose control systems.

Technical note: Extraction of proton pencil beam energy spectrum from measured integral depth dose in a cyclotron proton beam system.

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.

No effect of passive integrated transponder tagging method on survival or body condition in a northern population of Black-capped Chickadees (Poecile atricapillus).

Passive integrated transponder (PIT) tags allow a range of individual-level data to be collected passively and have become a commonly used technology in many avian studies. Although the potential adverse effects of PIT tags have been evaluated in several species, explicit investigations of their impacts on small (<12 g) birds are limited. This is important, because it is reasonable to expect that smaller birds could be impacted more strongly by application of PIT tags. In this study, we individually marked Black-capped Chickadees (Poecile atricapillus), a small (circa 10 g) passerine, at the University of Alberta Botanic Garden to evaluate potential lethal and sublethal effects of two PIT tagging methods: attachment to leg bands or subcutaneous implantation. We used a Cox proportional hazards model to compare the apparent survival of chickadees with leg band (N = 79) and implanted PIT tags (N = 77) compared with control birds that received no PIT tags (N = 76) over the subsequent 2 years based on mist net recaptures. We used radio-frequency identification (RFID) redetections of leg band PIT tags to evaluate sex-specific survival and increase the accuracy of our survival estimates. We also used a generalized linear regression model to compare the body condition of birds recaptured after overwintering with leg band PIT tags, implanted PIT tags, or neither. Our analysis found no evidence for adverse effects of either PIT tagging method on survival or body condition. While we recommend carefully monitoring study animals and evaluating the efficacy of different PIT tagging methods, we have shown that both leg band and subcutaneously implanted PIT tags ethical means of obtaining individualized information in a small passerine.

Clinical commissioning of intensity-modulated proton therapy systems: Report of AAPM Task Group 185.

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.

Impact of bowel and rectum air on target dose with robustly optimized intensity-modulated proton therapy plans.

PURPOSE: Pelvic target dose from intensity-modulated proton therapy (IMPT) is sensitive to patient bowel motion. Robustly optimized plans in regard to bowel filling may improve the dose coverage in the treatment course. Our purpose is to investigate the effect of air volume in large and small bowel and rectum on target dose from IMPT plans. METHODS AND MATERIAL: Data from 17 cancer patients (11 prostate, 3 gynecologic, 2 colon, and 1 embryonal rhabdomyosarcoma) with planning CT (pCT) and weekly or biweekly scanned quality assurance CTs (QACTs; 82 QACT scans total) were studied. Air in bowels and rectum traversed by proton pencil beams was contoured. The robust treatment plan was made by using 3 CT sets: the pCT set and 2 virtual CT sets that were copies of pCT but in which the fillings of bowels and rectum were overridden to be either air or muscle. Each plan had 2-5 beams with a mean of 3 beams. Targets in the pCT were mapped to the QACTs by deformable image registration, and the dose in QACTs was calculated. Dose coverage (D99 and D95) and correlations between dose coverage and changes in air volume were analyzed. The significance of the correlation was analyzed by t test. RESULTS: Mean changes of D99 in QACTs were within 3% of those in the pCT for all prostate and colon cases but >3% in 2 of the 3 gynecologic cases and in the embryonal rhabdomyosarcoma case. Of these three cases with mean change of D99 > 3%, air volume may be the main cause in 2. For the prostate cases, correlation coefficients were <0.7 between change in air volume and change in D99 and D95, because other anatomy changes also contributed to dose deviation. Correlation coefficients in the non-prostate cases were >0.9 between D99 change and rectum and between D95 change and small bowel, indicating a greater effect of the air volume on target dose. CONCLUSION: The air volume may still have an important effect on target dose coverage in treatment plans using 3 CT sets, particularly when the air is traversed by multiple beams.

Technical Note: Design and characterization of a large diameter parallel plate ionization chamber for accurate integral depth dose measurements with proton beams.

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.

Management of radiotherapy patients with implanted cardiac pacemakers and defibrillators: A Report of the AAPM TG-203†.

Managing radiotherapy patients with implanted cardiac devices (implantable cardiac pacemakers and implantable cardioverter-defibrillators) has been a great practical and procedural challenge in radiation oncology practice. Since the publication of the AAPM TG-34 in 1994, large bodies of literature and case reports have been published about different kinds of radiation effects on modern technology implantable cardiac devices and patient management before, during, and after radiotherapy. This task group report provides the framework that analyzes the potential failure modes of these devices and lays out the methodology for patient management in a comprehensive and concise way, in every step of the entire radiotherapy process.

AAPM task group 224: Comprehensive proton therapy machine quality assurance.

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.

New horizons in particle therapy systems.

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.

Feasibility study of range-based registration using daily cone beam CT for intensity-modulated proton therapy.

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.

Premature Ovarian Insufficiency in Childhood Cancer Survivors: A Report From the St. Jude Lifetime Cohort.

Context: Long-term follow-up data on premature ovarian insufficiency (POI) in childhood cancer survivors are limited. Objective: To describe the prevalence of POI, its risk factors, and associated long-term adverse health outcomes. Design: Cross-sectional. Setting: The St. Jude Lifetime Cohort Study, an established cohort in a tertiary care center. Patients: Nine hundred twenty-one participants (median age, 31.7 years) were evaluated at a median of 24.0 years after cancer diagnosis. Main Outcome Measure: POI was defined by persistent amenorrhea combined with a follicle-stimulating hormone level >30 IU/L before age 40. Multivariable Cox regression was used to study associations between demographic or treatment-related risk factors and POI. Multivariable logistic regression was used to study associations between POI and markers for cardiovascular disease, bone mineral density (BMD), and frailty. Exposure to alkylating agents was quantified using the validated cyclophosphamide equivalent dose (CED). Results: The prevalence of POI was 10.9%. Independent risk factors for POI included ovarian radiotherapy at any dose and CED ≥8000 mg/m2. Patients with a body mass index ≥30 kg/m2 at the time of the St. Jude Lifetime Cohort assessment were less likely to have a diagnosis of POI. Low BMD and frailty were independently associated with POI. Conclusion: High-dose alkylating agents and ovarian radiotherapy at any dose are associated with POI. Patients at the highest risk should be offered fertility preservation whenever feasible. POI contributes to poor general health outcomes in childhood cancer survivors; further studies are needed to investigate the role of sex hormone replacement in improving such outcomes.

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.

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 multiscale filter for noise reduction of low-dose cone beam projections.

The Poisson or compound Poisson process governs the randomness of photon fluence in cone beam computed tomography (CBCT) imaging systems. The probability density function depends on the mean (noiseless) of the fluence at a certain detector. This dependence indicates the natural requirement of multiscale filters to smooth noise while preserving structures of the imaged object on the low-dose cone beam projection. In this work, we used a Gaussian filter, exp(-x2/2σ(2)(f)) as the multiscale filter to de-noise the low-dose cone beam projections. We analytically obtained the expression of σ(f), which represents the scale of the filter, by minimizing local noise-to-signal ratio. We analytically derived the variance of residual noise from the Poisson or compound Poisson processes after Gaussian filtering. From the derived analytical form of the variance of residual noise, optimal σ(2)(f)) is proved to be proportional to the noiseless fluence and modulated by local structure strength expressed as the linear fitting error of the structure. A strategy was used to obtain the reliable linear fitting error: smoothing the projection along the longitudinal direction to calculate the linear fitting error along the lateral direction and vice versa. The performance of our multiscale filter was examined on low-dose cone beam projections of a Catphan phantom and a head-and-neck patient. After performing the filter on the Catphan phantom projections scanned with pulse time 4 ms, the number of visible line pairs was similar to that scanned with 16 ms, and the contrast-to-noise ratio of the inserts was higher than that scanned with 16 ms about 64% in average. For the simulated head-and-neck patient projections with pulse time 4 ms, the visibility of soft tissue structures in the patient was comparable to that scanned with 20 ms. The image processing took less than 0.5 s per projection with 1024 × 768 pixels.

Determining the optimal dosimetric leaf gap setting for rounded leaf-end multileaf collimator systems by simple test fields.

Individual QA for IMRT/VMAT plans is required by protocols. Sometimes plans cannot pass the institute's QA criteria. For the Eclipse treatment planning system (TPS) with rounded leaf-end multileaf collimator (MLC), one practical way to improve the agreement of planned and delivered doses is to tune the value of dosimetric leaf gap (DLG) in the TPS from the measured DLG. We propose that this step may be necessary due to the complexity of the MLC system, including dosimetry of small fields and the tongue-and-groove (T&G) effects, and report our use of test fields to obtain linac-specific optimal DLGs in TPSs. More than 20 original patient plans were reoptimized with the linac-specific optimal DLG value. We examined the distribution of gaps and T&G extensions in typical patient plans and the effect of using the optimal DLG on the distribution. The QA pass rate of patient plans using the optimal DLG was investigated. The dose-volume histograms (DVHs) of targets and organs at risk were checked. We tested three MLC systems (Varian millennium 120 MLC, high-definition 120 MLC, and Siemens 160 MLC) installed in four Varian linear accelerators (linacs) (TrueBEAM STx, Trilogy, Clinac 2300 iX, and Clinac 21 EX) and 1 Siemens linac (Artiste). With an optimal DLG, the individual QA for all those patient plans passed the institute's criteria (95% in DTA test or gamma test with 3%/3 mm/10%), even though most of these plans had failed to pass QA when using original DLGs optimized from typical patient plans or from the optimization process (automodeler) of Pinnacle TPS. Using either our optimal DLG or one optimized from typical patient plans or from the Pinnacle optimization process yielded similar DVHs.