Intrafraction Motion Research Articles

Intrafraction tumor motion monitoring and dose reconstruction for liver pencil beam scanning proton therapy

Pencil beam scanning (PBS) proton therapy can provide highly conformal target dose distributions and healthy tissue sparing. However, proton therapy of hepatocellular carcinoma (HCC) is prone to dosimetrical uncertainties induced by respiratory motion. This study aims to develop intra-treatment tumor motion monitoring during respiratory gated proton therapy and combine it with motion-including dose reconstruction to estimate the delivered tumor doses for individual HCC treatment fractions. Three HCC-patients were planned to receive 58 GyRBE (n=2) or 67.5 GyRBE (n=1) of exhale respiratory gated PBS proton therapy in 15 fractions. The treatment planning was based on the exhale phase of a 4-dimensional CT scan. Daily setup was based on cone-beam CT (CBCT) imaging of three implanted fiducial markers. An external marker block (RPM) on the patient's abdomen was used for exhale gating in free breathing. This study was based on 5 fractions (patient 1), 1 fraction (patient 2) and 6 fractions (patient 3) where a post-treatment control CBCT was available. After treatment, segmented 2D marker positions in the post-treatment CBCT projections provided the estimated 3D motion trajectory during the CBCT by a probability-based method. An external-internal correlation model (ECM) that estimated the tumor motion from the RPM motion was built from the synchronized RPM signal and marker motion in the CBCT. The ECM was then used to estimate intra-treatment tumor motion. Finally, the motion-including CTV dose was estimated using a dose reconstruction method that emulates tumor motion in beam's eye view as lateral spot shifts and in-depth motion as changes in the proton beam energy. The CTV homogeneity index (HI) The CTV homogeneity index (HI) was calculated as . The tumor position during spot delivery had a root-mean-square error of 1.3mm in left-right, 2.8mm in cranio-caudal and 1.7mm in anterior-posterior directions compared to the planned position. On average, the CTV HI was larger than planned by 3.7%-points (range: 1.0-6.6%-points) for individual fractions and by 0.7%-points (range: 0.3-1.1%-points) for the average dose of 5 or 6 fractions. A method to estimate internal tumor motion and reconstruct the motion-including fraction dose for PBS proton therapy of HCC was developed and demonstrated successfully clinically.

Empirical planning target volume modeling for high precision MRI guided intracranial radiotherapy

Magnetic resonance image-guided radiotherapy for intracranial indications is a promising advance; however, uncertainties remain for both target localization after translation-only MR setup and intrafraction motion. This investigation quantified these uncertainties and developed a population-based planning target volume (PTV) model to explore target and organ-at-risk (OAR) volumetric coverage tradeoffs. Sixty-six patients, 49 with a primary brain tumor and 17 with a post-surgical resection cavity, treated on a 1.5T-based MR-linac across 1329 fractions were included. At each fraction, patients were setup by translation-only fusion of the online T1 MRI to the planning image. Each fusion was independently repeated offline accounting for rotations. The six degree-of-freedom difference between fusions was applied to transform the planning CTV at each fraction (CTVfx). A PTV model parameterized by volumetric CTVfx coverage, proportion of fractions, and proportion of patients was developed. Intrafraction motion was quantified in a 412 fraction subset as the fusion difference between post- and pre-irradiation T1 MRIs. For the left-right/anterior-posterior/superior-inferior axes, mean±SD of the rotational fusion differences were 0.1±0.8/0.1±0.8/-0.2±0.9°. Covering 98% of the CTVfx in 95% of fractions in 95% of patients required a 3mm PTV margin. Margin reduction decreased PTV-OAR overlap; for example, the proportion of optic chiasm overlapped by the PTV was reduced up to 23.5% by margin reduction from 4mm to 3mm. An evidence-based PTV model was developed for brain cancer patients treated on the MR-linac. Informed by this model, we have clinically adopted a 3mm PTV margin for conventionally fractionated intracranial patients.

Frameless Gamma Knife Radiosurgery with Leksell ICON: Initial Experience.

Gamma knife radiosurgery saw the light of the day when the Swedish physician "Lars Leksell" postulated the salient first principles of stereotactic radiosurgery. Prior to being realized in its new 'avatar' "The ICON", Leksell Gamma Knife (LGK) "Perfexion" has been the most practiced model and is still in practice in most of the centers in India. The Gamma Knife ICON (the sixth generation model) utilizes the concept of the Cone-Beam Computed Tomography (CBCT) module, thus allowing non-invasive immobilization of the skull employing frameless treatments without jeopardizing accuracy to sub-millimeters. The LGK ICON however has the same stereotactic delivery and patient positioning system as Perfexion and mesmerizes the care givers with the added technically sound feature of the CBCT imaging arm, that is, CBCT and an intra-fraction motion management system. The experience with ICON on both the sub-sets of patients has been intriguing and awe-inspiring. Despite its challenges of being detected with significant intra-fraction errors, we realized that the non-invasive thermoplastic mask fixation system has its own set of specific characteristics: fairly simple dosimetry; short radiation delivery times; and calm, composed, co-operative patients. We have been successful in conducting frameless gamma knife surgeries in ~25% of patients planned for gamma knife surgery. We look forward to witness this avant-garde pioneering scientific automation being practiced in a higher number of patients.

Six degrees of freedom intrafraction cranial motion detection using a novel capacitive monitoring technique: evaluation with human subjects

The purpose of this work is to introduce and evaluate a capacitive monitoring array capable of continuous 6DOF cranial motion detection during high precision radiotherapy. The ring-shaped capacitive array consists of four equally sized conductive sensors positioned at the cranial vertex. The system is modular, non-contact, and provides continuous motion information through the thermoplastic immobilization mask without relying on skin monitoring or use of ionizing radiation. The array performance was evaluated through a volunteer study with a cohort of twenty-five individuals. The study was conducted in a linac suite and the volunteers were fitted with an S-frame thermoplastic mask. Each volunteer took part in one data acquisition session per day for three consecutive days. During the data acquisition, the conductive array was translated and rotated relative to their immobilized cranium in 1-millimetre and 1-degree steps to simulate cranial motion. Capacitive signals were collected at each position at a frequency of 20 Hz. The data from the first acquisition session was then used to train a classifier model and establish calibration equations. The classifier and calibration equations were then applied to data from the subsequent acquisition sessions to evaluate the system performance. The trained classifiers had an average success rate of 92.6% over the volunteer cohort. The average error associated with calibration had a mean value below 0.1 mm or 0.1 deg for all six motions. The capacitive array system provides a novel method to detect translational and rotational cranial motion through a thermoplastic mask.

Clinical outcomes and lung toxicities after lung SABR using dynamic conformal arc therapy: a single-institution cohort study

BackgroundStereotactic ablative radiotherapy (SABR) is a validated treatment for early stage lung cancer and pulmonary metastases. It provides a high local control rate with low symptomatic toxicities. Recently, Dynamic Conformal Arc Therapy (DCAT), a delivery option that differs from conventional DCA, has been implemented in the Monaco Treatment Planning System for SABR. The aim of the study was to report clinical outcomes and toxicities for patients treated for lung SABR with this new technique.MethodsWe retrospectively identified adult patients treated for primary or secondary lung tumors with DCAT-SABR and reported their clinical, radiological, histological characteristics and dosimetric parameters. Total dose was delivered in 3 or 5 fractions for 95% of patients and prescribed on the 80% isodose line to the PTV periphery.Results145 patients met inclusion criteria for a total of 152 lesions with a median follow up of 12 months. Local control for the irradiated site was 96.7% at 1 year. Overall survival was 93.1% at 1 year. Mean prescription dose in BED10 was 110 Gy. 92% of patients had a prescribed dose superior to 100 Gy BED10. Mean PTV coverage was 95.1%. There were 66 cases of grade 1 radiation pneumonitis (RP) (43%) and only 7 cases of symptomatic grade 2 RP (4.6%).ConclusionLung SABR for primary or metastatic lung tumors using dynamic conformal arc therapy provides efficient results of local control and low lung toxicities, similar to other SABR techniques. Advances in knowledge: SABR using DCAT is a safe technique to treat lung lesions, allowing intra-fraction motion limitation, potentially higher OARs protection and a shortened beam delivery.

Dosimetric Impact of Intrafraction Prostate Motion and Interfraction Anatomical Changes in Dose-Escalated Linac-Based SBRT

The dosimetric impact of intrafraction prostate motion and interfraction anatomical changes and the effect of beam gating and motion correction were investigated in dose-escalated linac-based SBRT. Fifty-six gated fractions were delivered using a novel electromagnetic tracking device with a 2 mm threshold. Real-time prostate motion data were incorporated into the patient's original plan with an isocenter shift method. Delivered dose distributions were obtained by recalculating these motion-encoded plans on deformed CTs reflecting the patient's CBCT daily anatomy. Non-gated treatments were simulated using the prostate motion data assuming that no treatment interruptions have occurred. The mean relative dose differences between delivered and planned treatments were -3.0% [-18.5-2.8] for CTV D99% and -2.6% [-17.8-1.0] for PTV D95%. The median cumulative CTV coverage with 93% of the prescribed dose was satisfactory. Urethra sparing was slightly degraded, with the maximum dose increased by only 1.0% on average, and a mean reduction in the rectum and bladder doses was seen in almost all dose metrics. Intrafraction prostate motion marginally contributed in gated treatments, while in non-gated treatments, further deteriorations in the minimum target coverage and bladder dose metrics would have occurred on average. The implemented motion management strategy and the strict patient preparation regimen, along with other treatment optimization strategies, ensured no significant degradations of dose metrics in delivered treatments.

Online Adaptive MRI-Guided Radiotherapy for Primary Tumor and Lymph Node Boosting in Rectal Cancer.

The purpose of this study was to characterize the motion and define the required treatment margins of the pathological mesorectal lymph nodes (GTVln) for two online adaptive MRI-guided strategies for sequential boosting. Secondly, we determine the margins required for the primary gross tumor volume (GTVprim). Twenty-eight patients treated on a 1.5T MR-Linac were included in the study. On T2-weighted images for adaptation (MRIadapt) before and verification after irradiation (MRIpost) of five treatment fractions per patient, the GTVln and GTVprim were delineated. With online adaptive MRI-guided radiotherapy, daily plan adaptation can be performed through the use of two different strategies. In an adapt-to-shape (ATS) workflow the interfraction motion is effectively corrected by redelineation and the only relevant motion is intrafraction motion, while in an adapt-to-position (ATP) workflow the margin (for GTVln) is dominated by interfraction motion. The margin required for GTVprim will be identical to the ATS workflow, assuming each fraction would be perfectly matched on GTVprim. The intrafraction motion was calculated between MRIadapt and MRIpost for the GTVln and GTVprim separately. The interfraction motion of the GTVln was calculated with respect to the position of GTVprim, assuming each fraction would be perfectly matched on GTVprim. PTV margins were calculated for each strategy using the Van Herk recipe. For GTVln we randomly sampled the original dataset 20 times, with each subset containing a single randomly selected lymph node for each patient. The resulting margins for ATS ranged between 3 and 4 mm (LR), 3 and 5 mm (CC) and 5 and 6 mm (AP) based on the 20 randomly sampled datasets for GTVln. For ATP, the margins for GTVln were 10-12 mm in LR and AP and 16-19 mm in CC. The margins for ATS for GTVprim were 1.7 mm (LR), 4.7 mm (CC) and 3.2 mm anterior and 5.6 mm posterior. Daily delineation using ATS of both target volumes results in the smallest margins and is therefore recommended for safe dose escalation to the primary tumor and lymph nodes.

Feasibility of delivered dose reconstruction for MR-guided SBRT of pancreatic tumors with fast, real-time 3D cine MRI

Background and purposeIn MR-guided SBRT of pancreatic cancer, intrafraction motion is typically monitored with (interleaved) 2D cine MRI. However, tumor surroundings are often not fully captured in these images, and motion might be distorted by through-plane movement. In this study, the feasibility of highly accelerated 3D cine MRI to reconstruct the delivered dose during MR-guided SBRT was assessed. Materials and methodsA 3D cine MRI sequence was developed for fast, time-resolved 4D imaging, featuring a low spatial resolution that allows for rapid volumetric imaging at 430 ms. The 3D cines were acquired during the entire beam-on time of 23 fractions of online adaptive MR-guided SBRT for pancreatic tumors on a 1.5 T MR-Linac. A 3D deformation vector field (DVF) was extracted for every cine dynamic using deformable image registration. Next, these DVFs were used to warp the partial dose delivered in the time interval between consecutive cine acquisitions. The warped dose plans were summed to obtain a total delivered dose. The delivered dose was also calculated under various motion correction strategies. Key DVH parameters of the GTV, duodenum, small bowel and stomach were extracted from the delivered dose and compared to the planned dose. The uncertainty of the calculated DVFs was determined with the inverse consistency error (ICE) in the high-dose regions. ResultsThe mean (SD) relative (ratio delivered/planned) D99% of the GTV was 0.94 (0.06), and the mean (SD) relative D0.5cc of the duodenum, small bowel, and stomach were respectively 0.98 (0.04), 1.00 (0.07), and 0.98 (0.06). In the fractions with the lowest delivered tumor coverage, it was found that significant lateral drifts had occurred. The DVFs used for dose warping had a low uncertainty with a mean (SD) ICE of 0.65 (0.07) mm. ConclusionWe employed a fast, real-time 3D cine MRI sequence for dose reconstruction in the upper abdomen, and demonstrated that accurate DVFs, acquired directly from these images, can be used for dose warping. The reconstructed delivered dose showed only a modest degradation of tumor coverage, mostly attainable to baseline drifts. This emphasizes the need for motion monitoring and development of intrafraction treatment adaptation solutions, such as baseline drift corrections.

Geometric and dosimetric consequences of intra-fractional movement in single isocenter non-coplanar stereotactic radiosurgery

PurposeTo investigate the geometric and dosimetric impacts of intra-fractional movement for patients with single or multiple brain metastasis treated using Varian Hyperarc™ mono-isocentric radiosurgery.MethodsA total of 50 single or hypo-fractionated Hyperarc™ treatment courses (118 lesions) were included in the analysis. Intra-fractional translational and rotational movements were quantified according to the post-treatment cone-beam CT (CBCT). Geometric displacements of all targets were calculated individually based on the assessed head movement in each treatment fraction and their relationships with treatment time and target-to-isocenter distances were studied. For dosimetric analysis, only single-fraction treatments (56 lesions) were included. Re-planning was performed with 0, 1, and 2 mm planning target volume (PTV) margins. Doses were then re-calculated on rotated CT images with isocenter shifted which emulate the change in patient treatment position. Target coverage, target and normal brain doses before and after intra-fractional movement were compared.ResultsThe mean 3D target displacements was 0.6 ± 0.3 (SD) mm. Target shifts for patients treated within 10 min were significantly smaller than those treated in longer sessions. No correlation was found between target shift and target-to-isocenter distance as the origin of head rotation was not located at the isocenter. Loss of target coverage and minimum Gross Tumor Volume (GTV) dose due to intra-fractional movement were apparent only when no margin was used, leading to an extra 23% of the targets violating the dose acceptance criteria, in contrast, the effects on normal brain V12Gy were negligible regardless of the margin used. The use of 1 mm PTV margin can compensate clinically significant geographical miss caused by intra-fractional movements while limiting V12Gy to within dose criteria for 88% of the cases. The plan acceptance rate (fulfillment of both target and normal brain dose criteria) after intra-fractional movement was also the highest with the 1 mm margin.ConclusionAlthough intra-fractional movements during Hyperarc™ treatments were small, there were substantial dosimetric effects due to the sharp dose fall-off near target boundaries. These effects could be mitigated by using a 1 mm PTV margin and maintaining the effective treatment time to within 10 min.

Feasibility of using a dual isocentre technique for treating cervical cancer on the 1.5 T MR-Linac

Objective. Patients treated for cervical cancer exhibit large inter and intra-fraction anatomical changes. The Unity MR-Linac (MRL) can image these patients with MR prior to and during treatment which enables daily plan adaptation. However, the MRL has a limited treatment field in the sup/inf direction of 22 cm which can restrict the treatment of patients who require longer treatment fields. Here we explore potential adaptive workflows in combination with a dual isocentre approach, to widen the range of cervix patients that can benefit from this treatment. Approach. Ten cervical cancer patients were retrospectively planned with a dual isocentre technique to deliver 45 Gy in 25 fractions. 5 node-negative and 5 node-positive patients were planned using the EMBRACE II protocol. A 2 cm overlap region between the two isocentres was positioned entirely in the nodal region. A treatment workflow was simulated to account for inter-fraction anatomical change. Isocentre shifts of 3 and 6 mm were applied to investigate the effect of intra-fraction motion. Main results. Dual isocentre adapted plans ensured significantly better coverage than non-adapted (recalculated) plans with a larger benefit seen for the node-negative cases. The difference to the reference plan for the V4275 cGy to the ITV was −0.8 cGy and −8.2 cGy for the adapted and recalculated plans respectively. Movements superiorly did not affect the coverage of the ITV by more than 1%, but shifting it inferiorly caused the ITV coverage on the plan to reduce by ∼2.4% per mm. Significance. A dual isocentre technique for cervical cancer treatments and adaptive workflows have been demonstrated to recover the required plan quality for inter-fraction changes. This illustrates the feasibility of a dual isocentre technique for the MRL.

Dosimetry implications of VMAT delivery angles and intra-fraction motion for SBRT prostate treatments.

For a cohort of retrospectively treated patients with prostate carcinoma using a Stereotactic body radiation therapy (SBRT) technique and with implanted localization fiducials, this study determined whether there were any correlations between theoretical fiducial visibility using intra-fraction megavoltage imaging and intra-fraction motion induced dosimetric effects. Treatment planning data for 20 retrospective patients that used a SBRT prostate technique were investigated in this study. An in-house script was created that split each of the 360-degree volumetric modulated arc therapy arcs into 12 sectors, with each sector being 30 degrees. The script created a total of 24 sectors for each SBRT plan ranging from 180 to 210 degrees to 180 to 150 degrees. Resulting data was assessed to determine whether there was dosimetric impact from intra-fractional prostate motion and if it correlated with the theoretical fiducial visibility. Forty percent of the 4 highest CTV D98% mean dose differences occurred within the angles of 240 and 270 degrees and 25% occurred between the angles of 90 and 120 degrees. The 4 highest average percentage differences in PTV D98% coverage during angular sectors of 270 to 240 degrees, 90 to 120 degrees, 240 to 270 degrees and 60 to 90 degrees were -1.19%, -1.14%, -1.10%, and 1.01% respectively. It was also the case for the PTV D95%in sectors 90 to 120 degrees, 240 to 270 degrees, 270 to 240 degrees and 270 to 300 degrees, which reduced by -0.97%, -0.93%, -0.92%, and -0.82% respectively. From the 4 highest rectal dose differences in V32Gy and V18Gy, it was found that 50% of the highest rectum V32Gyand V18Gy dose increases occurred between the angles of 90 and 120 degrees and 37.5% occurred between the angles of 240 and 270 degrees. The MU for each sector showed that 240_270, 240_210, 270_240, and 120_90 had the highest average MU with 150.8, 134.6, 129.2, and 124.3 respectively. This study demonstrated that there is a high correlation between dosimetric impact of intra-fractional motion and the theoretical fiducial visibility. As a result, modification to treatment plans to enable fiducial visibility at all angular sectors throughout a treatment may not be required. Further sector analysis tests are required to develop patient specific megavoltage imaging gantry angles for SBRT prostate patients.

Assessment of intra-fraction motion during frameless image guided Gamma Knife stereotactic radiosurgery

As frameless stereotactic radiosurgery increase in use, the aim of this study was to evaluate intra-fraction motion through cone-beam CT (CBCT) and high-definition motion management (HDMM) systems. Intra-fraction motion measured between localization, repeat localization and post-treatment CBCTs were correlated to intra-faction motion indicated by the HDMM files using the Pearson coefficient (r). A total of 302 plans were reviewed from 263 patients (114 male, 149 female); 216 pairs of localization-repeat localization, and 260 localization-post-treatment CBCTs were analyzed against HDMM logs. We found the magnitude of intra-fraction motion detected by the HDMM system were larger than the corresponding CBCT results.

Impact of Respiratory Motion on the Skin Dose for Breast Cancer in Tomotherapy: A Study in the In-house Moving Phantom.

Purpose: The dose expansion methods as the skin flash and virtual bolus were used to solve intrafraction movement for breast planning due to breathing motion. We investigated the skin dose in each planning method by using optically stimulated luminescence on an in-house moving phantom for breast cancer treatment in tomotherapy. The impact of respiratory motion on skin dose between static and dynamic phantom's conditions was evaluated. Methods: A phantom was developed with movement controlled by the respirator for generating the respiratory waveforms to simulate respiratory motion. Five optically stimulated luminescence dosimeters were placed on the phantom surface to investigate the skin dose for the TomoDirect and TomoHelical under static and dynamic conditions. Eight treatment plans were generated with and without skin flash or virtual bolus by varying the thickness. The difference in skin dose between the two phantom conditions for each plan was explored. Results: All plans demonstrated a skin dose of more than 87% of the prescription dose under static conditions. However, the skin dose was reduced to 84.1% (TomoDirect) and 78.9% (TomoHelical) for dynamic conditions. The treatment plans without skin flash or virtual bolus showed significant skin dose differences under static and dynamic conditions by 4.83% (TomoDirect) and 9.43% (TomoHelical), whereas the skin flash with two leaves (TomoDirect 2L) or virtual bolus of at least 1.0 cm thickness (VB1.0) application compensated the skin dose in case of intrafraction movements by presenting a skin dose difference of less than 2% between the static and dynamic conditions. Conclusion: The skin dose was reduced under dynamic conditions due to breathing motion. The skin flash method with TomoDirect 2L or virtual bolus application with 1.0 cm thickness was useful for maintaining skin dose following the prescription by compensating for intrafraction movement due to respiratory motion for breast cancer in tomotherapy.

Implementation of triggered kilovoltage imaging for stereotactic radiotherapy of the spine for patients with spinal fixation hardware.

Mitigation of intrafraction motion (IM) is valuable in stereotactic radiotherapy (SRT) radiotherapy where submillimeter accuracy is desired. The purpose of this study was to investigate the application of triggered kilovoltage (kV) imaging for spine SRT patients with hardware by correlating kV imaging with patient motion and summarizing implications of tolerance for IM based on calculated dose. Ten plans (33 fractions) were studied, correlating kV imaging during treatment with pre- and post-treatment cone beam computed tomography (CBCT). Images were taken at 20-degree gantry angle intervals during the arc-based treatment. The contour of the hardware with a 1mm expansion was displayed at the treatment console to manually pause treatment delivery if the hardware was visually detected outside the contour. The treatment CBCTs were compared using retrospective image registration to assess the validity of contour-based method for pausing treatment. Finally, plans were generated to estimate dose volume objective differences in case of 1mm deviation. When kV imaging during treatment was used with the 1mm contour, 100% of the post-treatment CBCTs reported consistent results. One patient in the cohort exhibited motion greater than 1mm during treatment which allowed intervention and re-setup during treatment. The average translational motion was 0.35mm. Treatment plan comparison at 1mm deviation showed little differences in calculated dose for the target and cord. Utilizing kV imaging during treatment is an effective method of assessing IM for SRT spine patients with hardware without increasing treatment time.

Analysis of the interplay effect in lung stereotactic ablative radiation therapy based on both breathing motion and plan characteristics

Abstract Introduction: Stereotactic ablative radiotherapy (SABR) is susceptible to challenges for tumours affected by intrafraction organ motion. This study aims to investigate the effect of breathing characteristics and plan complexity on the interplay effect. Methods: A patient-specific interplay effect evaluation was performed using in-house software with an alpha version of the treatment planning verification software Verisoft (PTW-Freiburg, Germany) on VMAT plans. The OCTAVIUS 4D phantom was used to acquire the static dose distribution, and the simulation approach was utilised to generate the moving dose distribution. The influence of plan complexity, PTV size, number of breaths, and motion amplitudes on the interplay effect were examined. The dose distribution of two extreme phases—end-inhale and end-exhale—was considered using the gamma criteria of 2%/2 mm for the interplay effect evaluation. Results: A strong correlation was found between the motion amplitude (p < 0.001) and the NBs (p < 0.001) with the gamma-passing rate. No correlation was found between the gamma-passing rate and the PTV size or plan complexity. Conclusion: The simulation tool allowed the analysis of a large number of breathing traces, demonstrating how free-breathing patients, suspected of high interplay, could be selected for other motion management solutions. The simulated cases showed strong interplay effects for long breathing periods with extended motion amplitudes in a small group of patients.

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

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

ExacTrac X-Ray 6D Imaging During Stereotactic Body Radiation Therapy of Spinal and Nonspinal Metastases.

The objective was to investigate the possibility of using ExacTrac X-ray (ETX) for 6D image guidance in stereotactic body radiation therapy (SBRT) of bone metastasis and to propose a patient management protocol. The analyses were first obtained from measurements on a pelvic phantom and on 19 patients treated for bone metastasis. The phantom study consisted of applying known offsets and evaluating the ETX level of accuracy, where results were compared with kV-cone beam computed tomography (kV-CBCT). Two groups of patients, 10 spinal and 9 nonspinal SBRT cases, were analyzed to evaluate ETX imaging for different bone localisations. A comparison was made between kV-CBCT and ETX prior to the treatment fractions. During treatments, two other kV-CBCT/ETX image pairs were also acquired and a total of 224 shifts were compared. A second study, using the ETX monitoring module, analyzed the intrafraction motion of 8 other patients. In the phantom study, the root mean square (RMS) of the translational and rotational discrepancies between ETX and kV-CBCT were < 0.6 mm and < 0.4°, respectively. For both groups of patients, the RMS of the discrepancies observed between the two imaging systems were greater than the phantom experiment while still remaining < 1 mm and < 0.7°. In the nonspinal group, three patients (2 scapulas and 1 humerus) did not have consistent shift values with ETX due to a lack of anatomical information. When ETX monitoring was used during irradiation, the setup errors measured were on average less than 1 mm/1°. The results obtained validated the use of ETX for 6D image guidance during bone SBRT. Real-time tracking of the target position improves the accuracy of the irradiation. This strategy allowed for faster correction of out-of-tolerance positioning errors. The registration of bone lesions with poor anatomical information is a limitation of this 2D-kV imaging system.

CBCT-based Prostate IGRT With and Without Implanted Markers: Assessment of Geometric Corrections and Time for Completion.

Cone-beam computed tomography (CBCT) is the most commonly used system in modern radiotherapy of prostate cancer for daily positioning verification. The use of intraprostatic radiopaque fiducials (FMs) may be added to CBCT. We wanted to investigate the possible advantage of using FMs in daily CBCT repositioning. We selected three CBCTs for each treatment course for 13 patients (seven with and six without use of FMs) treated at our centre. Seven experienced Radiation Oncologists retrospectively reviewed the CBCTs, recording couch movements for correct patient positioning, and time spent to do it. Analysis of variance and t-test were carried out for comparison of different groups and for differences in mean values of the movements recorded (with p<0.05 as significance level). No statistically significant difference was found between operators in the analysis of images with FMs nor of images without them. A difference was only found in the mean corrections in couch rotation and pitch angle, which were higher in the FM group, and in the mean time for image analysis, which was shorter in this group. Using the van Herk formula, we found a possible reduction of clinical target volume and planning target volume margins for the FM group. According to our study, the use of intraprostatic FMs in daily CBCT seems useful for better detection of and correction for non-negligible rotational errors. Furthermore, FMs reduced the time to treatment start, which is very important in reducing the risk of intrafraction organ motion. These results need to be confirmed by further studies.

Dosimetric Analysis of Intra-Fraction Motion Detected by Surface-Guided Radiation Therapy During Linac Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) immobilization with an open face mask is more comfortable and less invasive than frame based, but concerns about intrafraction motion must be addressed. Surface-guided radiation therapy (SGRT) is an attractive option for intrafraction patient monitoring because it is continuous, has submillimeter accuracy, and uses no ionizing radiation. The purpose of this study was to investigate the dosimetric consequences of uncorrected intrafraction patient motion detected during frameless linac-based SRS. Fifty-five SRS patients were monitored during treatment using SGRT between January 1, 2017, and September 30, 2020. If SGRT detected motion >1 mm, imaging was repeated and the necessary shifts were made before continuing treatment. For the 25 patients with intrafraction 3-dimensional vector shifts of ≥1 mm, we moved the isocenter in the planning system using the translational shifts from the repeat imaging and recalculated the plans to determine the dosimetric effect of the shifts. Planning target volume (PTV) coverage, minimum gross tumor volume (GTV) dose (relative and absolute), and normal brain V12 were evaluated. Wilcoxon signed rank tests were used to compare planned and simulated dosimetric parameters and median 2 sample tests were used to investigate these differences between cone and multileaf collimator (MLC) plans. For simulated plans, V12 increased by a median of 0.01 cc (P=.006) and relative GTV minimum dose and PTV coverage decreased by a median of 15.8% (P < .001) and 10.2 % (P < .001), respectively. Absolute minimum GTV dose was found to be significantly lower in the simulated plans (P < .001). PTV coverage decreased more for simulated cone plans than for simulated MLC plans (11.6% vs 4.7%, P=.011) but median V12 differences were found to be significantly larger for MLC plans (-0.34 cc vs -0.01 cc, P=.011). Differences in GTV minimum dose between cone and MLC plans were not statistically significant. SGRT detected clinically meaningful intrafraction motion during frameless SRS, which could lead to large underdoses and increased normal brain dose if uncorrected.

Commissioning and performance evaluation of commercially available mobile imager for image guided total body irradiation.

The setup of lung shield (LS) in total body irradiation (TBI) with the computed radiography (CR) system is a time-consuming task and has not been quantitatively evaluated. The TBI mobile imager (TBI-MI) can solve this problem through real-time monitoring. Therefore, this study aimed to perform commissioning and performance evaluation of TBI-MI to promote its use in clinical practice. The source-axis distance in TBI treatment, TBI-MI (CNERGY TBI, Cablon Medical B.V.), and the LS position were set to 400, 450, and 358cm, respectively. The evaluation items were as follows: accuracy of image scaling and measured displacement error of LS, image quality (linearity, signal-to-noise ratio, and modulation transfer function) using an EPID QC phantom, optimal thresholding to detect intra-fractional motion in the alert function, and the scatter radiation dose from TBI-MI. The accuracy of image scaling and the difference in measured displacement of the LS was<4mm in any displacements and directions. The image quality of TBI imager was slightly inferior to the CR image but was visually acceptable in clinical practice. The signal-to-noise ratio was improved at high dose rate. The optimal thresholding value to detect a 10-mm body displacement was determined to be approximately 5.0%. The maximum fraction of scattering radiation to irradiated dose was 1.7% at patient surface. MI-TBI can quantitatively evaluate LS displacement with acceptable image quality. Furthermore, real-time monitoring with alert function to detect intrafraction patient displacement can contribute to safe TBI treatment.