Serial and parallel organ-at-risk-specific noncoplanar arc optimization for small versus large target volumes in liver SBRT.

Noncoplanar arc optimization has been shown to reduce OAR doses in SRS/SRT and has the potential to reduce doses to OARs in SBRT. Extracranial targets have additional considerations, including large OARs and, in the case of the liver, volume constraints on the healthy liver. Considering pathlengths through OARs that encompass target volumes may lead to specific dose reductions as in the encompassing healthy liver tissue. These optimizations must also leverage delivery efficiency and trajectory sampling to ensure ease of clinical translation. The purpose of this research is to generate optimized static-couch arcs that separately consider serial and parallel OARs and arc delivery efficiency, with a trajectory sampling metric, towards the aim of reducing dose to OARs and the surrounding healthy liver tissue. Separate BEV cost maps were created for parallel, and serial OARs by means of a fast ray-triangle intersection algorithm. An additional BEV cost map was created for the liver which, by definition, encompasses the liver tumors. The individual costs of these maps were summed and combined with the sampling metric for 100 000 random combinations of arc trajectories. A search algorithm was applied to find an arc trajectory solution that satisfied BEV cost and sampling optimization, while also ensuring an efficient delivery was possible with a low number of arcs. This method of arc selection was evaluated for 16 liver SBRT patients characterized by small and large target volumes. Comparisons were made with a clinical arc template of coplanar arcs. Dosimetric plan quality was evaluated using published guidelines and metrics from RTOG1112. Four of five plan quality metrics for the liver were significantly reduced when planned with optimized noncoplanar arcs. Median (range) reductions of the volumes receiving 10, 18, and 21 Gy were found of 140.4 (295.8) cc (p = 0.001), 28.2 (230.6) cc (p = 0.002) and 18.5 (155.5) cc (p = 0.04). A significant increase in median (range) dose to the right kidney of 0.2 ± 0.9 Gy (p = 0.03) was also found using optimized noncoplanar arcs, which was below the tolerance of 10 Gy for all cases. The average number of arcs chosen was 4 ± 1. Optimizing serial and parallel OARs separately during static couch noncoplanar arc selection significantly reduced the dose to the liver during SBRT using a moderate number of arcs.

Static couch non-coplanar arc selection optimization for lung SBRT treatment planning.

Objective. Non-coplanar arc geometry optimizations that take advantage of beam's eye view (BEV) geometric overlap information have been proven to reduce dose to healthy organs-at-risk (OARs). Recently, a metric called mean arc distance (MAD) has been developed that quantifies the arc geometry sampling of 4πspace. The purpose of this research is to combine improved BEV overlap information with MAD to generate static couch lung stereotactic body radiotherapy (SBRT) treatment plans deliverable on a C-arm linear accelerator.Approach. An algorithm utilizing the Moller-Trumbore ray-triangle intersection method was employed to compute a cost surrogate for dose to overlapping OARs using distances interpolated onto a PDD. Cost was combined with MAD for 100 000 random combinations of arc trajectories. A pathfinding algorithm for arc selection was created, balancing the contributions of MAD and 4πcost for the final trajectory. This methodology was evaluated for 18 lung SBRT patients. Cases were also planned with arcs from a clinical treatment template protocol for dosimetric and plan quality comparison. Results were evaluated using dose constraints in the context of RTOG0915.Main results. Five of six OARs had maximum dose reductions when planned with the arc trajectory optimization algorithm. Significant maximum dose reductions were found for esophagus (7.41 ± 0.91 Gy,p= 0.00019), trachea (5.56 ± 1.55 Gy,p= 0.0025), spinal cord (2.87 ± 1.13 Gy,p= 0.039), large bronchus (3.47 ± 1.49 Gy,p= 0.0075), and aorta (3.13 ± 0.99 Gy,p= 0.012). Mean dose to contralateral lung was also significantly reduced (0.50 ± 0.06 Gy,p= 0.00019). There were two significant increases in OAR doses: mean dose to ipsilateral lung (0.40 ± 0.09,p= 0.00086) and V5Gyto ipsilateral lung (1.95 ± 0.70%,p= 0.011). Paddick conformity index increased by 0.03 ± 0.02 (p= 0.14), remaining below a limit of 1.2 for both techniques.Significance. Static couch non-coplanar optimization yielded maximum dose reductions to OARs while maintaining target conformity for lung SBRT.

Assessing the impact of intrafraction motion correction on PTV margins and target and OAR dosimetry for single-fraction free-breathing lung stereotactic body radiation therapy.

The objective of this research is to investigate intrafraction motion correction on planning target volume (PTV) margin requirements and target and organ-at-risk (OAR) dosimetry in single-fraction lung stereotactic body radiation therapy (SBRT). Sixteen patients (15 with upper lobe lesions, 1 with a middle lobe lesion) were treated with single-fraction lung SBRT. Cone-beam computed tomography (CBCT) images were acquired before the treatment, between the arcs, and after the delivery of the treatment fraction. Shifts from the reference images were recorded in anterior-posterior (AP), superior-inferior (SI), and lateral (LAT) dimensions. The deviations from the reference image were calculated for 3 clinical scenarios: not applying intratreatment couch shifts and not correcting for pretreatment deviations < 3 mm ( scenario 1), not applying intratreatment couch shifts and correcting for pretreatment deviations < 3 mm ( scenario 2), and applying all pre- and intratreatment couch shifts (scenario 3). PTV margins were determined using the van Herk formalism for each scenario and maximum and average deviations were assessed. The clinical scenarios were modelled in the treatment planning system based on each patient dataset to assess target and OAR dosimetry. Calculated lower-bound PTV margins in the AP, SI, and LAT dimensions were [4.6, 3.5, 2.3] mm in scenario 1, [4.6, 2.4, 2.2] mm in scenario 2, and [1.7, 1.2, 1.0] mm in scenario 3. The margins are lower bounds because they do not include contributions from nonmotion related errors. Average and maximum intrafraction deviations were larger in the AP dimension compared to the SI and LAT dimensions for all scenarios. A unidimensional movement (several mm) in the negative AP dimension was observed in clinical scenarios 1 and 2 but not scenario 3. Average intrafraction deviation vectors were 1.2, 1.1, and 0.3 mm for scenarios 1, 2, and 3, respectively. Modelled clinical scenarios revealed that using scenario 3 yields significantly fewer treatment plan objective failures compared to scenarios 1 and 2 using a Wilcoxon signed-rank test. Intratreatment motion correction between each arc may enable reductions PTV margin requirements. It may also compensate for unidimensional negative AP movement, and improve target and OAR dosimetry.

Evaluation of plan quality and treatment efficiency in cranial stereotactic radiosurgery treatment plans with a variable source-to-axis distance.

INTRODUCTION: Radiotherapy deliveries with dynamic couch motions that shorten the source-to-axis distance (SAD) on a C-arm linac have the potential to increase treatment efficiency through the increase of the effective dose rate. In this investigation, we convert clinically deliverable volumetric modulated arc therapy (VMAT) and dynamic conformal arc (DCA) plans for cranial radiosurgery into virtual isocenter plans through implementation of couch trajectories that maintain the target at a shortened but variable SAD throughout treatment. MATERIALS AND METHODS: A randomly sampled population of patients treated with cranial radiosurgery from within the last three years were separated into groups with one, two, and three lesions. All plans had a single isocenter (regardless of number of targets), and a single prescription dose. Patient treatment plans were converted from their original delivery at a standard isocenter to a dynamic virtual isocenter in MATLAB. The virtual isocenter plan featured a variable isocenter position based upon the closest achievable source-to-target distance (referred to herein as a virtual source-to-axis distance [vSAD]) which avoided collision zones on a TrueBeam STx platform. Apertures were magnified according to the vSAD and monitor units at a given control point were scaled based on the inverse square law. Doses were calculated for the plans with a virtual isocenter in the Eclipse (v13.6.23) treatment planning system (TPS) and were compared with the clinical plans. Plan metrics (MU, Paddick conformity index, gradient index, and the volume receiving 12 Gy or more), normal brain dose-volume differences, as well as maximum doses received by organs at risk (OARs) were assessed. The values were compared between standard and virtual isocenter plans with Wilcoxon Sign Ranked tests to determine significance. A subset of the plans were mapped to the MAX-HD anthropomorphic phantom which contained an insert housing EBT3 GafChromic film and a PTW 31010 microion chamber for dose verification on a linac. RESULTS: Delivering plans at a virtual isocenter resulted in an average reduction of 20.9% (p = 3×10-6 ) and 20.6% (p = 3.0×10-6 ) of MUs across all VMAT and all DCA plans, respectively. There was no significant change in OAR max doses received by plans delivered at a virtual isocenter. The low dose wash (1.0-2.0 Gy or 5-11% of the prescription dose) was increased (by approximately 20 cc) for plans with three lesions. This was equivalent to a 2.7%-3.8% volumetric increase in normal tissue receiving the respective dose level when comparing the plan with a virtual isocenter to a plan with a standard isocenter. Gamma pass rates with a 5%/1mm analysis criteria were 96.40% ± 2.90% and 95.07% ± 3.10% for deliveries at standard and virtual isocenter, respectively. Absolute point dose agreements were within -0.36% ± 3.45% and -0.55% ± 3.39% for deliveries at a standard and virtual isocenter, respectively. Potential time savings per arc were found to have linear relationship with the monitor units delivered per arc (savings of 0.009 s/MU with an r2 = 0.866 when fit to plans with a single lesion). CONCLUSIONS: Converting clinical plans at standard isocenter to a virtual isocenter design did not show any losses to plan quality while simultaneously improving treatment efficiency through MU reductions.

Comparison of anatomically informed class solution template trajectories with patient-specific trajectories for stereotactic radiosurgery and radiotherapy.

Class solution template trajectories are used clinically for efficiency, safety, and reproducibility. The aim was to develop class solutions for single cranial metastases radiotherapy/radiosurgery based on intracranial target positioning and compare to patient-specific trajectories in the context of 4π optimization. Template trajectories were constructed based on the open-source Montreal Neurological Institute (MNI) average brain. The MNI brain was populated with evenly spaced spherical target volumes (2 cm diameter, N = 243) and organs-at-risk (OARs) were identified. Template trajectories were generated for six anatomical regions (frontal, medial, and posterior, each with laterality dependence) based on previously published 4π optimization methods. Volumetric modulated arc therapy (VMAT) treatment plans generated using anatomically informed template 4π trajectories and patientspecific 4π trajectories were compared against VMAT plans from a standard four-arc template. Four-arc optimization techniques were compared to the standard VMAT template by placing three spherical targets in each of six anatomical regions of a test patient. This yielded 54 plans to compare various plan quality metrics. Increasing plan technique complexity, the total number of OAR maximum dose reductions compared to the standard arc template for the 6 anatomical classes was 4+/-2 (OFIXEDc) and 7+/-2 (OFIXEDi). In 65.6% of all cases, optimized fixed-couch positions outperformed the standard-arc template. Of the three comparisons, the most complex (OFIXEDi) showed the greatest statistical significance compared to the least complex (VMATi) across 12 plan quality metrics of maximum dose to each OAR, V12Gy, total plan Monitor Units, conformity index, and gradient index (p < 0.00417). In approximately 70% of all cases, 4π optimization methods outperformed the standard-arc template in terms of maximum dose reduction to OAR, by exclusively changing the arc geometry. We conclude that a tradeoff exists between complexity of a class solution methodology compared to patient-specific methods for arc selection, in the context of plan quality improvement.

Region-of-interest intra-arc MV imaging to facilitate sub-mm positional accuracy with minimal imaging dose during treatment deliveries of small cranial lesions.

PURPOSE: To automate the generation of region-of-interest (ROI) apertures for use with megavoltage imaging for online positional corrections during cranial stereotactic radiosurgery. MATERIALS AND METHODS: Digitally reconstructed radiographs (DRRs) were created for a 3D-printed skull phantom at 5 degree gantry angle increments for a three-arc beam arrangement. At each angle, 3000 random rectangular apertures were generated, and 100 shifts on a grid were applied to the anatomy within the frame. For all shifts, the mutual information (MI) between the shifted and unshifted DRR was calculated to derive an average MI gradient. The top 10% of apertures that minimized registration errors were overlaid and discretely thresholded to generate imaging plans. Imaging was acquired with the skull while implementing simulated patient motion on a linac. Control point-specific couch motions were derived to align the skull to its planned positioning. RESULTS: Apertures with a range of repositioning errors less than 0.1 mm possessed a 42% larger average MI gradient when compared with apertures with a range greater than 1 mm. Dose calculations with Monte Carlo exhibited an 84% reduction in the dose received by 50% of the skull with the 50% thresholded plan when compared to a constant 22 × 22 cm2 imaging plan. For all different imaging plans (with and without motion), the calculated median 3D-errors with respect to the tracking of a metal-BB fiducial positioned at isocenter in the skull were sub-mm except for the 80% thresholded plan. CONCLUSIONS: Sub-mm positional errors are achievable with couch motions derived from control point-specific ROI imaging. Smaller apertures that conform to an anatomical ROI can be utilized to minimize the imaging dose incurred at the expense of larger errors.

Machine learning for contour classification in TG-263 noncompliant databases.

A large volume of medical data are labeled using nonstandardized nomenclature. Although efforts have been made by the American Association of Physicists in Medicine (AAPM) to standardize nomenclature through Task Group 263 (TG-263), there remain noncompliant databases. This work aims to create an algorithm that can analyze anatomical contours in patients with head and neck cancer and classify them into TG-263 compliant nomenclature. To create an accurate algorithm capable of such classification, a combined approaching using both binary images of individual slices of anatomical contours themselves, as well as center of mass coordinates of the structures are input into a neural network. The center of mass coordinates were scaled using two normalization schemes, a simple linear normalization scheme agnostic of the patient anatomy, and an anatomical normalization scheme dependent on patient anatomy. The results of all of the individual slice classifications are then aggregated into a single classification by means of a voting algorithm. The total classification accuracy of the final algorithms was 97.6% mean accuracy per class for nonanatomically normalization scheme, and 97.9% mean accuracy per class for anatomically normalization scheme. The total accuracy was 99.0% (13 errors in 1302 structures) for the nonanatomically normalization scheme, and 98.3% (22 errors in 1302 structures) for the anatomically normalization scheme.

Mean Arc Distance (MAD): a quantity to compare trajectory 4πsampling in single target cranial stereotactic radiotherapy.

Purpose.C-arm linac-based radiotherapy has seen a recent interest in 4πmethods of delivery using simultaneous rotations of couch and gantry to reduce doses to organs-at-risk (OARs) and increase dose compactness. While many methods use heuristics to generate trajectories that avoid OARs, combined with arbitrary trajectory restrictions to prevent oversampling, a quantity has not yet been developed to succinctly compare sampling of the 4πspace for candidate trajectories as a surrogate for dosimetric compactness.Methods.Evenly spaced sampling points were distributed across a 4πsphere centred on isocentre. A metric, mean arc distance (MAD), was defined that quantifies the average arc distance between all sampling points and their nearest field in a radiotherapy trajectory. The relationship between isodose volume and MAD was examined in 2,047 plans: 900 unique trajectories of fixed port DCA plans, 900 unique trajectories of contiguous field DCA plans, 192 VMAT plans (eight volumes in four locations, each with six trajectories) in matRad with 5 VMAT plans repeated for validation in a clinical planning system, and 10 clinical VMAT cases replanned with five trajectories in a clinical treatment planning system.Results.All isodose volumes greater than 10% of the prescription dose decreased with decreasing MAD in all comparisons. In the range of 10% to 100% of the prescription dose, the rate of isodose volume decrease was exponential as a function of MAD in all comparisons. Reduction of absolute isodose volume is seen with increased 4πsampling, with larger target volumes exhibiting larger absolute reductions. Very low isodoses (0% to 10% of prescription) increased with decreasing MAD.Conclusions.MAD is a 4πsampling quantity useful in quantifying the decrease of isodose volume, relevant for sparing normal tissues. By quantifying this feature, candidate dynamic trajectories can be efficiently compared for 4πsampling. This quantity is explored here for single target cranial radiotherapy but may have applications to other radiotherapy treatment sites.

Intra-arc binary collimation with dynamic axes trajectory optimization for the SRS treatment of multiple metastases with multiple prescriptions.

PURPOSE: This work generates multi-metastases cranial stereotactic radiosurgery/radiotherapy (SRS/SRT) plans using a novel treatment planning technique in which dynamic couch, collimator, and gantry trajectories are used with periodic binary target collimation. The performance of this planning technique is evaluated against conventional volumetric arc therapy (VMAT) planning in terms of various dose and plan quality metrics. METHODS: A 3D cost space (referred to herein as the combined optimization of dynamic axes or CODA cube) was calculated based on an overlap between targets and organs-at-risk (OARs) and uncollimated areas between targets (island blocking) for all combinations of couch, gantry, and collimator angles. Gradient descent through the cube was applied to determine dynamic trajectories. At each control point (CP), each target can either be conformally treated or blocked by the multi-leaf collimator (referred to as intra-arc binary collimation, iABC). Simulated annealing was used to optimize the collimation patterns throughout the arcs as well as the monitor units (MUs) delivered at each CP. Seven previously treated VMAT plans were selected for comparison against the CODA-iABC planning technique. Two CODA-iABC plans were developed: a single gantry arc plan and a plan with one gantry arc and two couch arcs. Plan quality comparison metrics included maximum and mean dose to OARs (brainstem, chiasm, optic nerves, eyes, and lenses), the volume of normal brain receiving 12 Gy (V12Gy), total MUs, target conformity, and dose-gradient index. RESULTS: Treatment plans generated with 1-arc CODA-iABC plans delivered an average of 21% and 30% higher maximum and mean doses to brainstem, respectively, when compared to VMAT plans. Treatment plans generated with 3-arc CODA-iABC used an average of 24% fewer MUs and resulted in an average reduction of 48% maximum dose and 50% mean dose to the OARs, when compared to VMAT. Target conformity values were worse in both CODA-iABC plans than VMAT by an average of 35% and 15%, respectively. There are no significant differences in V12Gy for all three planning techniques; however, 3-arc CODA-iABC is more effective at reducing dose to normal brain in the low-dose region (<12 Gy). CONCLUSION: CODA-iABC is a novel planning technique that has been developed to automatically generate patient-specific multi-axis trajectories for multiple metastases cranial SRS/SRT. This work has demonstrated the feasibility of planning with this novel method. The 1-arc CODA-iABC planning technique is slightly dosimetric inferior to VMAT. With an increased sampling of a three-dimensional CODA cube by using a 3-arc CODA-iABC technique, there was improved total dose sparing to all the OARs and increased MU efficiency, but with a cost in target conformity, when compared to VMAT.

Precision dosimetry in yttrium-90 radioembolization through CT imaging of radiopaque microspheres in a rabbit liver model.

PURPOSE: To perform precision dosimetry in yttrium-90 radioembolization through CT imaging of radiopaque microspheres in a rabbit liver model and to compare extracted dose metrics to those produced from conventional PET-based dosimetry. MATERIALS AND METHODS: A CT calibration phantom was designed containing posts with nominal microsphere concentrations of 0.5 mg/mL, 5.0 mg/mL, and 25.0 mg/mL. The mean Hounsfield unit was extracted from the post volumes to generate a calibration curve to relate Hounsfield units to microsphere concentration. A nominal bolus of 40 mg of microspheres was administered to the livers of eight rabbits, followed by PET/CT imaging. A CT-based activity distribution was calculated through the application of the calibration curve to the CT liver volume. Post-treatment dosimetry was performed through the convolution of yttrium-90 dose-voxel kernels and the PET- and CT-based cumulated activity distributions. The mean dose to the liver in PET- and CT-based dose distributions was compared through linear regression, ANOVA, and Bland-Altman analysis. RESULTS: A linear least-squares fit to the average Hounsfield unit and microsphere concentration data from the calibration phantom confirmed a strong correlation (r2 > 0.999) with a slope of 14.13 HU/mg/mL. A poor correlation was found between the mean dose derived from CT and PET (r2 = 0.374), while the ANOVA analysis revealed statistically significant differences (p < 10-12) between the MIRD-derived mean dose and the PET- and CT-derived mean dose. Bland-Altman analysis predicted an offset of 15.0 Gy between the mean dose in CT and PET. The dose within the liver was shown to be more heterogeneous in CT than in PET with an average coefficient of variation equal to 1.99 and 1.02, respectively. CONCLUSION: The benefits of a CT-based approach to post-treatment dosimetry in yttrium-90 radioembolization include improved visualization of the dose distribution, reduced partial volume effects, a better representation of dose heterogeneity, and the mitigation of respiratory motion effects. Post-treatment CT imaging of radiopaque microspheres in yttrium-90 radioembolization provides the means to perform precision dosimetry and extract accurate dose metrics used to refine the understanding of the dose-response relationship, which could ultimately improve future patient outcomes.

Patient-specific collision zones for 4π trajectory optimized radiation therapy.

PURPOSE: The 4π methodology determines optimized noncoplanar subarcs for stereotactic radiation therapy that minimize dose to organs-at-risk. Every combination of treatment angle is examined, but some angles are not appropriate as a collision would occur between the gantry and the couch or the gantry and the patient. Those combinations of couch and gantry angles are referred to as collision zones. A major barrier to applying 4π to stereotactic body radiation therapy (SBRT) is the unknown shape of the collision zones, which are significant as patients take up a large volume within the 4π sphere. This study presents a system that determines patient-specific collision zones, without additional clinical steps, to enable safe and deliverable noncoplanar treatment trajectories for SBRT patients. METHODS: To augment patient's computed tomography (CT) scan, full body scans of patients in treatment position were acquired using an optical scanner. A library of a priori scans (N = 25) was created. Based on the patients' treatment position and their body dimensions, a library scan is selected and registered to the CT scan of the patient. Next, a model of the couch and immobilization equipment is added to the patient model. This results in a patient model that is then aligned with a model of the treatment LINAC in a "virtual treatment room," where both components can be rotated to test for collisions. To test the collision detection algorithm, an end-to-end test was performed using a cranial phantom. The registration algorithm was tested by comparing the registered patient collision zones to those generated by using the patient's matching scan. RESULTS: The collision detection algorithm was found to have a 97.80% accuracy, a 99.99% sensitivity, and a 99.99% negative predictive value (NPV). Analysis of the registration algorithm determined that a 6 cm buffer was required to achieve a 99.65% mean sensitivity, where a sensitivity of unity is considered to be a requirement for safe treatment delivery. With a 6 cm buffer, the mean accuracy was 86.70% and the mean NPV was 99.33%. CONCLUSIONS: Our method of determining patient-specific collision zones can be accomplished with minimal user intervention based on an a priori library of body surface scans, thus enabling the safe application of 4π SBRT.

Measuring small field profiles and output factors with a stemless plastic scintillator array.

PURPOSE: To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors. METHODS: An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95 cm SSD, 5 cm depth, and 10 × 10 cm2 field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5 × 0.5 cm2 , 1 × 1 cm2 , and 2 × 2 cm2 . Using the central pixel of the array, output factors were measured for field sizes of 0.5 × 0.5 cm2 to 25 × 25 cm2 . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements. RESULTS: The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1 × 1 cm2 to 25 × 25 cm2 . At 0.5 × 0.5 cm2 the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5 × 5 cm2 . In comparison to a bulk scintillator of dimensions 5 × 5 × 0.5 cm3 the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5 × 0.5 cm2 , 1 × 1 cm2 , and 2 × 2 cm2 , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees. CONCLUSION: A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1 × 1 cm2 field size) when compared to micro ion chamber and diamond detector measurements.

SBRT of ventricular tachycardia using 4pi optimized trajectories.

PURPOSE: To investigate the possible advantages of using 4pi-optimized arc trajectories in stereotactic body radiation therapy of ventricular tachycardia (VT-SBRT) to minimize exposure of healthy tissues. METHODS AND MATERIALS: Thorax computed tomography (CT) data for 15 patients were used for contouring organs at risk (OARs) and defining realistic planning target volumes (PTVs). A conventional trajectory plan, defined as two full coplanar arcs was compared to an optimized-trajectory plan provided by a 4pi algorithm that penalizes geometric overlap of PTV and OARs in the beam's-eye-view. A single fraction of 25 Gy was prescribed to the PTV in both plans and a comparison of dose sparing to OARs was performed based on comparisons of maximum, mean, and median dose. RESULTS: A significant average reduction in maximum dose was observed for esophagus (18%), spinal cord (26%), and trachea (22%) when using 4pi-optimized trajectories. Mean doses were also found to decrease for esophagus (19%), spinal cord (33%), skin (18%), liver (59%), lungs (19%), trachea (43%), aorta (11%), inferior vena cava (25%), superior vena cava (33%), and pulmonary trunk (26%). A median dose reduction was observed for esophagus (40%), spinal cord (48%), skin (36%), liver (72%), lungs (41%), stomach (45%), trachea (53%), aorta (45%), superior vena cava (38%), pulmonary veins (32%), and pulmonary trunk (39%). No significant difference was observed for maximum dose (p = 0.650) and homogeneity index (p = 0.156) for the PTV. Average values of conformity number were 0.86 ± 0.05 and 0.77 ± 0.09 for the conventional and 4pi optimized plans respectively. CONCLUSIONS: 4pi optimized trajectories provided significant reduction to mean and median doses to cardiac structures close to the target but did not decrease maximum dose. Significant improvement in maximum, mean and median doses for noncardiac OARs makes 4pi optimized trajectories a suitable delivery technique for treating VT.

Investigating the impacts of intrafraction motion on dosimetric outcomes when treating small targets with virtual cones.

PURPOSE: Intrafraction patient motion is a well-documented phenomenon in radiation therapy. In stereotactic radiosurgery applications in which target sizes can be very small and dose gradients very steep, patient motion can significantly impact the magnitude and positional accuracy of the delivered dose. This work investigates the impact of intrafraction motion on dose metrics for small targets when treated with a virtual cone. MATERIALS AND METHODS: Monte Carlo simulations were performed to calculate dose kernels for treatment apertures ranging from 1 × 2.5 mm2 to 10 × 10 mm2 . The phantom was an 8.2-cm diameter sphere and isotropic voxels had lengths of 0.25 mm. Simulated treatments consisted of 3 arcs: 1 axial arc (360° gantry rotation, couch angle 0°) and 2 oblique arcs (180° gantry rotation, couch angle ±45°). Dose distributions were calculated via superposition of the rotated kernels. Two different collimator orientations were considered to create a virtual cone: (a) each treatment arc was delivered twice, once each with a static collimator angle of ±45°, and (b) each treatment arc was delivered once, with dynamic collimator rotation throughout the arc. Two different intrafraction motion patterns were considered: (a) constant linear motion and (b) sudden, persistent motion. The impact of motion on dose distributions for target sizes ranging from 1 to 10 mm diameter spheres was quantified as a function of the aperture size used to treat the lesions. RESULTS: The impact of motion on both the target and the surrounding tissue was a function of both aperture shape and target size. When a 0.5-mm linear drift along each dimension occurred during treatment, targets ≥5 mm saw less than a 10% decrease in coverage by the prescription dose. Smaller apertures accrued larger penalties with respect to dosimetric hotspots seen in the tissues surrounding the target volume during intrafraction motion. For example, treating a 4-mm-sized target that undergoes 2.60 mm (3D vector) of continuous linear motion, the D5 in the concentric shells that extend 1, 2, and 3 mm from the surface of the target was 39%, 24%, and 14% smaller, respectively when comparing the delivery of a larger aperture (6 × 10 mm2 ) to a smaller aperture (2 × 5 mm2 ). Using a static collimator for shaping a virtual cone during treatment minimized the dosimetric impact of motion in the majority of cases. For example, the volume that is covered by 70% or more of the prescription dose is smaller in 60.4% of cases when using the static collimator. The volume covered by 50, and 30% or more of the prescription dose is also smaller when treating with a static collimator, but the clinical significance of this finding is unknown. CONCLUSIONS: In this work, the dosimetric trade-offs between aperture size and target size when irradiating with virtual cones has been demonstrated. These findings provide information about the tradeoffs between target coverage and normal tissue sparing that may help inform clinical decision making when treating smaller targets with virtual cones.

Post-administration dosimetry in yttrium-90 radioembolization through micro-CT imaging of radiopaque microspheres in a porcine renal model.

The purpose of this study is to perform post-administration dosimetry in yttrium-90 radioembolization through micro-CT imaging of radiopaque microsphere distributions in a porcine renal model and explore the impact of spatial resolution of an imaging system on the extraction of specific dose metrics. Following the administration of radiopaque microspheres to the kidney of a hybrid farm pig, the kidney was explanted and imaged with micro-CT. To produce an activity distribution, 400 MBq of yttrium-90 activity was distributed throughout segmented voxels of the embolized vasculature based on an established linear relationship between microsphere concentration and CT voxel value. This distribution was down-sampled to coarser isotropic grids ranging in voxel size from 2.5 to 15 mm to emulate nominal resolutions comparable to those found in yttrium-90 PET and Bremsstrahlung SPECT imaging. Dose distributions were calculated through the convolution of activity distributions with dose-voxel kernels generated using the GATE Monte Carlo toolkit. Contours were computed to represent normal tissue and target volumes. Dose-volume histograms, dose metrics, and dose profiles were compared to a ground truth dose distribution computed with GATE. The mean dose to the target for all studied voxel sizes was found to be within 5.7% of the ground truth mean dose.D70was shown to be strongly correlated with image voxel size of the dose distribution (r2 = 0.90).D70is cited in the literature as an important dose metric and its dependence on voxel size suggests higher resolution dose distributions may provide new perspectives on dose-response relationships in yttrium-90 radioembolization. This study demonstrates that dose distributions with large voxels incorrectly homogenize the dose by attributing escalated doses to normal tissues and reduced doses in high-dose target regions. High-resolution micro-CT imaging of radiopaque microsphere distributions can provide increased confidence in characterizing the absorbed dose heterogeneity in yttrium-90 radioembolization.

Influence of intra- and interfraction motion on planning target volume margin in liver stereotactic body radiation therapy using breath hold.

PURPOSE: This study aimed to investigate intra- and interfraction motion during liver stereotactic body radiation therapy for the purpose of planning target volume (PTV) margin estimation, comparing deep inspiration breath hold (DIBH) and deep expiration breath hold (DEBH). METHODS AND MATERIALS: Pre- and posttreatment kV cone beam computed tomography (CT) images were acquired for patients with liver cancer who were treated using stereotactic body radiation therapy with DIBH or DEBH. A total of 188 images were analyzed from 18 patients. Positioning errors were determined based on a comparison with planning CT images and matching to the liver. Treatment did not proceed until errors were ≤3 mm. Standard deviations of random and systematic errors resulting from this image matching process were used to calculate PTV margin estimates. RESULTS: DIBH errors are generally larger than DEBH errors, especially in the anterior-posterior and superior-inferior directions. Posttreatment errors tend to be larger than pretreatment errors, especially for DIBH. Standard deviations of random errors are larger than those of systematic errors. Considering both pre- and posttreatment cone beam CT images, PTV margins for DIBH and DEBH are estimated as anterior-posterior, superior-inferior, right-left = (5.7, 6.3, 3.0) mm and (3.1, 3.4, 2.8) mm, respectively. CONCLUSIONS: This study suggests that DEBH results in more reproducible target positioning, which could in turn justify the use of smaller PTV margins.

Fabrication and characterization of a stemless plastic scintillation detector.

PURPOSE: To fabricate a stemless plastic scintillation detector (SPSD) and characterize its linearity and reproducibility, and its dependence on energy and dose per pulse; and to apply it to clinical PDD and output factor measurements. METHODS: An organic bulk heterojunction photodiode was fabricated by spin coating a blend of P3HT and PCBM onto an ITO-coated glass substrate and depositing aluminum top contacts. Eljen scintillators (~5 × 5 × 5 mm3 ; EJ-204, EJ-208, and EJ-260) or Saint-Gobain scintillators (~3 × 3 × 2 mm3 ; BC-400 and BC-412) were placed on the opposite side of the glass using a silicone grease (optical coupling agent) creating the SPSD. Energy dependence was measured by using 100, 180, and 300 kVp photon beams from an orthovoltage treatment unit (Xstrahl 300) and 6 and 10 MV photons from a Varian TrueBeam linear accelerator. Linearity, dose per pulse dependence, output factors, and PDDs were measured using a 6 MV photon beam. PDDs and output factors were compared to ion chamber measurements. A control device was fabricated by substituting polystyrene (PS) for the P3HT/PCBM layer. No photocurrent should be generated in the control device and so any current measured is due to Compton current in the electrodes, wires, and surroundings from the irradiation. Output factors were corrected by subtracting the signal measured using the control device from the photodiode measured signal to yield the photocurrent. RESULTS: Each SPSD had excellent linearity with dose having an r2 of 1 and sensitivities of 1.07 nC/cGy, 1.04 nC/cGy, 1.00 nC/cGy and 0.10 nC/cGy, and 0.10 nC/cGy for EJ-204, EJ-208, EJ-260 (5 × 5 × 5 mm3 volumes), BC-400, and BC-412 (3 × 3 × 2 mm3 volumes), respectively. No significant dose per pulse dependence was measured. Output factors matched within 1% for the large scintillators for field sizes of 5 × 5 cm2 to 25 × 25 cm2 , but there was a large under-response at field sizes below 3 × 3 cm2 . After correcting the signal of the small scintillators by subtracting the current measured using the PS control, the output factors agreed with the ion chamber measurements within 1% from field sizes 1 × 1 cm2 to 20 × 20 cm2 . The impact of Cerenkov emissions in the scintillator was effectively corrected with a simple reflective coating on the scintillator. In comparison to a 6 MV photon beam, the large scintillator SPSDs exhibited 37%, 52%, and 73% of the response at energies 100 kVp, 180 kVp and 300 kVp, respectively. CONCLUSION: The principle of the SPSD was demonstrated. Devices had excellent linearity, reproducibility, and no significant dose per pulse dependence, and a simple reflective coating was sufficient to correct for Cerenkov emissions from within the scintillator. The devices demonstrated similar energy dependence to other scintillator detectors used in a radiotherapy setting.

Method for the differentiation of radiation-induced photocurrent from total measured current in P3HT/PCBM BHJ photodiodes.

Thin film radiation-detecting diodes fabricated in the laboratory, such as an organic bulk heterojunction, often contain conductive leads, indium tin oxide traces and metallic interconnects which are exposed to the high-energy photon beam during operation. These components generate extraneous radiation-induced currents, that, if not accounted for, will erroneously be attributed to the detector. In commercial devices, these contributions are mitigated by minimizing the size of these components, an approach that is often not feasible in a research lab. Here we demonstrate a method to measure these extraneous signals, and by subtraction, correct the gross signal to accurately reflect the signal generated in the active volume of the diode itself. The method can effectively correct the extraneous signal. The method showed promise over a range of photon beam energies, dose rates, and field sizes.

Toward the combined optimization of dynamic axes (CODA) for stereotactic radiotherapy and radiosurgery using fixed couch trajectories.

PURPOSE: To develop a novel system for patient-specific combined optimization of couch, collimator, and gantry angles for use in volumetric modulated arc therapy (VMAT) treatment planning. The system was designed to produce highly compact dose distributions by extensively sampling the 4π space. Automated fixed couch trajectory planning was used to reduce normal tissue doses by avoiding beams-eye-view (BEV) overlap with organs-at-risk (OARs) and improve monitor unit (MU) efficiency through collimator angle optimization. METHODS: By merging distinct BEV objective functions used to optimize the couch rotation angle and collimator angle, a three-dimensional (3D) cost space (the CODA cube) was constructed with axes of gantry, couch, and collimator rotation angles. At each voxel in this CODA cube, the cost of implementing this combination of axes positions in fixed couch trajectories was quantified. The CODA cube was sampled and explored using a modified constrained Bellman-Ford algorithm to suggest low-cost fixed candidate arcs on each plane of the space, from which 10-arcs are chosen throughout the 3D space using a k-means clustering algorithm. These fixed couch trajectories were then imported into the Eclipse treatment planning system (v.11) and inverse-optimized according to clinical standards. Eight artificial cranial targets were contoured in a test-patient anatomy, and seven treatment plans were generated from combinations of three and four targets. The CODA cube optimized plans were compared to standard 4-arc VMAT plans for cranial stereotactic radiotherapy/surgery that were optimized for the same sets of targets; maximum dose to each OAR, V12Gy to normal brain, conformity, and total MUs were compared. Both planning methods were inverse-optimized with identical dosimetric objectives. RESULTS: CODA plans resulted in a reduction in maximum dose to OARs of 20.6% (P < 0.01), with maximum brainstem dose decreased by 2.63 Gy (P = 0.031) on average when compared to the standard arc arrangement. The mean reduction in total MU was 8.6% (P = 0.156), the mean increase in the inverse of the van't Riet conformation number was 0.1%, (P = 0.67) and the mean decrease in normal brain tissue receiving 12 Gy or higher was 3.9% (P = 0.16), when compared to the standard VMAT arc configuration (n = 7). CONCLUSIONS: The optimization of couch, collimator, and gantry angles simultaneously using a 3D optimization space achieved improvement on multiple clinical metrics when compared to the standard VMAT arc configuration. A statistically significant sparing to OAR maximum doses was seen. Combining these optimizations may yield superior results to independent optimization.