Technical note: A simple method for patient-specific quality assurance for lateral targets on a 1.5 T MR-Linac.

The Elekta Unity magnetic resonance (MR) linac is limited to longitudinal couch motion and a sagittal-only laser, which restricts the ability to perform patient-specific quality assurance (PSQA) intensity-modulated radiotherapy (IMRT) measurements for very lateral targets. This work introduces a simple method to perform PSQA using the Sun Nuclear ArcCheck-MR phantom at left and right lateral positions without additional equipment or in-house construction. The proposed setup places the center of the phantom 1.3 cm vertical and 12.9 cm lateral to isocenter in either the left or right direction. Computed tomography (CT) scans are used to simulate the setup and create a QA plan template in the Monaco treatment planning system (TPS). The workflow is demonstrated for four patients, with an average axial distance from the center of the bore to the planning target volume (PTV) of 12.4 cm. Gamma pass rates were above 94% for all plans using global 3%/2 mm gamma criterion with a 10% threshold. Setup uncertainties are slightly larger for the proposed lateral setup compared to the centered setup on the Elekta platform (∼1 mm compared to ∼0.5 mm), but acceptable pass rates are achievable without optimizing shifts in the gamma analysis software. In general, adding the left and right lateral positions increases the axial area in the bore encompassed by the cylindrical measurement array by 147%, substantially increasing the flexibility of measurements for offset targets. Based on this work, we propose using the lateral QA setup if the closest distance to the PTV edge from isocenter is larger than the array radius (10.5 cm) or the percent of the PTV encompassed by the diode array would be increased with the lateral setup compared to the centered setup.

Reduced blood flow by laser speckle flowgraphy after 125I-plaque brachytherapy for uveal melanoma.

BACKGROUND: To determine whether reductions in retinal and choroidal blood flow measured by laser speckle flowgraphy are detected after 125I-plaque brachytherapy for uveal melanoma. METHODS: In a cross-sectional study, retinal and choroidal blood flow were measured using laser speckle flowgraphy in 25 patients after treatment with 125I-plaque brachytherapy for uveal melanoma. Flow was analyzed in the peripapillary region by mean blur rate as well as in the entire image area with a novel superpixel-based method. Relationships between measures were determined by Spearman correlation. RESULTS: Significant decreases in laser speckle blood flow were observed in both the retinal and choroidal vascular beds of irradiated, but not fellow, eyes. Overall, 24 of 25 patients had decreased blood flow compared to their fellow eye, including 5 of the 6 patients imaged within the first 6 months following brachytherapy. A significant negative correlation between blood flow and time from therapy was present. CONCLUSIONS: Decreases in retinal and choroidal blood flow by laser speckle flowgraphy were detected within the first 6 months following brachytherapy. Reduced retinal and choroidal blood flow may be an early indicator of microangiographic response to radiation therapy.

Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience.

MR image-guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR-linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR-to-MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator-triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end-to-end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350-mm-diameter spherical volume, the peak-to-peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator-triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS-calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 ± 1.1% (3%/ 2 mm) and end-to-end testing of adapted plans showed agreement within 0.4% between ion-chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end-to-end test using an IROC MRgRT phantom.

Stereotactic radiotherapy of appropriately selected meningiomas and metastatic brain tumor beds with gamma knife icon versus volumetric modulated arc therapy.

PURPOSE: To determine if the gamma knife icon (GKI) can provide superior stereotactic radiotherapy (SRT) dose distributions for appropriately selected meningioma and post-resection brain tumor bed treatments to volumetric modulated arc therapy (VMAT). MATERIALS AND METHODS: Appropriately selected targets were not proximal to great vessels, did not have sensitive soft tissue including organs-at-risk (OARs) within the planning target volume (PTV), and did not have concave tumors containing excessive normal brain tissue. Four of fourteen candidate meningioma patients and six of six candidate patients with brain tumor cavities were considered for this treatment planning comparison study. PTVs were generated for GKI and VMAT by adding 1 mm and 3 mm margins, respectively, to the GTVs. Identical PTV V100% -values were obtained for the GKI and VMAT plans for each patient. Meningioma and tumor bed prescription doses were 52.7-54.0 in 1.7-1.8 Gy fractions and 25 Gy in 5 Gy fractions, respectively. GKI dose rate was 3.735 Gy/min for 16 mm collimators. RESULTS: PTV radical dose homogeneity index was 3.03 ± 0.35 for GKI and 1.27 ± 0.19 for VMAT. Normal brain D1% , D5% , and D10% were lower for GKI than VMAT by 45.8 ± 10.9%, 38.9 ± 11.5%, and 35.4 ± 16.5% respectively. All OARs considered received lower maximum doses for GKI than VMAT. GKI and VMAT treatment times for meningioma plans were 12.1 ± 4.13 min and 6.2 ± 0.32 min, respectively, and, for tumor cavities, were 18.1 ± 5.1 min and 11.0 ± 0.56 min, respectively. CONCLUSIONS: Appropriately selected meningioma and brain tumor bed patients may benefit from GKI-based SRT due to the decreased normal brain and OAR doses relative to VMAT enabled by smaller margins. Care must be taken in meningioma patient selection for SRT with the GKI, even if they are clinically appropriate for VMAT.

The commissioning and validation of Monaco treatment planning system on an Elekta VersaHD linear accelerator.

Accurate beam modeling is essential to help ensure overall accuracy in the radiotherapy process. This study describes our experience with beam model validation of a Monaco treatment planning system on a Versa HD linear accelerator. Data were collected such that Monaco beam models could be generated using three algorithms: collapsed cone (CC) and photon Monte Carlo (MC) for photon beams, and electron Monte Carlo (eMC) for electron beams. Validations are performed on measured percent depth doses (PDDs) and profiles, for open-field point-doses in homogenous and heterogeneous media, and for obliquely incident electron beams. Gamma analysis is used to assess the agreement between calculation and measurement for intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) plans, including volumetric modulated arc therapy for stereotactic body radiation therapy (VMAT SBRT). For all relevant conditions, gamma index values below 1 are obtained when comparing Monaco calculated PDDs and profiles with measured data. Point-doses in a water medium are found to be within 2% agreement of commissioning data in 99.5% and 98.6% of the points computed by MC and CC, respectively. All point-dose calculations for the eMC algorithm in water are within 4% agreement of measurement, and 92% of measurements are within 3%. In heterogeneous media of air and cortical bone, both CC and MC yielded better than 3% agreement with ion chamber measurements. eMC yielded 3% agreement to measurement downstream of air with oblique beams of up to 27°, 5% agreement distal to bone, and within 4% agreement at extended source to surface distance (SSD) for all electron energies except 6 MeV. The 6-MeV point of measurement is on a steep dose gradient which may impact the magnitude of discrepancy measured. The average gamma passing rate for IMRT/VMAT plans is 96.9% (±2.1%) and 98.0% (±1.9%) for VMAT SBRT when evaluated using 3%/2 mm criteria. Monaco beam models for the Versa HD linac were successfully commissioned for clinical use.

Commissioning and performance evaluation of RadCalc for the Elekta unity MRI-linac.

Recent availability of MRI-guided linear accelerators has introduced a number of clinical challenges, particularly in the context of online plan adaptation. Paramount among these is verification of plan quality prior to patient treatment. Currently, there are no commercial products available for monitor unit verification that fully support the newly FDA cleared Elekta Unity 1.5 T MRI-linac. In this work, we investigate the accuracy and precision of RadCalc for this purpose, which is a software package that uses a Clarkson integration algorithm for point dose calculation. To this end, 18 IMRT patient plans (186 individual beams) were created and used for RadCalc point dose calculations. In comparison with the primary treatment planning system (Monaco), mean point dose deviations of 0.0 ± 1.0% (n = 18) and 1.7 ± 12.4% (n = 186) were obtained on a per-plan and per-beam basis, respectively. The dose plane comparison functionality within RadCalc was found to be highly inaccurate, however, modest improvements could be made by artificially shifting jaws and multi leaf collimator positions to account for the dosimetric shift due to the magnetic field (67.3% vs 96.5% mean 5%/5 mm gamma pass rate).

Implementation of respiratory-gated VMAT on a Versa HD linear accelerator.

The accurate delivery of respiratory-gated volumetric modulated arc therapy (VMAT) treatment plans presents a challenge since the gantry rotation and collimator leaves must be repeatedly stopped and set into motion during each breathing cycle. In this study, we present the commissioning process for an Anzai gating system (AZ-733VI) on an Elekta Versa HD linear accelerator and make recommendations for successful clinical implementation. The commissioning tests include central axis dose consistency, profile consistency, gating beam-on/off delay, and comparison of gated versus nongated gamma pass rates for patient-specific quality assurance using four clinically commissioned photon energies: 6 MV, 6 FFF, 10 MV, and 10 FFF. The central axis dose constancy between gated and nongated deliveries was within 0.6% for all energies and the analysis of open field profiles for gated and nongated deliveries showed an agreement of 97.8% or greater when evaluated with a percent difference criteria of 1%. The measurement of the beam-on/off delay was done by evaluating images of a moving ball-bearing phantom triggered by the gating system and average beam-on delays of 0.22-0.29 s were observed. No measurable beam-off delay was present. Measurements of gated VMAT dose distributions resulted in decrements as high as 9% in the gamma passing rate as compared to nongated deliveries when evaluated against the planned dose distribution at 3%/3 mm. By decreasing the dose rate, which decreases the gantry speed during gated delivery, the gamma passing rates of gated and nongated treatments can be made equivalent. We present an empirically derived formula to limit the maximum dose rate during VMAT deliveries and show that by implementing a reduced dose rate, a gamma passing rate of greater than 95% (3%/3 mm) was obtained for all plan measurements.

Clinical experience with adaptive MRI-guided pancreatic SBRT and the use of abdominal compression to reduce treatment volume.

Introduction: This work presents a method to treat stereotactic body radiation therapy (SBRT) for pancreatic cancer on a magnetic resonance-guided linear accelerator (MR-linac) using daily adaptation, real-time motion monitoring, and abdominal compression. Methods: The motion management and treatment planning process involves a magnetic resonance imaging (MRI) simulation with cine and 3D images, a computed tomography (CT) simulation with a breath-hold CT and a 4DCT, pre-treatment verification and planning MRI, and intrafraction MRI cine images. Results: The results from 26 patients were included in this work. Our motion management process results in consistent motion analysis on the CT simulation, MRI simulation, and each treatment fraction. The liver dome was found to be an overestimate of tumor superior/inferior (SI) motion for most patients. Adding compression reduced SI liver dome motion by 6.2 mm on average. Clinical outcomes are similar to those observed in the literature. Conclusions: In this work, we demonstrate how pancreatic SBRT can be successfully treated on an MR-linac using abdominal compression. This allows for an increased duty cycle compared to gating and/or breath-hold techniques.

A Technique to Enable Efficient Adaptive Radiation Therapy: Automated Contouring of Prostate and Adjacent Organs.

Purpose: The purpose of this work was to investigate the use of a segmentation approach that could potentially improve the speed and reproducibility of contouring during magnetic resonance-guided adaptive radiation therapy. Methods and Materials: The segmentation algorithm was based on a hybrid deep neural network and graph optimization approach that also allows rapid user intervention (Deep layered optimal graph image segmentation of multiple objects and surfaces [LOGISMOS] + just enough interaction [JEI]). A total of 115 magnetic resonance-data sets were used for training and quantitative assessment. Expert segmentations were used as the independent standard for the prostate, seminal vesicles, bladder, rectum, and femoral heads for all 115 data sets. In addition, 3 independent radiation oncologists contoured the prostate, seminal vesicles, and rectum for a subset of patients such that the interobserver variability could be quantified. Consensus contours were then generated from these independent contours using a simultaneous truth and performance level estimation approach, and the deviation of Deep LOGISMOS + JEI contours to the consensus contours was evaluated and compared with the interobserver variability. Results: The absolute accuracy of Deep LOGISMOS + JEI generated contours was evaluated using median absolute surface-to-surface distance which ranged from a minimum of 0.20 mm for the bladder to a maximum of 0.93 mm for the prostate compared with the independent standard across all data sets. The median relative surface-to-surface distance was less than 0.17 mm for all organs, indicating that the Deep LOGISMOS + JEI algorithm did not exhibit a systematic under- or oversegmentation. Interobserver variability testing yielded a mean absolute surface-to-surface distance of 0.93, 1.04, and 0.81 mm for the prostate, seminal vesicles, and rectum, respectively. In comparison, the deviation of Deep LOGISMOS + JEI from consensus simultaneous truth and performance level estimation contours was 0.57, 0.64, and 0.55 mm for the same organs. On average, the Deep LOGISMOS algorithm took less than 26 seconds for contour segmentation. Conclusions: Deep LOGISMOS + JEI segmentation efficiently generated clinically acceptable prostate and normal tissue contours, potentially limiting the need for time intensive manual contouring with each fraction.

Radiation effects on retinal layers revealed by OCT, OCT-A, and perimetry as a function of dose and time from treatment.

Optical coherence tomography (OCT) has become a key method for diagnosing and staging radiation retinopathy, based mainly on the presence of fluid in the central macula. A robust retinal layer segmentation method is required for identification of the specific layers involved in radiation-induced pathology in individual eyes over time, in order to determine damage driven by radiation injury to the microvessels and to the inner retinal neurons. Here, we utilized OCT, OCT-angiography, visual field testing, and patient-specific dosimetry models to analyze abnormal retinal layer thickening and thinning relative to microvessel density, visual function, radiation dose, and time from radiotherapy in a cross-sectional cohort of uveal melanoma patients treated with 125I-plaque brachytherapy. Within the first 24 months of radiotherapy, we show differential thickening and thinning of the two inner retinal layers, suggestive of microvessel leakage and neurodegeneration, mostly favoring thickening. Four out of 13 eyes showed decreased inner retinal capillary density associated with a corresponding normal inner retinal thickness, indicating early microvascular pathology. Two eyes showed the opposite: significant inner retinal layer thinning and normal capillary density, indicating early neuronal damage preceding a decrease in capillary density. At later time points, inner retinal thinning becomes the dominant pathology and correlates significantly with decreased vascularity, vision loss, and dose to the optic nerve. Stable multiple retinal layer segmentation provided by 3D graph-based methods aids in assessing the microvascular and neuronal response to radiation, information needed to target therapeutics for radiation retinopathy and vision loss.

Patient-specific quality assurance of dynamically-collimated proton therapy treatment plans.

BACKGROUND: The dynamic collimation system (DCS) provides energy layer-specific collimation for pencil beam scanning (PBS) proton therapy using two pairs of orthogonal nickel trimmer blades. While excellent measurement-to-calculation agreement has been demonstrated for simple cube-shaped DCS-trimmed dose distributions, no comparison of measurement and dose calculation has been made for patient-specific treatment plans. PURPOSE: To validate a patient-specific quality assurance (PSQA) process for DCS-trimmed PBS treatment plans and evaluate the agreement between measured and calculated dose distributions. METHODS: Three intracranial patient cases were considered. Standard uncollimated PBS and DCS-collimated treatment plans were generated for each patient using the Astroid treatment planning system (TPS). Plans were recalculated in a water phantom and delivered at the Miami Cancer Institute (MCI) using an Ion Beam Applications (IBA) dedicated nozzle system and prototype DCS. Planar dose measurements were acquired at two depths within low-gradient regions of the target volume using an IBA MatriXX ion chamber array. RESULTS: Measured and calculated dose distributions were compared using 2D gamma analysis with 3%/3 mm criteria and low dose threshold of 10% of the maximum dose. Median gamma pass rates across all plans and measurement depths were 99.0% (PBS) and 98.3% (DCS), with a minimum gamma pass rate of 88.5% (PBS) and 91.2% (DCS). CONCLUSIONS: The PSQA process has been validated and experimentally verified for DCS-collimated PBS. Dosimetric agreement between the measured and calculated doses was demonstrated to be similar for DCS-collimated PBS to that achievable with noncollimated PBS.

Advances in MRI-Guided Radiation Therapy.

Image guidance for radiation therapy (RT) has evolved over the last few decades and now is routinely performed using cone-beam computerized tomography (CBCT). Conventional linear accelerators (LINACs) that use CBCT have limited soft tissue contrast, are not able to image the patient's internal anatomy during treatment delivery, and most are not capable of online adaptive replanning. RT delivery systems that use MRI have become available within the last several years and address many of the imaging limitations of conventional LINACs. Herein, the authors review the technical characteristics and advantages of MRI-guided RT as well as emerging clinical outcomes.

Edge Creep: Increased Pigmentation at the Border of Choroidal Melanomas Treated with Plaque Brachytherapy.

Introduction: There is an increase in pigmentation that occurs in many tumors following plaque brachytherapy for choroidal melanoma. Correctly distinguishing between increased pigment at the tumor border versus true growth is imperative. We performed a retrospective review of patients treated with I-125 brachytherapy for choroidal melanoma at our institution to study this phenomenon. Methods: Records were reviewed for all patients undergoing plaque brachytherapy for uveal melanoma for a 5-year period (N = 195). Patients with iris and anterior tumors were excluded. Tumors treated more than 31 days after presentation were excluded. Fundus images for patients with increased pigmentation at any of the borders of the tumor at 6-month follow-up that extended beyond the initial pigmented margin were included (N = 20; 8 F, 12 M). Imaging at the last follow-up was reviewed, and it was confirmed that all tumors involuted appropriately with no evidence of local recurrence. The date of initial exam, time to treatment, and follow-up interval were recorded for each included patient. Results: Twenty patients (10%) exhibited increased pigment deposition at any of the borders of the tumor at 6-month follow-up that extended beyond the initial pigmented margin. Average tumor thickness was 3.2 mm (1.3-5.1); average largest tumor basal diameter was 11.6 mm (7-15.5). Average time from diagnosis to treatment was 25 days (17-31). Average length of follow-up was 35 months (16-68). No patient developed recurrence during the duration of follow-up, and 1 patient had developed metastasis. Conclusion: We describe the phenomenon of increased pigment deposition, "edge creep," at the borders of choroidal melanomas treated with plaque brachytherapy that gave the appearance of initial tumor growth but then subsequently remained stable over time. It is important that treating ocular oncologists be aware of this phenomenon to avoid unnecessary diagnosis of local recurrence.

Postoperative Echography for Optimization of Radiation Dosimetry in Patients with Uveal Melanoma Treated with Plaque Brachytherapy.

PURPOSE: (1) To describe the technique of postoperative echography to confirm the intended treatment dose to the tumor apex in patients with uveal melanoma treated with plaque brachytherapy. (2) To describe the local tumor control rate and visual outcomes with the brachytherapy strategies used at our institution. DESIGN: Retrospective review. SUBJECTS: Three hundred seventy-two consecutive patients with uveal melanoma (small, medium, and large) treated with plaque brachytherapy at the University of Iowa from August 2008 to February 2019. METHODS: Patient demographics and tumor characteristics were recorded for each patient. Patients with posterior tumors treated with plaque brachytherapy (n = 355) underwent intraoperative ultrasound to confirm plaque placement, and additional postoperative ultrasound on day 1 to 3 postplaque insertion. In cases where intratumor/episcleral plaque edema or hemorrhage shifted the dose to the prescription point to < 85 Gray (Gy), the duration of plaque brachytherapy was increased to compensate. Statistical analysis was performed to compare variables associated with the need for plaque adjustment. MAIN OUTCOMES MEASURES: Variables associated with plaque dose needing to be recalculated, local tumor control, and visual acuity outcomes. RESULTS: In 31 (8.3%) cases, postoperative echography showed that the tumor apex had shifted outside the 85 Gy isodose curve, requiring adjustment of the duration of brachytherapy (28 cases) or repositioning of the plaque (3 cases). Collaborative Ocular Melanoma Study tumor size was significantly associated with need to adjust the plaque prescription dose (P = 0.03), with large tumors having the highest rate of adjustment. Tumor thickness was larger in cases requiring plaque adjustment compared with those that were not adjusted (median 4.9 mm vs. 3.0 mm, P < 0.01). Local tumor control was 99% (95% confidence interval, 97%-100%) at 5 years and 99% (95% confidence interval, 97%-100%) at 10 years. The percentage of patients who had experienced a visual acuity decline of ≥ 3 lines of vision or had < 20/200 acuity was 14.9% at 1 year after brachytherapy, 35.3% at 3 years, and 51.6% at 5 years. CONCLUSIONS: Postoperative ultrasound performed on postoperative day 1 to 3 after plaque insertion for patients undergoing brachytherapy for uveal melanoma may result in improved local tumor control, particularly in the setting of thicker or larger tumors. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

PETRA: A pencil beam trimming algorithm for analytical proton therapy dose calculations with the dynamic collimation system.

BACKGROUND: The Dynamic Collimation System (DCS) has been shown to produce superior treatment plans to uncollimated pencil beam scanning (PBS) proton therapy using an in-house treatment planning system (TPS) designed for research. Clinical implementation of the DCS requires the development and benchmarking of a rigorous dose calculation algorithm that accounts for pencil beam trimming, performs monitor unit calculations to produce deliverable plans at all beam energies, and is ideally implemented with a commercially available TPS. PURPOSE: To present an analytical Pencil bEam TRimming Algorithm (PETRA) for the DCS, with and without its range shifter, implemented in the Astroid TPS (.decimal, Sanford, Florida, USA). MATERIALS: PETRA was derived by generalizing an existing pencil beam dose calculation model to account for the DCS-specific effects of lateral penumbra blurring due to the nickel trimmers in two different planes, integral depth dose variation due to the trimming process, and the presence and absence of the range shifter. Tuning parameters were introduced to enable agreement between PETRA and a measurement-validated Dynamic Collimation Monte Carlo (DCMC) model of the Miami Cancer Institute's IBA Proteus Plus system equipped with the DCS. Trimmer position, spot position, beam energy, and the presence or absence of a range shifter were all used as variables for the characterization of the model. The model was calibrated for pencil beam monitor unit calculations using procedures specified by International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398). RESULTS: The integral depth dose curves (IDDs) for energies between 70 MeV and 160 MeV among all simulated trimmer combinations, with and without the ranger shifter, agreed between PETRA and DCMC at the 1%/1 mm 1-D gamma criteria for 99.99% of points. For lateral dose profiles, the median 2-D gamma pass rate for all profiles at 1.5%/1.5 mm was 99.99% at the water phantom surface, plateau, and Bragg peak depths without the range shifter and at the surface and Bragg peak depths with the range shifter. The minimum 1.5%/1.5 mm gamma pass rates for the 2-D profiles at the water phantom surface without and with the range shifter were 98.02% and 97.91%, respectively, and, at the Bragg peak, the minimum pass rates were 97.80% and 97.5%, respectively. CONCLUSION: The PETRA model for DCS dose calculations was successfully defined and benchmarked for use in a commercially available TPS.

Dosimetric delivery validation of dynamically collimated pencil beam scanning proton therapy.

Objective. Pencil beam scanning (PBS) proton therapy target dose conformity can be improved with energy layer-specific collimation. One such collimator is the dynamic collimation system (DCS), which consists of four nickel trimmer blades that intercept the scanning beam as it approaches the lateral extent of the target. While the dosimetric benefits of the DCS have been demonstrated through computational treatment planning studies, there has yet to be experimental verification of these benefits for composite multi-energy layer fields. The objective of this work is to dosimetrically characterize and experimentally validate the delivery of dynamically collimated proton therapy with the DCS equipped to a clinical PBS system.Approach. Optimized single field, uniform dose treatment plans for 3 × 3 × 3 cm3target volumes were generated using Monte Carlo dose calculations with depths ranging from 5 to 15 cm, trimmer-to-surface distances ranging from 5 to 18.15 cm, with and without a 4 cm thick polyethylene range shifter. Treatment plans were then delivered to a water phantom using a prototype DCS and an IBA dedicated nozzle system and measured with a Zebra multilayer ionization chamber, a MatriXX PT ionization chamber array, and Gafchromic™ EBT3 film.Main results. For measurements made within the SOBPs, average 2D gamma pass rates exceeded 98.5% for the MatriXX PT and 96.5% for film at the 2%/2 mm criterion across all measured uncollimated and collimated plans, respectively. For verification of the penumbra width reduction with collimation, film agreed with Monte Carlo with differences within 0.3 mm on average compared to 0.9 mm for the MatriXX PT.Significance. We have experimentally verified the delivery of DCS-collimated fields using a clinical PBS system and commonly available dosimeters and have also identified potential weaknesses for dosimeters subject to steep dose gradients.

Integration and dosimetric validation of a dynamic collimation system for pencil beam scanning proton therapy.

Objective.To integrate a Dynamic Collimation System (DCS) into a pencil beam scanning (PBS) proton therapy system and validate its dosimetric impact.Approach.Uncollimated and collimated treatment fields were developed for clinically relevant targets using an in-house treatment plan optimizer and an experimentally validated Monte Carlo model of the DCS and IBA dedicated nozzle (DN) system. The dose reduction induced by the DCS was quantified by calculating the mean dose in 10- and 30-mm two-dimensional rinds surrounding the target. A select number of plans were then used to experimentally validate the mechanical integration of the DCS and beam scanning controller system through measurements with the MatriXX-PT ionization chamber array and EBT3 film. Absolute doses were verified at the central axis at various depths using the IBA MatriXX-PT and PPC05 ionization chamber.Main results.Simulations demonstrated a maximum mean dose reduction of 12% for the 10 mm rind region and 45% for the 30 mm rind region when utilizing the DCS. Excellent agreement was observed between Monte Carlo simulations, EBT3 film, and MatriXX-PT measurements, with gamma pass rates exceeding 94.9% for all tested plans at the 3%/2 mm criterion. Absolute central axis doses showed an average verification difference of 1.4% between Monte Carlo and MatriXX-PT/PPC05 measurements.Significance.We have successfully dosimetrically validated the delivery of dynamically collimated proton therapy for clinically relevant delivery patterns and dose distributions with the DCS. Monte Carlo simulations were employed to assess dose reductions and treatment planning considerations associated with the DCS.

Development and implementation of an EPID-based method for localizing isocenter.

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter to an accuracy of less than 1 mm using the EPID (Electronic Portal Imaging Device). The proposed solution uses a collimator setting of 10 × 10 cm2 to acquire EPID images of a new phantom constructed from LEGO blocks. Images from a number of gantry and collimator angles are analyzed by automated analysis software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180° collimator rotation to determine if the phantom is centered in the dimension being investigated. Repeated tests show that the system is reproducibly independent of the imaging session, and calculated offsets of the phantom from radiation isocenter are a function of phantom setup only. Accuracy of the algorithm's calculated offsets were verified by imaging the LEGO phantom before and after applying the calculated offset. These measurements show that the offsets are predicted with an accuracy of approximately 0.3 mm, which is on the order of the detector's pitch. Comparison with a star-shot analysis yielded agreement of isocenter location within 0.5 mm. Additionally, the phantom and software are completely independent of linac vendor, and this study presents results from two linac manufacturers. A Varian Optical Guidance Platform (OGP) calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment and OGP calibration on a monthly basis.