Couch Angles Research Articles

Radiobiological assessment of nasopharyngeal cancer IMRT using various collimator angles and non-coplanar fields

AbstractAim:The aim of this study was to evaluate clinical efficacy and radiobiological outcome of intensity-modulated radiation therapy (IMRT) modalities using various collimator angles and non-coplanar fields for nasopharyngeal cancer (NPC).Materials and methods:A 70-Gy planning target volume dose was administered for 30 NPC patients referred for IMRT. Standard IMRT plans were constructed based on the target and organs at risk (OARs) volume; and dose constraints recommended by Radiation Therapy Oncology Group (RTOG). Using various collimator angles and non-coplanar fields, 11 different additional IMRT protocols were investigated. Homogeneity indexes (HIs) and conformation numbers (CNs) were calculated. Poisson and relative seriality models were utilised for estimating tumour control probability (TCP) and normal tissue complication probabilities (NTCPs), respectively.Results:Various collimator angles and non-coplanar fields had no significant effect on HI, CN and TCP, while significant effects were noted for some OARs, with a maximum mean dose (Dmax). No significant differences were observed among the calculated NTCPs of all the IMRT protocols. However, the protocol with 10° collimator angle (for five fields out of seven) and 8° couch angle had the lowest NTCP. Furthermore, the standard and some of non-coplanar IMRT protocols led to the reduction in OARs Dmax.Conclusions:Using appropriate standard/non-coplanar IMRT protocols for NPC treatment could potentially reduce the dose to the OARs and the probability of inducing secondary cancer in patients.

Comprehensive radiation and imaging isocenter verification using NIPAM kV‐CBCT dosimetry

To develop and demonstrate a comprehensive method to directly measure radiation isocenter uncertainty and coincidence with the cone-beam computed tomography (kV-CBCT) imaging coordinate system that can be carried out within a typical quality assurance (QA) time slot. An N-isopropylacrylamide (NIPAM) three-dimensional (3D) dosimeter for which dose is observed as increased electron density in kV-CBCT is irradiated at eight couch/gantry combinations which enter the dosimeter at unique orientations. One to three CBCTs are immediately acquired, radiation profile is detected per beam, and displacement from imaging isocenter is quantified. We performed this test using a 5mm diameter MLC field, and 7.5 and 4mm diameter cones, delivering approximately 16Gy per beam. CBCT settings were 1035-4050mAs, 80-125kVs, smooth filter, 1mm slice thickness. The two-dimensional (2D) displacement of each beam from the imaging isocenter was measured within the planning system, and Matlab code developed in house was used to quantify relevant parameters based on the actual beam geometry. Detectability of the dose profile in the CBCT was quantified as the contrast-to-noise ratio (CNR) of the irradiated high-dose regions relative to the surrounding background signal. Our results were compared to results determined by the traditional Winston-Lutz test, film-based "star shots," and the vendor provided machine performance check (MPC). The ability to detect alignment errors was demonstrated by repeating the test after applying a 0.5mm shift to the MLCs in the direction of leaf travel. In addition to radiation isocenter and coincidence with CBCT origin, the analysis also calculated the actual gantry and couch angles per beam. Setup, MV irradiation, and CBCT readout were carried out within 38min. After subtracting the background signal from the pre-CBCT, the CNR of the dosimeter signal from the irradiation with the MLCs (125kVp, 1035mAs, n=3), 7.5mm cone (125kVp, 1035mAs, n=3), and 4mm cone (80kVp, 4050mAs, n=1) was 5.4, 5.9, and 2.9, respectively. The minimum radius that encompassed all beams calculated using the automated analysis was 0.38, 0.48, and 0.44mm for the MLCs, 7.5mm cone, and 4mm cone, respectively. When determined manually, these values were slightly decreased at 0.28, 0.41, and 0.40mm. For comparison, traditional Winston-Lutz test with MLCs and MPC measured the 3D isocenter radius to be 0.24mm. Lastly, when a 0.5mm shift to the MLCs was applied, the smallest radius that intersected all beams increased from 0.38 to 0.90mm. The mean difference from expected value for gantry angle was 0.19±0.29°, 0.17±0.23°, and 0.12±0.14° for the MLCs, 7.5mm cone, and 4mm cone, respectively. The mean difference from expected for couch angle was -0.07±0.28°, -0.08±0.66°, and 0.04±0.25°. This work demonstrated the feasibility of a comprehensive isocenter verification using a NIPAM dosimeter with sub-mm accuracy which incorporates evaluation of coincidence with imaging coordinate system, and may be applicable to all SRS cones as well as MLCs.

Impact of patient positioning uncertainty in noncoplanar intracranial stereotactic radiotherapy.

The aim of this study is to evaluate the patient positioning uncertainty in noncoplanar stereotactic radiosurgery or stereotactic radiotherapy (SRS/SRT) for intracranial lesions with the frameless 6D ExacTrac system. In all, 28 patients treated with SRS/SRT of 70 treatment plans at our institution were evaluated in this study. Two X‐ray images with the frameless 6D ExacTrac system were first acquired to correct (XC) and verify (XV) the patient position at a couch angle of 0º. Subsequently, the XC and XV images were also acquired at each planned couch angle for using noncoplanar beams to detect position errors caused by rotating a couch. The translational XC and XV shift values at each couch angle were calculated for each plan. The percentages of the translational XC shift values within 1.0 mm for each planned couch angle for using noncoplanar beams were 77.86%, 72.26%, and 98.47% for the lateral, longitudinal, and vertical directions, respectively. Those within 2.0 mm were 98.22%, 97.96%, and 99.75% for the lateral, longitudinal, and vertical directions, respectively. The maximum absolute values of the translational XC shifts among all planned couch angles for using noncoplanar beams were 2.69, 2.45, and 2.17 mm for the lateral, longitudinal, and vertical directions, respectively. The overall absolute values of the translational XV shifts were less than 1.0 mm for all directions except for one case in the longitudinal direction. The patient position errors were detected after couch rotation for using noncoplanar beams, and they exceeded a planning target volume (PTV) margin of 1.0–2.0 mm used commonly in SRS/SRT treatment. These errors need to be corrected at each planned couch angle, or the PTV margin should be enlarged.

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

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. 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. CODA plans resulted in a reduction in maximum dose to OARs of 20.6% (P<0.01), with maximum brainstem dose decreased by 2.63Gy (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 12Gy or higher was 3.9% (P=0.16), when compared to the standard VMAT arc configuration (n=7). 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.

Lower doses to hippocampi and other brain structures for skull-base meningiomas with intensity modulated proton therapy compared to photon therapy

Background and purposeRadiotherapy of skull-base meningiomas is challenging due to the close proximity of multiple sensitive organs at risk (OARs). This study systematically compared intensity modulated proton therapy (IMPT), non-coplanar volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT) based on automated treatment planning. Differences in OARs sparing, with specific focus on the hippocampi, and low-dose delivery were quantified. Materials and methodsTwenty patients, target diameter >3 cm, were included. Automated plan generation was used to calculate a VMAT plan with three non-coplanar arcs, an IMRT plan with nine non-coplanar beams with optimized gantry and couch angles, and an IMPT plan with three patient-specific selected non-coplanar beams. A prescription dose of 50.4 GyRBE in 28 fractions was used. The same set of constraints and prioritized objectives was used. All plans were rescaled to the same target coverage. Repeated measures ANOVA was used to assess the statistical significance of differences in OAR dose parameters between planning techniques. ResultsCompared to VMAT and IMRT, IMPT significantly improved dose conformity to the target volume. Consequently, large dose reductions in OARs were observed. With respect to VMAT, the mean dose and D40% in the bilateral hippocampus were on average reduced by 48% and 74%, respectively (p ≤ 0.005). With IMPT, the mean dose in the normal brain and volumes receiving 20–30 Gy were up to 47% lower (p ≤ 0.01). When comparing IMPT and IMRT, even larger dose differences in those OARs were observed. ConclusionFor skull-base meningiomas IMPT allows for a considerable dose reduction in the hippocampi, normal brain and other OARs compared to both non-coplanar VMAT and IMRT, which may lead to a clinically relevant reduction of late neurocognitive side effects.

Optimized Volumetric Modulated Arc Technique for Noninvasive Cardiac Ablation to Treat Ventricular Tachycardia

Recent publications (Cuculich et al., NEJM-2017, Robinson et al., Circulation-2019) have shown promising outcomes of radioablation using a single-fraction SBRT to treat patients with refractory, life-threatening, VT. This prompted growing interest across the world to use SBRT to treat VT. The primary goal of this study is to explore an alternative VMAT planning technique to achieve greater Non-Target Heart-Tissue (NTHT) sparing while simultaneously allowing for breath hold (BH) treatments. 13 patients were selected who previously received a single fraction dose of 35 Gy to ITV and 25 Gy to PTV utilizing 3 non-coplanar VMAT arcs of 6 MV photon beams using HD-MLC on a radiosurgery-optimized linac. To further improve NTHT sparing we retrospectively re-planned each patient using 10 ipsi-lateral arcs with optimal gantry, couch and collimator angles. Two plans were generated using 6X FFF and 10X FFF beams with dynamic jaw-tracking. Gantry arc lengths were limited to 90° (< 15s/arc) for feasibility of a potential BH treatments. Couch angles were limited to ± 15° to avoid collisions. Plans were optimized to maximize the NTHT sparing while meeting the protocol constraints for the stomach, esophagus and colon (OAR). Plan quality metrics for both clinical and reoptimized plans are summarized in Table 1. Revised plans showed adequate target coverage (V95%Rx=>95%), better dose conformity and gradient, while meeting the dose constraints of the OARs. Excellent NTHT sparing is achieved by reducing the volumes of V20Gy by 43 cc and V5Gy by 250 cc. Average mean dose reduction of 2.5 (range: 0.6 – 6.0) Gy and 2.2 (0.7 – 5.7 Gy) was observed in 6X FFF and 10X FFF plans, respectively. Revised plans showed the possibility of treating each arc in a single BH. The standardized technique used in this study showed improved NTHT sparing and better dose conformity. This result may be important for the group of patients where cardiac toxicity may play a vital role in long-term survival. BH treatments may reduce the required PTV margins resulting in further NTHT sparing.Abstract 3778; Table 1Vol. (cc)Plan TypeTarget CoverageConformityMean D (Gy)%VRx%V95%RxConformity IndexGradient MeasureITV48.1 ± 29Clinical38.4 ± 1.197.6 ± 2.499.7 ± 0.66X FFF36.9 ± 0.494.2 ± 3.199.3 ± 0.810X FFF36.8 ± 0.890.5 ± 11.698.7 ± 2.0PTV138.9 ± 67.8Clinical33.6 ± 1.199.0 ± 0.899.7 ± 0.41.14 ± 0.122.01 ± 0.476X FFF32.3 ± 0.493.8 ± 2.596.8 ± 2.21.02 ± 0.041.76 ± 0.4710X FFF32.3 ± 0.494.2 ± 2.397.1 ± 1.81.03 ± 0.031.55 ± 0.26Non-Target Heart TissueMean D (Gy)%V25%V20%V15%V10%V5%V2Clinical6.2 ± 2.10.9 ± 0.84.9 ± 2.511.2 ± 5.421.4 ± 9.140.5 ± 13.966.7 ± 17.86X FFF3.7 ± 1.10.3 ± 0.31.9 ± 2.74.9 ± 1.610.1 ± 3.622.5 ± 8.945.9 ± 15.7Vol. spared (cc)9 ± 1143.± 3590 ± 76160 ± 113250 ± 146284 ± 20910X FFF4.0 ± 1.10.4 ± 0.32.2 ± 0.75.5 ± 1.711.4 ± 3.825.5 ± 9.649.6 ± 16.5Vol. spared (cc)8 ± 1039 ± 3381 ± 75142 ± 108208 ± 111233 ± 150 Open table in a new tab

Development and clinical implementation of eclipse scripting-based automated patient-specific collision avoidance software.

PurposeIncreased use of Linac‐based stereotactic radiosurgery (SRS), which requires highly noncoplanar gantry trajectories, necessitates the development of efficient and accurate methods of collision detection during the treatment planning process. This work outlines the development and clinical implementation of a patient‐specific computed tomography (CT) contour‐based solution that utilizes Eclipse Scripting to ensure maximum integration with clinical workflow.MethodsThe collision detection application uses triangle mesh structures of the gantry and couch, in addition to the body contour of the patient taken during CT simulation, to virtually simulate patient treatments. Collision detection is performed using Binary Tree Hierarchy detection methods. Algorithm accuracy was first validated for simple cuboidal geometry using a calibration phantom and then extended to an anthropomorphic phantom simulation by comparing the measured minimum distance between structures to the predicted minimum distance for all allowable orientations. The collision space was tested at couch angles every 15° from 90 to 270 with the gantry incremented by 5° through the maximum trajectory. Receiver operating characteristic curve analysis was used to assess algorithm sensitivity and accuracy for predicting collision events. Following extensive validation, the application was implemented clinically for all SRS patients.ResultsThe application was able to predict minimum distances between structures to within 3 cm. A safety margin of 1.5 cm was sufficient to achieve 100% sensitivity for all test cases. Accuracy obtained was 94.2% with the 5 cm clinical safety margin with 100% true positive collision detection. A total of 88 noncoplanar SRS patients have been currently tested using the application with one collision detected and no undetected collisions occurring. The average time for collision testing per patient was 2 min 58 s.ConclusionsA collision detection application utilizing patient CT contours was developed and successfully clinically implemented. This application allows collisions to be detected early during the planning process, avoiding patient delays and unnecessary resource utilization if detected during delivery.

Optimization of couch angles and number of arcs in non-coplanar VMAT for pituitary adenomas

Non-coplanar VMAT spares critical organs with short delivery time while maintaining PTV coverage. Due to the growth patterns of pituitary adenoma, the beam angle selection step was simplified by finding the optimal couch angles and number of arcs. The 2 clinical cases with difference PTV sizes were selected to study the results of dose distribution between coplanar and non-coplanar techniques. Later, couch angles were varied from 30° to 90° and 270° to 330° with a 15° interval. The numbers of arcs were varied from 3 to 4 arcs. Then, the selected protocol was applied to 5 pituitary adenoma patients. It was found that 90° and 270° couch angle showed the highest scores. However, these beams passed directly through the body, so the second couch angles of 75° and 285° were selected instead. For number of arcs optimization, 3-arc plan and 4-arc plan demonstrated a similar score. However, the 3-arc plan illustrated an obviously shorter treatment time. After the selection was proven to the 5 patients, there were 4 patients who passed the criteria, while 1 patient did not pass the hippocampal planning criteria. In conclusion, the suggested couch angles were 75° and 285°. The optimal number of arcs for pituitary adenoma was 3 arcs.

Quality assurance of non-coplanar, volumetric-modulated arc therapy employing a C-arm linear accelerator, featuring continuous patient couch rotation

PurposeTo perform quality assurance of non-coplanar, volumetric-modulated arc therapy featuring continuous couch rotation (CCR-VMAT) using a C-arm linear accelerator.MethodsWe planned and delivered CCR-VMAT using the TrueBeam Developer Mode. Treatment plans were created for both a C-shaped phantom and five prostate cancer patients using seven CCR trajectories that lacked collisions; we used RayStation software (ver. 4.7) to this end. Subsequently, verification plans were generated. The mean absolute error (MAE) between the center of an MV-imaged steel ball and the radiation field was calculated using the Winston–Lutz test. The MAEs between planned and actual irradiation values were also calculated from trajectory logs. In addition, correlation coefficients (r values) among the MAEs of gantry angle, couch angle, and multi-leaf collimator (MLC) position, and mechanical parameters including gantry speed, couch speed, MLC speed, and beam output, were estimated. The dosimetric accuracies of planned and measured values were also assessed using ArcCHECK.ResultsThe MAEs ±2 standard deviations as revealed by the Winston–Lutz test for all trajectories were 0.3 ± 0.3 mm in two dimensions. The MAEs of the gantry, couch, and MLC positions calculated from all trajectory logs were within 0.04°, 0.08°, and 0.02 mm, respectively. Deviations in the couch angle (r = 0.98, p < 0.05) and MLC position (r = 0.86, p < 0.05) increased significantly with speed. The MAE of the beam output error was less than 0.01 MU. The mean gamma passing rate ± 2 SD (range) of the 3%/3 mm, 3%/1 mm, and 5%/1 mm was 98.1 ± 1.9% (95.7–99.6%), 87.2 ± 2.8% (80.2–96.7%), and 96.3 ± 2.8% (93.9–99.6%), respectively.ConclusionsCCR-VMAT delivered via the TrueBeam Developer Mode was associated with high-level geometric and mechanical accuracy, thus affording to high dosimetric accuracy. The CCR-VMAT performance was stable regardless of the trajectory chosen.

Stereotactic radiosurgery with MLC-defined arcs: Verification of dosimetry, spatial accuracy, and end-to-end tests.

PurposeTo measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator‐(MLC) equipped linear accelerator without cones or other special attachments or modifications.MethodsDosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ‐index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC‐shaped fields ranging from 1.6 × 5 to 30 × 30 mm2 were planned and delivered to an in‐house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple‐arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm2 fields applied at six different couch angles. For end‐to‐end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine‐arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy.ResultsIn the single‐arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ‐index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm2 and rose to 4.4% as field size increased to 20 × 20 mm2. Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine‐arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end‐to‐end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained <1 at all pixels. To date, more than 50 functional SRS treatments using MLC‐shaped static field arcs have been delivered.ConclusionStereotactic radiosurgery (SRS) can be planned and delivered on a standard linac without cones or other modifications with better than 0.5 mm spatial and 5% dosimetric accuracy.

Part I: Out-of-field dose mapping for 6X and 6X-flattening-filter-free beams on the TrueBeam for extended distances.

With increasing cancer treatment success rates, many patients go on to live long, productive lives following recovery. Therefore, minimizing potential side effects due to dose outside the treated field is becoming a significant consideration in radiation therapy. With many potential treatment configurations available, it is important to quantify how out-of-field dose varies with common variables such as distance from isocenter, couch angle, jaw size, and flattening-filter setting. The accurate quantification of out-of-field dose at extended distances could also benefit researchers and detector developers. While data exist for out-of-field dose from older linear accelerator (Linac) models, the phenomenon has not been described for the latest generation of machines, such as the Varian TrueBeam. The purpose of this study was to comprehensively quantify out-of-field dose for the Varian TrueBeam Linac low energy photons in a wide range of positions and treatment geometries. Out-of-field doses were measured using two phantom setups: (a) A large volume ion chamber with a buildup sleeve to quantify head leakage and collimator scatter background dose; and (b) A farmer ion chamber in solid water to incorporate phantom scatter in addition to collimator scatter, and head leakage background dose. In both cases, the ion chamber was positioned with its length along the slowly varying transverse direction (perpendicular to the radial from isocenter). Doses were measured for four symmetric jaw settings (2×2cm2 , 4×4cm2 , 10×10cm2 , and 20×20cm2 ) for a range of distances from the isocenter (0-100cm). The angular dependence of the out-of-field dose was measured using four different angles: 0°, 45°, 90°, and 135° with respect to the in-plane direction. All measurements were performed for both 6X and 6X-flattening-filter-free (FFF) beams. The lowest out-of-field doses were observed at 60cm away from isocenter in both in-plane and cross-plane directions for fields smaller than 10×10cm2 . Out-of-field dose decreased with decreasing jaw size (a factor of 4.7 for 6X-FFF and a factor of 3.1 for 6X going from 20×20cm2 to 2×2cm2 at 60cm from isocenter in the in-plane direction). The 6X-FFF beam produced out-of-field doses as low as 64% of the 6X beam. This study presents a comprehensive description of 6X and 6X-FFF out-of-field doses on a Varian TrueBeam Linac including measurements at a range of positions, angles, and jaw settings and with and without phantom scatter.

Dosimetric Comparison of Four Volumetric-Modulated Arc Therapy Beam Arrangements Utilized for Hippocampal-Sparing Whole-Brain Radiation Therapy.

Purpose:In the present study, the performance of four VMAT beam arrangements used for hippocampal-sparing whole-brain radiation therapy is addressed.Material and Methods:Data corresponding to 20 patients were utilized so as to generate plans for every beam configuration. A preliminary study was conducted to assess the optimal distance between optimization structures (PTVx) and hippocampi. V25, V30, D50%, D2%, D98%, homogeneity index (HI) and Paddick conformity factor (CF) were evaluated for PTV. D100% and Dmax were considered for hippocampi. All plans were required to perform at least as recommended in RTOG 0933 trial regarding organs at risk (OAR) sparing and PTV objectives.Results:Considerable hippocampi sparing alongside with a reasonably low decrease in PTV coverage was achieved using a 7 mm distance between hippocampi and PTV optimization structure. Beam setup 3 (comprised of two full arcs with 0° couch angle and two half arcs with 90° couch angle) achieved the best PTV coverage, HI and CF, while it performed the second-best sparing in hippocampi and lenses. Moreover, beam setup 3 was the second-fastest treatment, although it resulted in the highest number of delivered MU among all beam setups. Beam setup 1 (comprised of two full arcs with no couch angles) was the fastest and it delivered a significantly less amount of monitor units compared with the other beam setups evaluated. Furthermore, a higher robustness was obtained by using no couch angles. Although beam setup 1 was the least optimal considering OAR sparing, it still performed better than required in the RTOG 0933 trial.Conclusions:Overall, beam setup 3 was considered to be the best. It is worth mentioning that, apart from our results, the election of one of these beam arrangements might be dependent on the amount of patient workload at a specific institution.

Goal-Driven Beam Setting Optimization for Whole-Breast Radiation Therapy.

Purpose:To develop an automated optimization program to generate optimal beam settings for whole-breast radiation therapy driven by clinically oriented goals.Materials and Methods:Forty patients were retrospectively included in this study. Each patient’s planning images, contoured structures of planning target volumes, organs-at-risk, and breast wires were used to optimize for patient-specific–beam settings. Two beam geometries were available tangential beams only and tangential plus supraclavicular beams. Beam parameters included isocenter position, gantry, collimator, couch angles, and multileaf collimator shape. A geometry-based goal function was defined to determine such beam parameters to minimize out-of-field target volume and in-field ipsilateral lung volume. For each geometry, the weighting in the goal function was trained with 10 plans and tested on 10 additional plans. For each query patient, the optimal beam setting was searched for different gantry-isocenter pairs. Optimal fluence maps were generated by an in-house automatic fluence optimization program for target coverage and homogeneous dose distribution, and dose calculation was performed in Eclipse. Automatically generated plans were compared with manually generated plans for target coverage and lung and heart sparing.Results:The program successfully produced a set of beam parameters for every patient. Beam optimization time ranged from 10 to 120 s. The automatic plans had overall comparable plan quality to manually generated plans. For all testing cases, the mean target V95% was 91.0% for the automatic plans and 88.5% for manually generated plans. The mean ipsilateral lung V20Gy was lower for the automatic plans (15.2% vs 17.9%). The heart mean dose, maximum dose of the body, and conformity index were all comparable.Conclusion:We developed an automated goal-driven beam setting optimization program for whole-breast radiation therapy. It provides clinically relevant solutions based on previous clinical practice as well as patient specific anatomy on a substantially faster time frame.

Commissioning and Evaluation of a Third-Party 6 Degrees-of-Freedom Couch Used in Radiotherapy.

Purposes:The newly released Protura 6 degrees-of-freedom couch (CIVCO) has limited quality assurance protocols and pertinent publications. Herein, we report our experiences of the Protura system acceptance, commissioning, and quality assurance.Methods:The Protura system integration was tested with peripheral equipment on the following items: couch movement range limit, 6 degrees-of-freedom movement accuracy, weight test and couch sagging, system connection with Linac, isocentricity of couch and rotation alignment, kV and cone-beam computed tomography imaging of HexaCHECK with MIMI phantom (Standard Imaging), and an in-house custom 6 degrees-of-freedom quality assurance phantom. A couch transmission measurement was also performed.Results:The vertical, longitudinal, and lateral ranges of the 6 degrees-of-freedom couch pedestal are 43.9 to 0.0 cm, 24.6 to 149.5 cm, −20.6 to 20.7 cm, respectively. The couch movement accuracy was within 1 mm in all directions. The couch sagging with a 200 lbs (∼91 kg) evenly distributed object is 1.0 cm and 0.4° pitch in the distal end of the couch. The isocentricity of the couch was about 0.5 mm in diameter of all crosshair projections on the couch isocenter level, and the largest couch rotation alignment observed was (0.3°) at the couch angle of 90°. The deviation from the reference position (zero position) of the HexaCHECK phantom, measured by matching the cone-beam computed tomography with the reference planning computed tomography, was found to be below 0.2 mm in the anterior–posterior and right–left dimensions, 0.4 mm in superior–inferior dimension, and 0.1° in roll, pitch, and yaw directions.Conclusions:A 6 degrees-of-freedom quality assurance phantom is helpful for the commissioning and routine quality assurance tests. Due to the third-party integration with Linac, the system is prone to “double-correction” errors. A rigorous quality assurance program is the key to a successful clinical implementation of the Protura system.

4π Radiotherapy Using a Linear Accelerator: A Misnomer in Violation of the Solid Geometric Boundary Conditions in Three-Dimensional Euclidean Space.

Purpose:The concept of 4πc radiotherapy is a radiotherapy planning technique receiving much attention in recent times. The aim of this article is to disprove the feasibility of the 4π radiotherapy using a cantilever-type linear accelerator or any other external-beam delivery machines.Materials and Methods:A surface integral-based mathematical derivation for the maximum achievable solid angle for a linear accelerator was carried out respecting the rotational boundary conditions for gantry and couch in three-dimensional Euclidean space. The allowed movements include a gantry rotation of 0–2πc and a table rotation of .Results:Total achievable solid angle by cantilever-type linear accelerator (or any teletherapy machine employing a cantilever design) is , which is applicable only for the foot and brain radiotherapy where the allowed table rotation is 90°–0°–270°. For other sites such as pelvis, thorax, or abdomen, achievable solid angle as the couch rotation comes down significantly. Practically, only suitable couch angle is 0° by avoiding gantry–couch–patient collision.Conclusions:Present cantilever design of linear accelerator prevents achieving a 4π radian solid angle at any point in the patient. Even the most modern therapy machines like CyberKnife which has a robotic arm also cannot achieve 4π geometry. Maximum achievable solid angle under the highest allowable boundary condition(s) cannot exceed 2πc, which is restricted for only extremities such as foot and brain radiotherapy. For other parts of the body such as pelvis, thorax, and abdomen, the solid angle is reduced to 1/5th (maximum value) of the 4πc. To obtain a 4πc solid angle in a three-dimensional Euclidean space, the patient has to be a zero-dimensional point and X-ray head of the linear accelerator has a freedom to rotate in every point of a hypothetical sphere of radius 1 m. This article establishes geometrically why it is not possible to achieve a 4πc solid angle.

Extended SSD VMAT treatment for total body irradiation.

In this work, we develop a total body irradiation technique that utilizes arc delivery, a buildup spoiler, and inverse optimized multileaf collimator (MLC) motion to shield organs at risk. The current treatment beam model is verified to confirm its applicability at extended source‐to‐surface distance (SSD). The delivery involves 7–8 volumetric modulated arc therapy arcs delivered to the patient in the supine and prone positions. The patient is positioned at a 90° couch angle on a custom bed with a 1 cm acrylic spoiler to increase surface dose. Single‐step optimization using a patient CT scan provides enhanced dose homogeneity and limits organ at risk dose. Dosimetric data of 109 TBI patients treated with this technique is presented along with the clinical workflow. Treatment planning system (TPS) verification measurements were performed at an extended SSD of 175 cm. Measurements included: a 4‐point absolute depth‐dose curve, profiles at 1.5, 5, and 10 cm depth, absolute point‐dose measurements of an treatment field, 2D Gafchromic® films at four locations, and measurements of surface dose at multiple locations of a Alderson phantom. The results of the patient DVH parameters were: Body‐5 mm D98 95.3 ± 1.5%, Body‐5 mm D2 114.0 ± 3.6%, MLD 102.8 ± 2.1%. Differences between measured and calculated absolute depth‐dose values were all <2%. Profiles at extended SSD had a maximum point difference of 1.3%. Gamma pass rates of 2D films were greater than 90% at 5%/1 mm. Surface dose measurements with film confirmed surface dose values of >90% of the prescription dose. In conclusion, the inverse optimized delivery method presented in the paper has been used to deliver homogenous dose to over 100 patients. The method provides superior patient comfort utilizing a commercial TPS. In addition, the ability to easily shield organs at risk is available through the use of MLCs.

Direct comparison between surface imaging and orthogonal radiographic imaging for SRS localization in phantom.

PurposeSurface imaging (SI) offers a nonionizing, near real time alternative to radiographic imaging for intrafraction radiosurgery localization. In this work, we systematically compared a commercial SI system vs a commercial room mounted x‐ray localization system in phantom.MethodsAn anthropomorphic head phantom with fiducial markers was imaged with linear accelerator on‐board x‐ray imaging, SI, and room mounted x‐ray imaging (RM) at ±45° and ±90° couch angles for three different head tilts and six different isocenters (72 total positions). The shifts generated by the three systems were compared as functions of couch angle, head tilt, and isocenter position with the on‐board imaging shifts used as ground truth. Two sample Kolmogorov–Smirnov tests were used to evaluate equivalence of the groups.ResultsThe magnitude of the displacement vectors for RM minus on‐board imaging and SI minus on‐board imaging over all 72 phantom positions were 0.7 ± 0.3 mm for both cases. The RM and SI showed no significant difference based on couch angle or isocenter position. Both systems showed decreasing accuracy with increasing couch angle, but both systems agreed with ground truth to <=1.1 mm at all couch angles. The exaggerated chin‐up head orientation showed significantly different shifts for SI and RM based on increased variance in the SI measurements, although both had submillimeter accuracy on average. The standard deviation of the real time SI displacement vector was <0.06 mm over all measurements, during which the on‐board imaging panels partially blocked the lateral camera pods for half the time.Conclusions RM and SI showed similar accuracy over measurements at 72 different phantom positions. SI showed minimal performance loss with camera pods blocked. SI is a feasible option for intra‐fraction radiosurgery localization based on these phantom measurements.

196. Preliminary comparison between two different stereotactic machines, robot based and linac based, by means of a 3D phantom

Purpose Two machines, Accuray Cyberknife and Varian linac TrueBeam Novalis STx, equipped with 6MV Flattering Filter Free (FFF) and stereotactic cones were used to compare similar plans on a 3D phantom in order to evaluate differences in the dose distributions. Moreover, this work aims to investigate the possibility to use with Cyberknife the same 3D phantom correction factors already applied with conventional linacs setup. Methods Analysis was carried out with Scandidos Delta4 Phantom Plus (1069 p diodes types with 0.78 mm2 active area) on which a Cyberknife plan and a TrueBeam plan with identical ballistics were superimposed for a series of measurements of narrow beams. The setup, originally selected from Cyberknife, was converted into linac coordinates (gantry and couch angles) with the same cone size. The plan for the TrueBeam was calculated and delivered using two energies, 6MV and 6MV FFF. Results First, measurements were carried out on TrueBeam with separate dose fields, in order to evaluate the feasibility of narrow beams to be intercepted by diodes arrays in the phantom with different phantom tilt setups (0°, −4°), obtaining these gamma values: 99.0% with 6 MV, 98.8% with 6 MV FFF (local gamma, 2%-2 mm, threshold 20%). Then the delivered doses were compared obtaining a gamma value of 82.1% for 6 MV and 84.1% for 6 MV FFF. Conclusion The two machines were similar in terms of dose fall (with a steeper gradient for Cyberknife), dose coverage of target and in general clinical acceptance of the plans. The 3D phantom was initially used on both machines with the same correction factors and it was able to detect dose differences when properly irradiated with beams with a no-parallel incidence on plates (4° was enough in the setup, varying agreement for the same plan from 83% gamma value to 99%).

Treating Benign Cranial Lesions - Treatment Planning System Comparison

To compare three treatment planning systems for treating benign cranial lesions using stereotactic radiosurgery (SRS). Three treatment planning software systems were compared for treating benign cranial lesions. iPlan (Brainlab, Germany) is a dedicated software for SRS planning using non-coplanar dynamic conformal arcs. The dose prescription, arc setup and MLC positions are planner controlled. Cranial SRS (Brainlab, Germany) (CSRS) is a new software for treating benign cranial lesions using up to 5 intensity-modulated arcs. The user can select the degree of modulation, (from minimal modulation to full VMAT). The couch angles, number of arcs and their lengths are optimization parameters and the treatment planning is highly automated. Eclipse RapidArc (Varian, Palo Alto) (RA) uses an inverse planning approach where the planner determines the number of arcs, their lengths and couch angles, as well as the optimization objectives for the PTV and organs at risk (OARs). The optimization parameters can be adjusted "on the fly" as the optimization progresses. The comparison metrics were conformity index (CI), gradient index (GI), volume of brain receiving over 12 Gy (V12) and mean brain dose between the three TPSs. We compared 10 plans for acoustic neuromas and meningiomas to plans generated and treated by iPlan, our current clinical standard. We also compared 4 cases that were treated with RA due to unusual shape, for which iPlan did not yield acceptable plans. All CSRS and RA plans for all indications were judged clinically acceptable. The average CI was 1.38 and 1.30 for CSRS and RA, respectively, and the average GI was 3.76 and 4.11 for CSRS and RA, respectively. No metrics were significantly different compared to iPlan. However, for those lesions for which iPlan could not generate acceptable plans, treatment plans with CSRS were more homogenous than those generated by RA. Planning times were significantly shorter for CSRS compared to the other TPSs. This was especially noticeable for iPlan, where MLC leaf shaping is a manual, labor-intensive process. Eclipse took much longer to perform the actual calculation using a 1 mm grid than either Brainlab TPS. iPlan used less MUs compared to either RA or CSRS: an average of 1910 MUs for iPlan, 3883 MUs for RA and 2033 MUs for CSRS. Planning times for CSRS were much shorter than for either iPlan or RA and planning was more efficient due to the high level of automation. However, the need to pre-define settings in a template outside the CSRS workspace can be cumbersome. Higher modulation in CSRS led to lower CI and GI values and lower OAR doses without affecting planning times. For complex lesions for which iPlan was not adequate, both VMAT TPSs yielded highly conformal treatment plans that satisfied OAR constraints, however CSRS was faster to plan, gave more homogeneous dose distributions and used less MUs.

A Dosimetric Analysis of Dorsal Nerve Root Ablation Therapy using a Virtual Cone Technique

Virtual cone radiosurgery has been established as an effective treatment for trigeminal neuralgia. This delivery technique employs multiple static-field arcs with HDMLC-defined apertures to deliver high doses to small target volumes (<1cc) with sharp dose gradients. As a replacement to a physical cone, virtual cone delivery has the advantage of reduced treatment times and superior intra-fractional motion monitoring capability, while eliminating the need for Co-60 radiosurgery units. There is currently clinical interest in employing virtual cone techniques in the ablation of dorsal nerve root ganglia as a treatment for chronic thoraco-abdominal pain. We hypothesize that a standard template for single-fraction virtual cone treatment may be established to achieve ablative 90Gy point doses to the ganglia while adhering to conventional OAR dose constraints. This would enable the first such application of virtual cone treatment outside of cranial cases. Dosimetric analysis of virtual cone plans was performed within our institution’s treatment planning system using a linear Boltzman transport equation model on a 1mm grid. 10MV flattening filter-free beams were used at a 2400 MU/minute dose rate. Calculation accuracy was assessed using orthogonal film measurements in an anatomical phantom. Film-based verification indicated that dose discrepancies relative to the calculation model were <4%. Having established the accuracy of our calculations, various arc arrangements, HDMLC-defined field sizes, and collimator angles were investigated. The optimal field arrangement involved 10 equally weighted arcs at 0, ±7.5, and ±15 degree couch angles. For left-nerve root treatment, arcs ran from 10-170 degrees, with a 60-120 degree skip arc (with mirrored arcs for right-nerve root treatment). Delivery time was 25 minutes. The four innermost HDMLC leaves formed a 10mm x 4mm aperture, with jaws at 11mm x 5mm. ±5 degree collimator angles were used in repeated arcs. Doses were normalized to a 90Gy point dose at the target center. In both a thorax phantom and a test patient plan, this arrangement produced a 60Gy isodose surface approximately 5.5mm x 7.5mm x 7.5mm along the lateral, A-P, and S-I axes, respectively. The maximum dose to the spinal cord was 4.14Gy, and the maximum dose to the esophagus was 12.08Gy, with a V12 of <0.001cc. Results of this dosimetric study indicate that ablative doses can be delivered to dorsal nerve root ganglia while respecting normal tissue constrains. Furthermore, due to the robustly conformal dose profile achieved and the elimination of inverse-planning, it is probable that this virtual cone technique can be safely employed as a standardized template treatment for most patients (subject to dose constraint verification), eliminating the need for time-consuming optimization and allow for the expedited treatment of patients with intractable pain.