Couch Rotation Research Articles

Volumetric Modulated Arc Therapy (VMAT): A modern radiotherapy technique - A single institutional experience.

Objective:To evaluate VMAT plans for conformity and homogeneity of radiation dose to the target in order to share our experience as a pioneering institute to use VMAT technology in Pakistan.Methods:Since December 2014 to January 2018, 530 patients of various anatomical sites were treated by VMAT technique at Neurospinal Cancer Care Institute (NCCI) Karachi Pakistan. ERGO++ planning system (Version 1.7.2) was used to develop VMAT plans with single or multiple arcs by the rotation of couch and gantry. The plans were evaluated by calculating Conformity Index (CI) and Homogeneity Index (HI) and critical organ (OARs) doses of individual tumor sites.Results:The average CI of various sites was 1.4 (range: 1.0–2.0) and average HI of various sites was 1.20 (range: 1.07–1.374), respective critical organ doses were adequately achieved.Conclusions:VMAT treatment planning technique showed good conformal and homogeneous target coverage with sparing of organs at risk and reduced treatment delivery time. With these features, safety of VMAT technique may allow its routine clinical use, though it is still under investigation in many areas.

Simultaneous Optimization of Radiation-Imaging Coincidence for a Multi-Energy Linac.

Introduction:Medical physics guidelines stress the importance of radiation-imaging coincidence, especially for stereotactic treatments. However, multi-energy linear accelerators may only allow a single imaging isocenter. A procedure was developed to simultaneously optimize radiation-imaging isocenter coincidence for all linac photon energies on a Versa HD.Materials and Methods:First, the radiation beam center of each energy was adjusted to match the collimator rotation axis using a novel method that combined ion chamber measurements with a modified Winston-Lutz (WL) test using images only at gantry, couch, and collimator angles of 0°. With all energies properly steered, an 8-field WL test was performed to determine average linac isocenter position across all energies, gantry, and collimator angles. Lasers and the kV imaging isocenter were calibrated to the average linac isocenter of all photon energies. Finally, A 12-field WL test consisting of gantry, couch, and collimator rotations was used to adjust the couch rotation axis to the average linac isocenter, thereby minimizing overall radiation-imaging isocentricity of the system.Results:Using this method, the beam centers were calibrated within 0.10 mm of collimator rotation axis, and linac isocenter coincidence was within 0.20 mm for all energies. Couch isocenter coincidence was adjusted within 0.20 mm of average linac isocenter. Average radiation-imaging isocentricity for all energies was 0.89 mm (0.80–0.98 mm) for a single imaging isocenter.Conclusion:This work provides a method to adjust radiation-imaging coincidence within 1.0 mm for all energies on Elekta's Versa HD.

Non-coplanar VMAT plans for postoperative primary brain tumour to reduce dose to hippocampus, temporal lobe and cochlea: a planning study.

Objectives:This study aimed to compare radiotherapy plan quality of coplanar volumetric modulated arc therapy (CO-VMAT) and non-coplanar VMAT (NC-VMAT) for post-operative primary brain tumour.Methods:A total of 16 patients who were treated for primary brain tumours were retrospectively selected for this study. For each patient, identical CT sets with structures were used for both CO-VMAT and NC-VMAT planning. For CO-VMAT, one full arc and two coplanar half arcs were used. For NC-VMAT, one full coplanar and two non-coplanar half arcs with couch rotation of 315° or 45° were used. Dose constraints were adhered to the RTOG 0614, RTOG 0933 and TMH protocol. Dose volumetric parameters were collected for statistical analysis.Results:.NC-VMAT achieved significant dose reduction in contralateral hippocampus, both temporal lobes and cochleae, and other OARs while the plan qualities remained the same. In particular, NC-VMAT decreased contralateral hippocampus mean dose by 1.67Gy. Similarly, the NC-VMAT decreased temporal lobe mean dose by 6.29Gy and 2.8Gy for ipsilateral and contralateral side respectively. Furthermore, it decreased cochlea mean dose by 5.34Gy and 0.97Gy for ipsilateral and contralateral side respectively. Overall, there was a reduction of 5.4% of normal brain tissue volume receiving low dose irradiation.Conclusion:The proposed NC-VMAT showed more favourable plan quality than the CO-VMAT for primary brain tumours, in particular to hippocampus, temporal lobes, cochleae and OARs located to the contralateral side of tumours.Advances in knowledge:For primary brain tumours radiotherapy, NC-VMAT can reduce doses to the hippocampus, both temporal lobes, and cochleae, as well as OARs located to the contralateral side of tumours.

Radiobiological effects of the interruption time with Monte Carlo Simulation on multiple fields in photon beams.

PurposeThe interruption time is the irradiation interruption that occurs at sites and operations such as the gantry, collimator, couch rotation, and patient setup within the field in radiotherapy. However, the radiobiological effect of prolonging the treatment time by the interruption time for tumor cells is little evaluated. We investigated the effect of the interruption time on the radiobiological effectiveness with photon beams based on a modified microdosimetric kinetic (mMK) model.MethodsThe dose‐mean lineal energy yD (keV/µm) of 6‐MV photon beams was calculated by the particle and heavy ion transport system (PHITS). We set the absorbed dose to 2 or 8 Gy, and the interruption time (τ) was set to 1, 3, 5, 10, 30, and 60 min. The biological parameters such as α0, β0, and DNA repair constant rate (a + c) values were acquired from a human non‐small‐cell lung cancer cell line (NCI‐H460) for the mMK model. We used two‐field and four‐field irradiation with a constant dose rate (3 Gy/min); the photon beams were paused for interruption time τ. We calculated the relative biological effectiveness (RBE) to evaluate the interruption time's effect compared with no interrupted as a reference.ResultsThe yD of 6‐MV photon beams was 2.32 (keV/µm), and there was little effect by changing the water depth (standard deviation was 0.01). The RBE with four‐field irradiation for 8 Gy was decreased to 0.997, 0.975, 0.900, and 0.836 τ = 1, 10, 30, 60 min, respectively. In addition, the RBE was affected by the repair constant rate (a + c) value, the greater the decrease in RBE with the longer the interruption time when the (a + c) value was large.ConclusionThe ~10‐min interruption of 6‐MV photon beams did not significantly impact the radiobiological effectiveness, since the RBE decrease was <3%. Nevertheless, the RBE's effect on tumor cells was decreased about 30% by increasing the 60 min interruption time at 8 Gy with four‐field irradiation. It is thus necessary to make the interruption time as short as possible.

Validation and practical implementation of seated position radiotherapy in a commercial TPS for proton therapy.

This work aims to validate new 6D couch features and their implementation for seated radiotherapy in RayStation (RS) treatment planning system (TPS). In RS TPS, new 6D couch features are (i) chair support device, (ii) patient treatment option of "Sitting: face towards the front of the chair", and (iii) patient support pitch and roll capabilities. The validation of pitch and roll was performed by comparing TPS generated DRRs with planar x-rays. Dosimetric tests through measurement by 2D ion chamber array were performed for beams created with varied scanning and treatment orientation and 6D couch rotations. For the implementation of 6D couch features for treatments in a seated position, the TPS and oncology information system (Mosaiq) settings are described for a commercial chair. An end-to-end test using an anthropomorphic phantom was performed to test the complete workflow from simulation to treatment delivery. The 6D couch features were found to have a consistent implementation that met IEC 61712 standard. The DRRs were found to have an acceptable agreement with planar x-rays based on visual inspection. For dose map comparison between measured and calculated, the gamma index analysis for all the beams was >95% at a 3% dose-difference and 3mm distance-to-agreement tolerances. For an end-to end-testing, the phantom was successfully set up at isocenter in the seated position and treatment was delivered. Chair-based treatments in a seated position can be implemented in RayStation through the use of newly released 6D couch features.

Evaluation of the RUBY modular QA phantom for planar and non-coplanar VMAT and stereotactic radiations.

PurposeThis study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac.Material and MethodsThe daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated.ResultsAll systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria.ConclusionAll the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

Validating robotic couch isocentricity with 3D surface imaging.

BackgroundA proton therapy system with 190° gantries uses robotic couch rotations to change the treatment beam laterality. Couch rotations are typically validated clinically with post‐rotation radiographic imaging.AimsThis study assesses the specificity and sensitivity of a commercial 3D surface imaging system, AlignRT (Vision RT, London UK) for validating couch rotations.Materials & MethodsIn clinical operation, a reference surface image of the patient is acquired after radiographic setup with couch at 270°, perpendicular to the gantry axis of rotation. The couch is then rotated ±90° to a typical treatment angle, and AlignRT reports a 3D displacement vector. Patient motion, changes in patient surface, non‐coincidence between AlignRT and couch isocenter, and mechanical couch run‐out all contribute to the 3D vector magnitude. To assess AlignRT sensitivity in detecting couch run‐out, volunteers were positioned orthogonal to the proton gantry and reference surface images were captured without x‐ray localization. Subjects were repeatedly rotated ±90⁰ to typical treatment angles and displacement vectors were recorded. Additionally, measurements were performed in which intentional translations of 2, 4, 6, and 8 mm were combined with the intended isocentric rotations. Data sets were collected using a phantom; subjects with a thoracic isocenter and no immobilization; and subjects with a cranial isocenter and thermoplastic immobilization. A total of 300 rotations were measured.ResultsDuring isocentric rotations, the mean AlignRT displacement vectors for the phantom, immobilized, and non‐immobilized volunteers were 0.1 ± 0.1 mm, 0.8 ± 0.1 mm, and 1.1 ± 0.2 mm respectively. 95% of the AlignRT measurements for the immobilized and non‐immobilized subjects were within 1 mm and 2 mm of the actual displacement respectively.DiscussionAfter characterizing the accuracy using phantoms and volunteers, we have shown that a three‐pod surface imaging system can be used to identify gross non‐isocentric patient rotations. Significant positional deviations, either due to improper couch rotation or patient motion, should be followed by radiographic imaging and repositioning.ConculsionAlignRT can be used to verify patient positioning following couch rotations that are applied after the initial x‐ray guided patient setup. Using a three‐pod AlignRt system, positional deviations exceeding 4 mm were flagged with sensitivity and specificity of 90% and 100% respectively.

The potential of an optical surface tracking system in non-coplanar single isocenter treatments of multiple brain metastases.

To evaluate the accuracy of a commercial optical surface tracking (OST) system and to demonstrate how it can be implemented to monitor patient positioning during non‐coplanar single isocenter stereotactic treatments of brain metastases. A 3‐camera OST system was used (Catalyst HD™, C‐RAD) on a TruebeamSTx with a 6DoF couch. The setup accuracy and agreement between the OST system, and CBCT and kV‐MV imaging at couch angles 0° and 270°, respectively, were examined. Film measurements at 3 depths in the Rando‐Alderson phantom were performed using a single isocenter non‐coplanar VMAT plan containing 4 brain lesions. Setup of the phantom was performed with CBCT at couch 0° and subsequently monitored by OST at other couch angles. Setup data for 7 volunteers were collected to evaluate the accuracy and reproducibility of the OST system at couch angles 0°, 45°, 90°, 315°, and 270°. These results were also correlated to the couch rotation offsets obtained by a Winston‐Lutz (WL) test. The Rando‐Alderson phantom, as well as volunteers, were fixated using open face masks (Orfit). For repeated tests with the Rando‐Alderson phantom, deviations between rotational and translational isocenter corrections for CBCT and OST systems are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the film and TPS doses in the center of the 4 lesions were −1.2%, −0.1%, −0.0%, and −1.9%. Local gamma evaluation criteria of 2%/2 mm and 3%/1 mm yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for the 4 lesions. Regarding the volunteers, the mean translational and rotational isocenter shift values were (0.24 ± 0.09) mm and (0.15 ± 0.07) degrees. Largest isocenter shifts were found for couch angles 45˚ and 90˚, confirmed by WL couch rotation offsets. Patient monitoring during non‐coplanar VMAT treatments of brain metastases is feasible with submillimeter accuracy.

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.

Non-coplanar VMAT plans for lung SABR to reduce dose to the heart: a planning study.

This study aimed to compare the plan quality of non-coplanar partial arc (NPA) volumetric modulated arc therapy (VMAT) to that of coplanar partial arc (CPA) VMAT for stereotactic ablative radiotherapy (SABR) for lung cancer. A total of 20 patients treated for lung cancer with the SABR VMAT technique and whose lung tumors were close to the heart were retrospectively selected for this study. For the CPA VMAT, three coplanar half arcs were used while two coplanar half arcs and one noncoplanar arc rotating 315°-45° with couch rotations of 315° ± 5° were used for the NPA VMAT. For each patient, identical CT image sets and identical structures were used for both the CPA and NPA VMAT plans. Dose-volumetric parameters of each plan were analyzed. For the planning target volume and both lungs, no statistically significant differences between the CPA and NPA VMAT plans were observed in general. For the heart, average values of D0.1cc of the CPA and NPA VMAT plans were 29.42 ± 13.37 and 21.71 ± 9.20 Gy, respectively (p < 0.001). For whole body, the mean dose and the gradient index of the CPA VMAT plans were 1.2 ± 0.5 Gy and 4.356 ± 0.608 while those of the NPA VMAT plans were 1.1 ± 0.5 Gy and 4.111 ± 0.480, respectively (both with p < 0.001). The NPA VMAT proposed in this study showed more favorable plan quality than the CPA VMAT plans for lung SABR with tumors located close to the heart. For lung SABR, NPA VMAT can reduce doses to the heart as well as whole-body irradiation.

Automated Planning to Improve Linac-Based Stereotactic Radiosurgery Plan Quality in Patients with 4 up to 10 Brain Metastases

Stereotactic radiosurgery (SRS) is a promising treatment option for patients with multiple brain metastases (BM). We studied whether automated planning can improve LINAC-based SRS plan quality for multiple BM. Additionally, the use of one isocenter instead of separate isocentres for each metastasis was studied. For 12 patients with 4 up to 10 brain metastases, five non-coplanar LINAC-based SRS plans were created for 6MV photons: a manually planed dynamic conformal arc (DCA) plan with a separate isocenter for each metastasis, a dynamic IMRT plan with one isocenter, a VMAT plan with one isocenter, two DCA plans with one isocenter for 3 and 5 couch rotations. The last three plans were automatically generated. The maximum allowed gross tumor volume to planning target volume (PTV) margin was 1 mm. The prescription dose was 21 Gy or 18 Gy in a single fraction or 8.5 Gy in 3 fractions depending on the PTV volume. All plans were calculated with Monte Carlo, except the VMAT which was calculated with Collapsed Cone algorithm. The PTV coverage should be at least 98%. To assess SRS plan quality, the Paddick conformity index (CI), the Paddick gradient index (GI), the total V12Gy and V5Gy were studied and reported as MEAN±1SD. The number of monitor units (MU) and irradiation time were also compared. The mean total CI was the highest for dynamic IMRT (see table). The lowest GI was for DCA plans with the separate-isocenter approach (3.7±0.6) and automatically generated DCA plans with one isocenter, whereas the highest GI was for VMAT plans (7.0±3.0). Moreover, the GI was 4.4±0.6 and 4.2±0.6 for automatically generated DCA plans with 3 couch and 5 couch rotations, respectively. The total V12Gy of automatically generated DCA plans with one isocenter (4.7±5.6% and 4.1±4.3% for 3 and 5 couch rotations, respectively) and dynamic IMRT plans (4.4±4.5%) were comparable with the manual DCA plans (3.6±3.7%). For the total V5Gy, quite the same behaviour as for the GI was found. The number of MU was the smallest for VMAT plans, followed by automatically generated DCA and IMRT plans and much lower than the manual DCA plans. The irradiation time was the smallest for automatically planned DCA plans followed by VMAT and IMRT plans. Automatically generated LINAC-based SRS plans for multiple BM in one isocenter can make the number of MU and irradiation time smaller with a comparable plan quality to manual DCA plans with the separate-isocenter approach. Based on all compared parameters, dynamic IMRT and DCA plans with one isocenter were the best and comparable with manual DCA plans with separate isocentres for each metastasis. Moreover, we can conclude that automated DCA plans with 5 couch and 3 couch rotations were comparable.Abstract 3798; Table 1Conf IGIV12Gy [%]V5Gy [%]MUIrradiation time [s]Separate-isocenter DCA0.713.73.619146641832IMRT 1 isoc0.724.84.42675171235VMAT 1 isoc0.707.05.7374123976Multimets DCA 1 isoc 3 table rotations0.694.44.7206130820Multimets DCA 1 isoc 5 table rotations0.714.24.1185403724 Open table in a new tab

Evaluation of the Residual Rotation Error of a 6 DOF Robotic Couch Using Optical Surface and CBCT.

Rotation setup errors for SRS treatments are typically less than two degrees which usually causes less dosimetry discrepancy for a typical SRS treatment as the target size is likely less than 4 cm. However this might not be true for the single isocenter and multiple metastatic targets’ technique where, the distances between the isocenter of the treatment and those tumor centers can be very large. A 6DOF couch can be used to correct this setup error by using optical surface and/or Conebeam CT (CBCT) images. The accuracy and consistence between optical surface and CBCT images were studied. The CBCT system and optical surface images of an SRS phantom were taken at different pitch, roll, and rotation angles varied from -3 to 3 degrees by intentionally changing values on the service console. A total 39 combinations of pitch, roll, and rotations using the 6DOF couch were generated. The optical surface images were taken and Vision RT software was used to determine the rotation angles and isocenter shifts. Eight CBCT images corresponding to +- 3 degree changes in pitch, roll, and rotation were taken. The center positions of the 5 tungsten carbide BBs contained in the SRS QA phantom were determined using in-house software. A least square fit was used to determine translation/rotation parameters. Among the 39 calculations from optical images, the maximum rotation angle differences were 0.2, 0.1, and -0.2 degrees for pitch, roll, and couch rotation, respectively. The maximum isocenter shifts were -0.80 mm, 0.70 mm, and -0.10 mm in lateral, longitudinal, and vertical axes, respectively. The maximum residual rotation angles from analysis of CBCT images were 0.20 degree for pitch, roll, and rotation. By using 6DOF couch, the rotation errors can be reduced dramatically compared to initial rotation errors. To overcome this rotation errors without 6DOF couch, one has to introduce extra margin for target which is far away from the treatment iso-center. Six DOF can avoid extra CTV-PTV margin. The residual rotational errors studied for the couch were less than 0.2 degree. This accuracy for the 6DOF system is sufficient for SRS applications even for single isocenter multiple tumor technique and extra CTV-PTV margin might be not necessary for some cases.

The Impact of CBCT Imaging and Setup Accuracy on Spinal Cord Dose in SBRT Treatment

IGRT is an integral aspect of spine SBRT treatment delivery, allowing for accurate patient positioning and high dose to tumor while sparing dose to critical nerves. Meanwhile, intrafraction imaging with kV CBCT introduces additional radiation dose to the peripheral tissues, bone, and spinal cord, which is unaccounted for in treatment planning, with implications particularly for pediatric patients or those at high risk of myelopathy in setting of reirradiation. IGRT to verify our institutional tolerance of 1 mm and 1° between arcs may prolong treatment time and introduce further setup uncertainty. To optimize the imaging workflow (prioritizing KV-2D-3D vs KV-CBCT if sufficient) for achieving practical set up accuracy while limiting the imaging dose received, the cumulative CBCT imaging dose to the spinal cord and the dose perturbation induced by patient setup error were investigated. CBCT imaging dose to spinal cord was derived by correlating our institutional CTDI QA assessment on 4 linear accelerators and 3 C-Series LINACs with the reference value provided in the current literature. The clinical pelvis protocol CTDI was measured with the ACR torso phantom with 10cm pencil chamber, 125kVp, 1080mAs on liner accelerators and 125kVp, 80mA, 13ms on C-Series, respectively. Dosimetry change from setup uncertainty was analyzed by shifting the VMAT plan isocenter by 1 mm in Y direction and 1° couch rotation from the original planning isocenter on 5 spine SBRT patients, prescribed a course of 30Gy in 5 fractions. Dose differences were evaluated in terms of the target volume coverage D95, the spinal cord D0.035cc, D0.1cc and Dmax by re-calculating the plan with the same beam geometry, MLC motion, and MUs as the original plans. Differences between measured and vendor specified CTDI was 14% ± 11%SD (n = 7). Correlating the CBCT imaging dose reference based on Rando phantom measurement and Monte Carlo simulation in the literature, imaging dose to the spinal cord in our clinic was estimated as 3 - 4cGy per pelvis protocol CBCT. Dosimetry change with respect to target D95 showed an average deviation of 0.2% prescription dose with 1mm and 1° positioning uncertainty from the planning intents (n = 5). In the worst-case, the spinal cord with 1mm expansion had a max dose (D0.035cc) increase by an average of 132cGy (4.4% Rx) and with a max of 245cGy (8.2% Rx). CBCT dose from multiple scans (conventionally initial, mid and post) contribute to the total dose of the spinal cord. Mid-CBCTs may also prolong the treatment and introduce additional uncertainty. Our current patient positioning accuracy, effectively achieved with use of multiple 2D-3D kV images and minimized CBCT verification scans per fraction, does not produce any significant change in patient dose distributions in the target or spinal cord. Customized CBCT protocols, optical monitoring system (OSMS), or use of MR-LINAC could potentially further reduce imaging dose related to intrafraction monitoring for spine SBRT.

Intra-Fraction Motion Analysis for Frameless Linac-Based Stereotactic Radiosurgery with Three-Dimension (3D) Optical Surface Imaging for Intra-Fractional Motion Management

There has been a rapid increase in the use of frameless SRS in place of the frame-based SRS for treatment of brain metastases (BM). Compared to the frame-based SRS, intra-fraction motion management is a major challenge for the frameless SRS. With advanced immobilization and patient monitoring technologies, intra-fraction motion can be reduced. The purpose of this study is to analyze intra-fraction motion with LINAC-based SRS using a frameless radiosurgery platform that uses an integrated 3D optical surface monitoring system for intra-fraction motion management. This retrospective study includes 97 LINAC-SRS treatments delivered to 60 patients of BM with an integrated 3D surface imaging software for intra-fraction monitoring. Every treatment was delivered with a 6MV FFF beam at a dose-rate of 1400MU/min. Each treatment had at least 3 non-coplanar arcs with a minimum of 3 couch rotations. The patients were immobilized using an open-faced rigid SRS-thermoplastic mask with an opening encompassing the eyes, nose, and part of their forehead. Each patient was first aligned using a high-quality cone-beam CT (CBCT) with 1mm slice thickness and positioned using a 6 degrees of freedom couch. The patient’s face was monitored during treatment delivery through the surface monitoring system. During delivery, beam holds occurred if patient motion exceeded pre-defined tolerance (1mm translation/1° rotation) for > 5 seconds. An additional CBCT was performed following the delivering of two arcs to assess for any intra-fraction motion. Each CBCT was then registered to the planning CT using the automated intensity-based registration algorithm, followed by confirmation with a manual check. The intra-fraction shifts (RL, AP, and SI) and rotation (yaw, roll, and pitch) data were retrospectively recorded and analyzed. The total magnitude of the patient intra-fraction motion for all ninety-seven SRS treatments as recorded through shifts were 0.4 mm (mean) ± 0.3 mm (stv). Translational shifts were 0.0±0.3 mm in RL, 0.0±0.3 mm in AP, 0.0±0.3 mm in SI, while the rotational variations were 0.0±0.4° in yaw, 0.0±0.2° in roll, and 0.0±0.2° in pitch. In total, 93.8%(n=91/97) delivered treatments had intra-fraction motion that was ≤1mm and ≤1 degree. Minimal intra-fraction motion was observed during frameless-based LINAC-SRS treatments when using an integrated 3D surface image tracking; Only 6% percent of treatments experienced motion that was >1mm or >1 degree. A planning target margin of ≤1 mm may be considered adequate during the planning process to account for intra-fraction motion in the majority of all cases.

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.

Dosimetric accuracy of dynamic couch rotation during volumetric modulated arc therapy (DCR-VMAT) for primary brain tumours

Radiotherapy treatment plans using dynamic couch rotation during volumetric modulated arc therapy (DCR-VMAT) reduce the dose to organs at risk (OARs) compared to coplanar VMAT, while maintaining the dose to the planning target volume (PTV). This paper seeks to validate this finding with measurements. DCR-VMAT treatment plans were produced for five patients with primary brain tumours and delivered using a commercial linear accelerator (linac). Dosimetric accuracy was assessed using point dose and radiochromic film measurements. Linac-recorded mechanical errors were assessed by extracting deviations from log files for multi-leaf collimator (MLC), couch, and gantry positions every 20 ms. Dose distributions, reconstructed from every fifth log file sample, were calculated and used to determine deviations from the treatment plans. Median (range) treatment delivery times were 125 s (123–133 s) for DCR-VMAT, compared to 78 s (64–130 s) for coplanar VMAT. Absolute point doses were 0.8% (0.6%–1.7%) higher than prediction. For coronal and sagittal films, respectively, 99.2% (96.7%–100%) and 98.1% (92.9%–99.0%) of pixels above a 20% low dose threshold reported gamma <1 for 3% and 3 mm criteria. Log file analysis showed similar gantry rotation root-mean-square error (RMSE) for VMAT and DCR-VMAT. Couch rotation RMSE for DCR-VMAT was 0.091° (0.086–0.102°). For delivered dose reconstructions, 100% of pixels above a 5% low dose threshold reported gamma <1 for 2% and 2 mm criteria in all cases. DCR-VMAT, for the primary brain tumour cases studied, can be delivered accurately using a commercial linac.

Small animal image-guided radiotherapy

Small animal image-guided radiotherapy

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.

Evaluation of patient positional reproducibility on the treatment couch and its impact on dose distribution using rotating gantry system in scanned carbon-ion beam therapy

PurposeThe daily variations in patient setup may cause beam range uncertainties. We evaluated the reproducibility of relative position between the patient and the treatment couch throughout the treatment course and assessed its effects on dose distributions when a beam passes through treatment couch using rotating gantry system. MethodsWe enrolled 1023 patients (=13072 fractions) treated by carbon-ion pencil beam scanning therapy. Seven treatment sites including prostate, head and neck, bone and soft tissue, rectum, liver, lung, and pancreas were investigated. Inter-fractional changes in couch position relative to the patient were defined as translational errors. Changes in couch rotation were defined as rotational errors. Treatment planning was performed for 4 patients in each of the treatment sites. Dose distributions were then re-calculated after the couch was shifted according to average, 95th percentile, and maximum values of translational error. ResultsLarge positional errors (>1.5 cm) were observed in 5% of treatment fractions. Positional errors were largest in prostate and pancreas patients, while smallest in head and neck and lung patients. There were no or only small changes in PTV-D95 and CTV-D95 values for almost all treatment sites. Clinically significant changes were observed in the duodenum (difference in D2cc values ranged from −55% to 28% with maximum couch shift) in pancreas treatment. ConclusionsAlthough underdosage to the PTV or CTV was limited, significant overdoses to organs at risk were found. The improvement of immobilization technique and appropriate selection of gantry angles could reduce the uncertainties due to changes in patient position.