Martin J. Menten

Lung stereotactic body radiotherapy with an MR-linac – Quantifying the impact of the magnetic field and real-time tumor tracking

Background and purpose There are concerns that radiotherapy doses delivered in a magnetic field might be distorted due to the Lorentz force deflecting secondary electrons. This study investigates this effect on lung stereotactic body radiotherapy (SBRT) treatments, conducted either with or without multileaf collimator (MLC) tumor tracking. Material and methods Lung SBRT treatments with an MR-linac were simulated for nine patients. Two different treatment techniques were compared: conventional, non-tracked deliveries and deliveries with real-time MLC tumor tracking, each conducted either with or without a 1.5 T magnetic field. Results Slight dose distortions at air-tissue-interfaces were observed in the presence of the magnetic field. Most prominently, the dose to 2% of the skin increased by 1.4 Gy on average. Regardless of the presence of the magnetic field, MLC tracking was able to spare healthy tissue, for example by decreasing the mean lung dose by 0.3 Gy on average, while maintaining the target dose. Conclusions Accounting for the magnetic field during treatment plan optimization allowed for design and delivery of clinically acceptable lung SBRT treatments with an MR-linac. Furthermore, the ability of MLC tumor tracking to decrease dose exposure of healthy tissue, was not inhibited by the magnetic field.

Comparison of a multileaf collimator tracking system and a robotic treatment couch tracking system for organ motion compensation during radiotherapy

PURPOSE One limitation of accurate dose delivery in radiotherapy is intrafractional movement of the tumor or the entire patient which may lead to an underdosage of the target tissue or an overdosage of adjacent organs at risk. In order to compensate for this movement, different techniques have been developed. In this study the tracking performances of a multileaf collimator (MLC) tracking system and a robotic treatment couch tracking system were compared under equal conditions. METHODS MLC tracking was performed using a tracking system based on the Siemens 160 MLC. A HexaPOD robotic treatment couch tracking system was also installed at the same linac. A programmable 4D motion stage was used to reproduce motion trajectories with different target phantoms. Motion localization of the target was provided by the 4D tracking system of Calypso Medical Inc. The gained positional data served as input signal for the control systems of the MLC and HexaPOD tracking systems attempting to compensate for the target motion. The geometric and dosimetric accuracy for the tracking of eight different respiratory motion trajectories was investigated for both systems. The dosimetric accuracy of both systems was also evaluated for the tracking of five prostate motion trajectories. RESULTS For the respiratory motion the average root mean square error of all trajectories in y direction was reduced from 4.1 to 2.0 mm for MLC tracking and to 2.2 mm for HexaPOD tracking. In x direction it was reduced from 1.9 to 0.9 mm (MLC) and to 1.0 mm (HexaPOD). The average 2%/2 mm gamma pass rate for the respiratory motion trajectories was increased from 76.4% for no tracking to 89.8% and 95.3% for the MLC and the HexaPOD tracking systems, respectively. For the prostate motion trajectories the average 2%/2 mm gamma pass rate was 60.1% when no tracking was applied and was improved to 85.0% for MLC tracking and 95.3% for the HexaPOD tracking system. CONCLUSIONS Both systems clearly increased the geometric and dosimetric accuracy during tracking of respiratory motion trajectories. Thereby, the geometric accuracy was increased almost equally by both systems, whereas the dosimetric accuracy of the HexaPOD tracking system was slightly better for all considered respiratory motion trajectories. Substantial improvement of the dosimetric accuracy was also observed during tracking of prostate motion trajectories during an intensity-modulated radiotherapy plan. Thereby, the HexaPOD tracking system showed better results than the MLC tracking.

Assessment of MLC tracking performance during hypofractionated prostate radiotherapy using real-time dose reconstruction

Abstract By adapting to the actual patient anatomy during treatment, tracked multi-leaf collimator (MLC) treatment deliveries offer an opportunity for margin reduction and healthy tissue sparing. This is assumed to be especially relevant for hypofractionated protocols in which intrafractional motion does not easily average out. In order to confidently deliver tracked treatments with potentially reduced margins, it is necessary to monitor not only the patient anatomy but also the actually delivered dose during irradiation. In this study, we present a novel real-time online dose reconstruction tool which calculates actually delivered dose based on pre-calculated dose influence data in less than 10 ms at a rate of 25 Hz. Using this tool we investigate the impact of clinical target volume (CTV) to planning target volume (PTV) margins on CTV coverage and organ-at-risk dose. On our research linear accelerator, a set of four different CTV-to-PTV margins were tested for three patient cases subject to four different motion conditions. Based on this data, we can conclude that tracking eliminates dose cold spots which can occur in the CTV during conventional deliveries even for the smallest CTV-to-PTV margin of 1 mm. Changes of organ-at-risk dose do occur frequently during MLC tracking and are not negligible in some cases. Intrafractional dose reconstruction is expected to become an important element in any attempt of re-planning the treatment plan during the delivery based on the observed anatomy of the day.

MRI-guided lung SBRT: Present and future developments

Stereotactic body radiotherapy (SBRT) is rapidly becoming an alternative to surgery for the treatment of early-stage non-small cell lung cancer patients. Lung SBRT is administered in a hypo-fractionated, conformal manner, delivering high doses to the target. To avoid normal-tissue toxicity, it is crucial to limit the exposure of nearby healthy organs-at-risk (OAR). Current image-guided radiotherapy strategies for lung SBRT are mostly based on X-ray imaging modalities. Although still in its infancy, magnetic resonance imaging (MRI) guidance for lung SBRT is not exposure-limited and MRI promises to improve crucial soft-tissue contrast. Looking beyond anatomical imaging, functional MRI is expected to inform treatment decisions and adaptations in the future. This review summarises and discusses how MRI could be advantageous to the different links of the radiotherapy treatment chain for lung SBRT: diagnosis and staging, tumour and OAR delineation, treatment planning, and inter- or intrafractional motion management. Special emphasis is placed on a new generation of hybrid MRI treatment devices and their potential for real-time adaptive radiotherapy.

Using dual-energy x-ray imaging to enhance automated lung tumor tracking during real-time adaptive radiotherapy

PURPOSE Real-time, markerless localization of lung tumors with kV imaging is often inhibited by ribs obscuring the tumor and poor soft-tissue contrast. This study investigates the use of dual-energy imaging, which can generate radiographs with reduced bone visibility, to enhance automated lung tumor tracking for real-time adaptive radiotherapy. METHODS kV images of an anthropomorphic breathing chest phantom were experimentally acquired and radiographs of actual lung cancer patients were Monte-Carlo-simulated at three imaging settings: low-energy (70 kVp, 1.5 mAs), high-energy (140 kVp, 2.5 mAs, 1 mm additional tin filtration), and clinical (120 kVp, 0.25 mAs). Regular dual-energy images were calculated by weighted logarithmic subtraction of high- and low-energy images and filter-free dual-energy images were generated from clinical and low-energy radiographs. The weighting factor to calculate the dual-energy images was determined by means of a novel objective score. The usefulness of dual-energy imaging for real-time tracking with an automated template matching algorithm was investigated. RESULTS Regular dual-energy imaging was able to increase tracking accuracy in left-right images of the anthropomorphic phantom as well as in 7 out of 24 investigated patient cases. Tracking accuracy remained comparable in three cases and decreased in five cases. Filter-free dual-energy imaging was only able to increase accuracy in 2 out of 24 cases. In four cases no change in accuracy was observed and tracking accuracy worsened in nine cases. In 9 out of 24 cases, it was not possible to define a tracking template due to poor soft-tissue contrast regardless of input images. The mean localization errors using clinical, regular dual-energy, and filter-free dual-energy radiographs were 3.85, 3.32, and 5.24 mm, respectively. Tracking success was dependent on tumor position, tumor size, imaging beam angle, and patient size. CONCLUSIONS This study has highlighted the influence of patient anatomy on the success rate of real-time markerless tumor tracking using dual-energy imaging. Additionally, the importance of the spectral separation of the imaging beams used to generate the dual-energy images has been shown.

Real-time 4D dose reconstruction for tracked dynamic MLC deliveries for lung SBRT.

Purpose This study provides a proof of concept for real‐time 4D dose reconstruction for lung stereotactic body radiation therapy (SBRT) with multileaf collimator (MLC) tracking and assesses the impact of tumor tracking on the size of target margins. Methods The authors have implemented real‐time 4D dose reconstruction by connecting their tracking and delivery software to an Agility MLC at an Elekta Synergy linac and to their in‐house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40 ms. The dose is calculated in real‐time from 4DCT data directly after each reported aperture by utilization of precalculated dose‐influence data based on a Monte Carlo algorithm. The dose is accumulated onto the peak‐exhale (reference) phase using energy‐mass transfer mapping. To investigate the impact of a potentially reducible safety margin, the authors have created and delivered treatment plans designed for a conventional internal target volume (ITV) + 5 mm, a midventilation approach, and three tracking scenarios for four lung SBRT patients. For the tracking plans, a moving target volume (MTV) was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the reference phase, resulting in a unified maximum GTV to which a 1, 3, or 5 mm isotropic margin was added. All scenarios were planned for 9‐beam step‐and‐shoot IMRT to meet the criteria of RTOG 1021 (3 × 18 Gy). The GTV 3D center‐of‐volume shift varied from 6 to 14 mm. Results Real‐time dose reconstruction at 25 Hz could be realized on a single workstation due to the highly efficient implementation of dose calculation and dose accumulation. Decreased PTV margins resulted in inadequate target coverage during untracked deliveries for patients with substantial tumor motion. MLC tracking could ensure the GTV target dose for these patients. Organ‐at‐risk (OAR) doses were consistently reduced by decreased PTV margins. The tracked MTV + 1 mm deliveries resulted in the following OAR dose reductions: lung V 20 up to 3.5%, spinal cord D 2 up to 0.9 Gy/Fx, and proximal airways D 2 up to 1.4 Gy/Fx. Conclusions The authors could show that for patient data at clinical resolution and realistic motion conditions, the delivered dose could be reconstructed in 4D for the whole lung volume in real‐time. The dose distributions show that reduced margins yield lower doses to healthy tissue, whilst target dose can be maintained using dynamic MLC tracking.

Treating locally advanced lung cancer with a 1.5 T MR-Linac – Effects of the magnetic field and irradiation geometry on conventionally fractionated and isotoxic dose-escalated radiotherapy

Purpose This study investigates the feasibility and potential benefits of radiotherapy with a 1.5 T MR-Linac for locally advanced non-small cell lung cancer (LA NSCLC) patients. Material and methods Ten patients with LA NSCLC were retrospectively re-planned six times: three treatment plans were created according to a protocol for conventionally fractionated radiotherapy and three treatment plans following guidelines for isotoxic target dose escalation. In each case, two plans were designed for the MR-Linac, either with standard (∼7 mm) or reduced (∼3 mm) planning target volume (PTV) margins, while one conventional linac plan was created with standard margins. Treatment plan quality was evaluated using dose–volume metrics or by quantifying dose escalation potential. Results All generated treatment plans fulfilled their respective planning constraints. For conventionally fractionated treatments, MR-Linac plans with standard margins had slightly increased skin dose when compared to conventional linac plans. Using reduced margins alleviated this issue and decreased exposure of several other organs-at-risk (OAR). Reduced margins also enabled increased isotoxic target dose escalation. Conclusion It is feasible to generate treatment plans for LA NSCLC patients on a 1.5 T MR-Linac. Margin reduction, facilitated by an envisioned MRI-guided workflow, enables increased OAR sparing and isotoxic target dose escalation for the respective treatment approaches.

Online dose reconstruction for tracked volumetric arc therapy: Real‐time implementation and offline quality assurance for prostate SBRT

Purpose: Firstly, this study provides a real‐time implementation of online dose reconstruction for tracked volumetric arc therapy (VMAT). Secondly, this study describes a novel offline quality assurance tool, based on commercial dose calculation algorithms. Methods: Online dose reconstruction for VMAT is a computationally challenging task in terms of computer memory usage and calculation speed. To potentially reduce the amount of memory used, we analyzed the impact of beam angle sampling for dose calculation on the accuracy of the dose distribution. To establish the performance of the method, we planned two single‐arc VMAT prostate stereotactic body radiation therapy cases for delivery with dynamic MLC tracking. For quality assurance of our online dose reconstruction method we have also developed a stand‐alone offline dose reconstruction tool, which utilizes the RayStation treatment planning system to calculate dose. Results: For the online reconstructed dose distributions of the tracked deliveries, we could establish strong resemblance for 72 and 36 beam co‐planar equidistant beam samples with less than 1.2% deviation for the assessed dose‐volume indicators (clinical target volume D98 and D2, and rectum D2). We could achieve average runtimes of 28–31 ms per reported MLC aperture for both dose computation and accumulation, meeting our real‐time requirement. To cross‐validate the offline tool, we have compared the planned dose to the offline reconstructed dose for static deliveries and found excellent agreement (3%/3 mm global gamma passing rates of 99.8%–100%). Conclusion: Being able to reconstruct dose during delivery enables online quality assurance and online replanning strategies for VMAT. The offline quality assurance tool provides the means to validate novel online dose reconstruction applications using a commercial dose calculation engine.

Tumour auto-contouring on 2d cine MRI for locally advanced lung cancer: A comparative study

Background and purpose Radiotherapy guidance based on magnetic resonance imaging (MRI) is currently becoming a clinical reality. Fast 2d cine MRI sequences are expected to increase the precision of radiation delivery by facilitating tumour delineation during treatment. This study compares four auto-contouring algorithms for the task of delineating the primary tumour in six locally advanced (LA) lung cancer patients. Material and methods Twenty-two cine MRI sequences were acquired using either a balanced steady-state free precession or a spoiled gradient echo imaging technique. Contours derived by the auto-contouring algorithms were compared against manual reference contours. A selection of eight image data sets was also used to assess the inter-observer delineation uncertainty. Results Algorithmically derived contours agreed well with the manual reference contours (median Dice similarity index: ⩾0.91). Multi-template matching and deformable image registration performed significantly better than feature-driven registration and the pulse-coupled neural network (PCNN). Neither MRI sequence nor image orientation was a conclusive predictor for algorithmic performance. Motion significantly degraded the performance of the PCNN. The inter-observer variability was of the same order of magnitude as the algorithmic performance. Conclusion Auto-contouring of tumours on cine MRI is feasible in LA lung cancer patients. Despite large variations in implementation complexity, the different algorithms all have relatively similar performance.

TH‐AB‐303‐06: Real‐Time 4D Dose Reconstruction for Tracked Dynamic MLC Deliveries in the Presence of Respiratory Motion

Purpose: This work provides a proof-of-concept study for real-time 4D dose reconstruction on 4DCT lung data treated with MLC tracking and its impact on the size of target margins. Methods: We have implemented online dose reconstruction by connecting our research platform for dynamic dose delivery to an Agility MLC at an Elekta Synergy linac with our in-house treatment planning software (TPS). Actual MLC apertures and (simulated) target positions are reported to the TPS every 40ms and 33ms respectively. The dose is then accumulated in real-time by utilization of pre-calculated dose-influence matrices based on a collapsed-cone algorithm. To investigate the impact of a potentially reducible safety margin we have created and delivered treatment plans designed for a conventional internal target volume (ITV)+5mm and for a tracking scenario of a lung patient. Both were planned to meet the criteria of RTOG 1021 (3-Fx, 9-beam). For the tracking plan, a maximum target volume (MTV)+5mm was established by delineating the gross target volume (GTV) on every 4DCT phase. These were rigidly aligned to the chosen reference phase, resulting in a unified maximum GTV to which a 5mm isotropic margin was added. The dose accumulation on a reference phase of the 4DCT (2×2×2mm3) was accomplished by the use of 3D deformation vector fields. Results: D95 for GTV on the reference phase was 98% of the RTOG prescribed dose for both the ITV+5mm plan and the MTV+5mm plan with tracking. V20 for ipsilateral lung and mean chest wall dose decreased by 10% and 12% respectively for the tracking scenario. Dose calculation and accumulation were performed in <30ms per MLC aperture. Conclusion: For the patient data and motion conditions in this study, the reconstructed dose distributions show that target dose can be maintained using dynamic MLC tracking whilst the reduced margins yield lower doses to healthy tissue. We acknowledge support from Elekta AB under a research agreement. Research at The Institute of Cancer Research is also supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.