Dynamic Tumor Tracking Research Articles

3. Commissioning and Clinical Operation of Ring Radiation Therapy Systems with Dynamic Tumor Tracking System

3. Commissioning and Clinical Operation of Ring Radiation Therapy Systems with Dynamic Tumor Tracking System

Prospective external validation of radiomics-based predictive model of distant metastasis after dynamic tumor tracking stereotactic body radiation therapy in patients with non-small-cell lung cancer: A multi-institutional analysis.

This study aims to externally validate a predictive model for distant metastasis (DM) with computed tomography (CT)-based radiomics features in prospectively enrolled non-small-cell lung cancer patients undergoing dynamic tumor-tracking stereotactic body radiation therapy (DTT-SBRT). The study collected retrospective data from 567 patients across 11 institutions as the training dataset and prospectively enrolled 42 patients from four institutions asthe external test dataset. Four clinical features were collected, and 944 CT-based radiomic features were extracted from gross tumor volumes. After standardization and feature selection, DM predictive models were developed using fine and gray regression (FG) and random survival forest (RSF), incorporating clinical and radiomic features, and their combinations within the training dataset. Then, the model was applied to the test dataset, dividing patients into high- and low-risk groups based on medians of risk scores. Model performance was assessed using the concordance index (C-index), and the statistical significance between groups was evaluated using Gray's test. In the training dataset, 122 of 567 patients (21.5%) developed DM, compared to 9 of 42 patients (21.4%) in the test dataset. In the test dataset, the C-indices of the clinical, radiomics, and hybrid models with FG were 0.559, 0.544, and 0.560, respectively, whereas those with RSF were 0.576, 0.604, and 0.627, respectively. The hybrid model with RSF, which exhibited the best predictive performance of all models, identified 7 of 23 patients (30.4%) as high risk and 2 of 19 patients (10.5%) as low risk for DM incidence in the test dataset (p=0.116). Although predictive models for DM lack significance when applied to prospectively enrolled cases undergoing DTT-lung SBRT, the model with RSF exhibits a consistent capacity to effectively classify patients at a high risk of developing DM.

2079: Four-dimensional dose calculations and treatment planning strategies for dynamic tumour tracking

2079: Four-dimensional dose calculations and treatment planning strategies for dynamic tumour tracking

Four-dimensional treatment planning strategies for dynamic tumor tracking.

Dynamic tumor tracking (DTT) is a motion management technique where the radiation beam follows a moving tumor in real time. Not modelling DTT beam motion in the treatment planning system leaves an organ at risk (OAR) vulnerable to exceeding its dose limit. This work investigates two planning strategies for DTT plans, the "Boolean OAR Method" and the "Aperture Sorting Method," to determine if they can successfully spare an OAR while maintaining sufficient target coverage. A step-and-shoot intensity modulated radiation therapy (sIMRT) treatment plan was re-optimized for 10 previously treated liver stereotactic ablative radiotherapy patients who each had one OAR very close to the target. Two planning strategies were investigated to determine which is more effective at sparing an OAR while maintaining target coverage: (1) the "Boolean OAR Method" created a union of an OAR's contours from two breathing phases (exhale and inhale) on the exhale phase (the planning CT) and protected this combined OAR during plan optimization, (2) the "Aperture Sorting Method" assigned apertures to the breathing phase where they contributed the least to an OAR's maximum dose. All 10 OARs exceeded their dose constraints on the original plan four-dimensional (4D) dose distributions and average target coverage was V100%=91.3%±2.9% (ranging from 85.1% to 94.8%). The "Boolean OAR Method" spared 7/10 OARs, and mean target coverage decreased to V100%=87.1%±3.8% (ranging from 80.7% to 93.7%). The "Aperture Sorting Method" spared 9/10 OARs and the mean target coverage remained high at V100%=91.7%±2.8% (ranging from 84.9% to 94.5%). 4D planning strategies are simple to implement and can improve OAR sparing during DTT treatments. The "Boolean OAR Method" improved sparing of OARs but target coverage was reduced. The "Aperture Sorting Method" further improved sparing of OARs and maintained target coverage.

Markerless dynamic tumor tracking (MDTT) radiotherapy using diaphragm as a surrogate for liver targets.

To assess the feasibility of using the diaphragm as a surrogate for liver targets during MDTT. Diaphragm as surrogate for markers: a dome-shaped phantom with implanted markers was fabricated and underwent dual-orthogonal fluoroscopy sequences on the Vero4DRT linac. Ten patients participated in an IRB-approved, feasibility study to assess the MDTT workflow. All images were analyzed using an in-house program to back-project the diaphragm/markers position to the isocenter plane. ExacTrac imager log files were analyzed. Diaphragm as tracking structure for MDTT: The phantom "diaphragm" was contoured as a markerless tracking structure (MTS) and exported to Vero4DRT/ExacTrac. A single field plan was delivered to the phantom film plane under static and MDTT conditions. In the patient study, the diaphragm tracking structure was contoured on CT breath-hold-exhale datasets. The MDTT workflow was applied until just prior to MV beam-on. Diaphragm as surrogate for markers: phantom data confirmed the in-house 3D back-projection program was functioning as intended. In patients, the diaphragm/marker relative positions had a mean±RMS difference of 0.70±0.89, 1.08±1.26, and 0.96±1.06mm in ML, SI, and AP directions. Diaphragm as tracking structure for MDTT: Building a respiratory-correlation model using the diaphragm as surrogate for the implanted markers was successful in phantom/patients. During the tracking verification imaging step, the phantom mean±SD difference between the image-detected and predicted "diaphragm" position was 0.52±0.18mm. The 2D film gamma (2%/2mm) comparison (static to MDTT deliveries) was 98.2%. In patients, the mean difference between the image-detected and predicted diaphragm position was 2.02±0.92mm. The planning target margin contribution from MDTT diaphragm tracking is 2.2, 5.0, and 4.7mm in the ML, SI, and AP directions. In phantom/patients, the diaphragm motion correlated well with markers' motion and could be used as a surrogate. MDTT workflows using the diaphragm as the MTS is feasible using the Vero4DRT linac and could replace the need for implanted markers for liver radiotherapy.

Dosimetric Analysis of Patients Treated with Stereotactic Body Radiation Therapy for Malignant Portal Vein Tumor Thrombosis Using MRI-Guided vs. CT-Guided Linear Accelerator

The use of MRI-guided linear accelerators (MRI-LINAC) for radiation therapy has allowed for increased treatment precision and real-time tumor tracking. We hypothesized that the use of an MRI-LINAC, recently acquired by our institution, would allow us to treat malignant portal vein tumor thromboses (PVTT) closer to the origin of the portal vein with similar doses to the liver and nearby GI organs and with similar treatment toxicities compared to patients treated using a CT-guided linear accelerator (CT-LINAC). We retrospectively assessed patients who received SBRT for treatment of PVTT within the UVA health system using the electronic medical record. We contoured the origin of the portal vein on the simulation scans (MRI or CT) for each patient. The shortest distance from the GTV to the portal vein origin and to organs at risk (OAR; duodenum, stomach or "other intestine") were measured. The PTV, volume of the liver receiving less than 18 Gy during treatment, and the Dmax to the portal vein origin and OAR were determined. The means for each variable were then compared between CT-LINAC and MRI-LINAC using independent samples t-tests. Treatment toxicities were graded using CTCAE version 5.0 guidelines. All plans met institutional dose constraints for SBRT. A total of 6 patients treated on CT-LINAC and 3 patients treated on MRI-LINAC were identified (mean dose CT 37.7 Gy [range 21-50] in 2-5 fractions; MRI 48.3 Gy [range 45-50] in 5 fractions). The GTV was significantly closer to the portal vein origin in the MRI-LINAC group (mean CT 92.75 mm ± 45.89; MRI 32.00 mm ± 29.46; p = 0.04). The GTV trended towards being closer to OAR in the MRI-LINAC group (mean CT 43.81 mm ± 37.09; MRI 4.36 mm ± 1.85; p = 0.059). The Dmax to both the portal vein origin (mean CT 0.58 Gy ± 0.33; MRI 22.49 Gy ±24.16; p = 0.024) and OAR (mean CT 13.38 Gy ± 5.98; MRI 36.23 Gy ± 5.38; p = <0.001) was significantly higher in the MRI-LINAC group. The PTV (mean CT 152.11 cc ± 169.94; MRI 122.47 cc ± 114.65; p = 0.398) or volume of liver receiving less than 18 Gy during treatment (mean CT 1287.87 cc ± 285.20; MRI 1743.88 cc ±961.94 = 3; p = 0.147) were not significantly different between the two groups. No patients in either the MRI-LINAC or CT-LINAC group reported acute toxicities greater than Grade 1. We found that patients with PVTT treated with MRI-LINAC SBRT at our institution were more likely to have central tumors near dose-limiting OAR. However, due to superior tumor delineation and dynamic tumor tracking, these patients were able to be treated without decreasing treatment dose or increasing acute GI toxicity compared to patients treated with CT-LINAC. A larger sample size and longer follow-up period is warranted to corroborate these initial findings.

Evaluation of Patient-Specific Quality Control (QC) for Markerless Dynamic Tumor Tracking (MDTT) Deliveries

To report on the patient-specific QC for MDTT for liver and lung tumors treated using the dome-of-liver/diaphragm as a surrogate for the target position and compare to the QC of conventional fiducial-based tracking. Vero4DRT is a linear accelerator that has a gimbal-mounted waveguide and collimation system allowing the radiation beam to perform dynamic tumor tracking based on a patient's respiratory motion. A correlation model is built between an external IR marker placed on the chest and an internal structure, which may be implanted fiducial surrogates or in the case of MDTT, an anatomic landmark. During treatment an orthogonal kV image pair is taken every second. The auto detected internal structure position versus the predicted position is recorded in treatment log files. Five IMRT tracking plans were delivered to two commercial motion phantoms with in-house additions for patient-specific tracking QC. For point dose measurements, a 0.6cc farmer chamber was placed inside the dynamic tracking phantom which contains fiducials for fiducial-based tracking delivery and an anatomic landmark for MDTT. For 2D dose distribution measurements, a radiographic film insert for the Quasar Respiratory Motion Phantom was constructed. The phantom contains both fiducials and has a liver-dome shape at one end to facilitate MDTT. Both motion platforms were interfaced with software that enabled patient-specific respiratory motion traces acquired during a 4DCT scan. Each plan was delivered three times on each phantom: 1) in static mode, 2) during fiducial-based tracking, and 3) during MDTT. Chamber measurements taken during static and tracked delivery were compared to the average chamber dose from the treatment planning software (TPS). Film distributions measured during tracked deliveries were compared to the static film distribution using gamma analysis with FILMQA PRO software. Finally, dynamic tracking treatment statistics, extracted from log files, were compared between fiducial and markerless tracking deliveries. All chamber measurements resulted in dose differences <2.5% compared with the TPS. Dose differences between fiducial-based and MDTT were 0.26% on average (range 0.03-0.62%). For film dose map analysis, gamma pass rates were >95% for all plans for all tracking methods. The average gamma pass rate for 3%/3mm was 99.4% (fiducial-based) vs 98.4% (markerless). For 2%/2mm, the average gamma pass rate was 96.4% (fiducial-based tracking) vs 95.6% (MDTT). Dynamic treatment statistics from logs files reported an average 3D absolute deviation from predicted position of 0.52mm (±0.24) for fiducial tracking and 1.19mm (±0.50) for MDTT. Both fiducial-based and MDTT plans meet passing criteria for patient-specific QC. 3D absolute deviation of detected versus predicted position are larger for MDTT compared to fiducial tracking, however no significant impact on dose delivery was measured.

Long-Term Results of a Multi-Institutional Study of Dynamic Tumor Tracking-Stereotactic Body Radiotherapy for Lung Tumors

We had conducted a multi-institutional phase II study to evaluate the safety and efficacy of dynamic tumor tracking-stereotactic body radiotherapy (DTT-SBRT) for lung tumors. The primary endpoint was 2-year local control, which was reported to be 95.2%. After the initial evaluation of the primary endpoint, the study was extended as an observational study which was designed to evaluate 5-year survival and late toxicities. We present the long-term results of DTT-SBRT for lung tumors. The main eligibility criteria for the study were as follows: (1) primary or metastatic lung cancer with a diameter of 5 cm or less, and up to 3 lesions without any extrapulmonary lesions; (2) ineligibility to standard surgery, or patient's refusal of surgery; (3) ECOG-PS of 0 to 2; and (4) expected range of respiratory motion of 10 mm or more. The study included 48 patients from four institutions with the median age of 80 years (range, 49-90 years). Forty-two patients had primary non-small-cell lung cancer, and 6 patients had metastatic lung tumors. Forty-eight tumors (median diameter, 23.5 mm; range, 5-47 mm) in 48 patients were targeted for DTT-SBRT using a gimbal-mounted linear accelerator. Prior to treatment planning, spherical gold markers were placed around the tumor to detect internal tumor motion using fluoroscopy. The prescribed dose was 50 Gy in four fractions. Treatment beams were delivered with DTT according to a 4D model that predicts internal tumor motion with abdominal wall motion. DTT-SBRT was successfully delivered to all but one patient who had poor correlation between abdominal wall and tumor motion. Median follow-up period at data cutoff was 5.0 years (interquartile range, 3.1-6.3 years). Twenty-nine patients died; the causes of death were cancer-specific in 10 patients, comorbidity in 14 patients (pulmonary disease, renal failure, cerebral infarction, other malignancies, etc.), and unknown in 5 patients without cancer recurrence. Overall survival at 5 years was 51.5% (95% confidence interval [CI], 36.5-64.6%). Progression-free survival and local control at 5 years were 41.0% (95% CI, 27.0-54.5%) and 92.6% (95% CI, 78.7-97.6%), respectively. There were no grade 4-5 toxicities. One patient (2%) developed grade 3 radiation pneumonitis at 3 months. Grade 2 toxicities were observed in 9 patients (19%), including dyspnea, radiation pneumonitis, pleural effusion, rib fracture, and dermatitis. Dynamic tumor tracking SBRT achieved the long-term efficacy with low incidence of severe toxicities in lung tumors with respiratory motion. In this elderly patient cohort, non-cancer deaths were observed more than cancer-specific deaths.

Risk-Adapted SABR for Ultra-Central Lung Tumors Attaining High BED10 to Targets in a Steep Dose-Response Window of 72-105 Gy under Tumor Tracking: A Planning Study

Higher rate of durable local control in stereotactic ablative radiotherapy (SABR) for ultra-central lung tumors (UCLTs) strongly correlates to a BED10 ≥ 100 Gy to the tumors. However, higher rates of grade 5 toxicities are reported to be directly associated with higher dose to the organs at risk (OARs) abutting or in close distance to the UCLTs, including trachea, proximal bronchial tree (PBT) and esophagus. This study evaluated two risk-adapted SABR schemes in a steep dose-response window of BED10 at 72 -105 Gy with isotoxic optimization and dynamic tumor tracking to safely attain high BED10 to the tumors while keeping dose to the OARs within evidence-based tolerances. A total of 12-16 patients are included for planning under two risk-adapted-SABR schemes of 60 Gy in 8 (scheme A) and15 (scheme B) fractions to attain high BED10 of ≥100 Gy to the distal sections of the PTV and yet potent BED10 at 72-84 Gy to the proximal sections of the PTV. Such inhomogeneous dose plans use a 3-5 mm GTV to PTV margins under a scenario that patients have at least one fiducial marker placed in or near the GTV for real-time tumor tracking on a robotic SRS-SABR system. All plans used fixed cone collimators and a planning tool's dose calculation to reach highest BED10 dose coverage achievable to the target volumes while respecting the OAR dose tolerances, including V105 Gy, V100 Gy, V84 Gy and V72 Gy in BED10 for target volumes and the maximum EQD2 dose tolerances (α/β = 3 Gy) in D0.03cc for the trachea/PBT ≤80.5-82.5 Gy and esophagus ≤64.0-77.6 Gy in scheme-A; and trachea/PBT≤97.7 Gy and esophagus ≤64.3 Gy in scheme-B. Median and range of plan dosimetry metrics are compared between the two schemes. For scheme-A, mean BED10 to PTV are 118.0 Gy (median) in105.5-133.5 Gy (range), and mean BED10 dose to GTV are 132.2 (118.3-149.4 Gy). EQD2 dose to trachea are 80.5 Gy (68.0-81.7 Gy), PBT 80.5 Gy (49.1-82.2 Gy), esophagus 67.2 Gy (51.6-77.3 Gy). PTV coverage by BED10 of 72 Gy, 84 Gy, 100 Gy and 105 Gy are 98.9% (87.5-100%), 97.1% (81.8-99.8%), 91.9% (72.5-98.6%) and 85.3% (68.5-97.9%). For scheme-B, mean BED10 to PTV are 102.2 Gy (93.4-106.0 Gy), and mean BED10 dose to GTV are 115.4 (103.4-117.3 Gy). EQD2 dose to trachea are 73.3 Gy (67.5-96.3 Gy), PBT 90.9 Gy (74.4-97.2 Gy), esophagus 61.4 Gy (41.9-64.1 Gy). PTV coverage by BED10 of 72 Gy, 84 Gy, 100 Gy and 105 Gy are 100% (94.9-100%), 95.5% (83.9-99.0%), 54.8% (31.2-65.4%) and 42.8% (17.9-53.2%). Two risk-adapted SABR of 7.5 Gyx8 and 4 Gyx15 are implemented for treating ultra-central lung tumors with BED10 ≥100 Gy to the distal sections of PTV and yet attain potent BED10 ≥72-84 Gy to the proximal sections of PTV that abut or overlap with trachea, proximal bronchial tree or esophagus. Accurate dose calculation by a planning tool and real-time tumor tracking are essential for safe and accurate delivery of such high-risk SABR treatments.

278 Evaluation of Patient-Specific Quality Control (QC) for Markerless Dynamic Tumour Tracking (MDTT) Deliveries

278 Evaluation of Patient-Specific Quality Control (QC) for Markerless Dynamic Tumour Tracking (MDTT) Deliveries

Comparison of dynamic tumor tracking error measurement methods for robotic radiosurgery.

Dynamic tumor motion tracking is used in robotic radiosurgery for targets subject to respiratory motion, such as lung and liver cancers. Different methods of measuring tracking error have been reported, but the differences among these methods have not been studied, and the optimal method is unknown. The purpose of this study was to assess and compare tracking errors encountered with individual patients using different evaluation methods for method optimization. We compared the beam's eye view (BEV), machine learning (ML), log (addition error: AE), and log (root sum square: RSS) methods. Log (AE) and log (RSS) were calculated from log files. These tracking errors were compared, and the optimal evaluation method was ascertained. A t-test was performed to evaluate statistically significant differences. Here, the significance level was set at 5%. The mean values of BEV, log (AE), log (RSS), and ML were 2.87, 3.91, 2.91, and 3.74mm, respectively. The log (AE) and ML were higher than BEV (p<0.001), and log (RSS) was equivalent to the BEV, suggesting that the log (RSS) calculated with the log file method can substitute for the BEV calculated with the BEV method. As RSS error calculation is simpler than BEV calculation, using it may improve clinical practice throughput. This study clarified differences among three tracking error evaluation methods for dynamic tumor tracking radiotherapy using a robotic radiosurgery system. The log (RSS) calculated by the log file method was found to be the best alternative to BEV method, as it can calculate tracking errors more easily than the BEV method.

In-vivo quality assurance of dynamic tumor tracking (DTT) for liver SABR using EPID images.

To assess dynamic tumor tracking (DTT) target localization uncertainty for in-vivo marker-based stereotactic ablative radiotherapy (SABR) treatments of the liver using electronic-portal-imaging-device (EPID) images. The Planning Target Volume (PTV) margin contribution for DTT is estimated. Phantom and patient EPID images were acquired during non-coplanar 3DCRT-DTT delivered on a Vero4DRT linac. A chain-code algorithm was applied to detect Multileaf Collimator (MLC)-defined radiation field edges. Gold-seed markers were detected using a connected neighbor algorithm. For each EPID image, the absolute differences between the measured center-of-mass (COM) of the markers relative to the aperture-center (Tracking Error, (ET )) was reported in pan, tilt, and 2D-vector directions at the isocenter-plane. An acrylic cube phantom implanted with gold-seed markers was irradiated with non-coplanar 3DCRT-DTT beams and EPID images collected. Patient Study: Eight liver SABR patients were treated with non-coplanar 3DCRT-DTT beams. All patients had three to four implanted gold-markers. In-vivo EPID images were analyzed. Phantom Study: On the 125 EPID images collected, 100% of the markers were identified. The average±SD of ET were 0.24±0.21, 0.47±0.38, and 0.58±0.37mm in pan, tilt and 2D directions, respectively. Patient Study: Of the 1430 EPID patient images acquired, 78% had detectable markers. Over all patients, the average±SD of ET was 0.33±0.41mm in pan, 0.63±0.75mm in tilt and 0.77±0.80mm in 2D directions The random 2D-error, σ, for all patients was 0.79mm and the systematic 2D-error, Σ, was 0.20mm. Using the Van Herk margin formula 1.1mm planning target margin can represent the marker based DTT uncertainty. Marker-based DTT uncertainty can be evaluated in-vivo on a field-by-field basis using EPID images. This information can contribute to PTV margin calculations for DTT.

Dynamic tumor-tracking stereotactic body radiotherapy with real-time monitoring of liver tumors using a gimbal-mounted linac: A multi-institutional phase II study.

This prospective multicenter phase II study aimed to evaluate the safety and efficacy of dynamic tumor tracking (DTT) stereotactic body radiotherapy (SBRT) with real-time monitoring of liver tumors using a gimbal-mounted system. Patients with<4 primary or metastatic liver tumors with diameters≤50mm and expected to have a respiratory motion of≥10mm were eligible. The prescribed dose was 40Gy in five fractions. The primary endpoint was local control (LC) at 2years. The secondary endpoints were overall survival (OS), progression-free survival (PFS), treatment-related toxicity, and tracking accuracy. Between September 2015 and March 2019, 48 patients (48 lesions) with a median age of 74years were enrolled from four institutions. Of these, 39 were diagnosed with hepatocellular carcinoma and nine with metastatic liver cancer. The median tumor diameter was 17.5mm. DTT-SBRT was successfully performed in all patients; the median treatment time was 28min/fraction. The median follow-up period was 36.5months. The 2-year LC, OS, and PFS rates were 98.0%, 88.8%, and 55.1%, respectively. Disease progression was observed in 33 (68.8%) patients. One patient (0.2%) had local recurrence, 31 (64.6%) developed new hepatic lesions outside the irradiation field, and nine (18.8%) had distant metastases (including overlap). Grade 3 late adverse events were observed in seven patients (14.5%). No grade 4 or 5 treatment-related toxicity was observed. The median tracking accuracy was 2.9mm. Employing DTT-SBRT to treat liver tumors results in excellent LC with acceptable adverse-event incidence.

Monte Carlo Modeling of Dynamic Tumor Tracking on a Gimbaled Linear Accelerator.

The Vero4DRT (Brainlab AG) linear accelerator is capable of dynamic tumor tracking (DTT) by panning/tilting the radiation beam to follow respiratory-induced tumor motion in real time. In this study, the panning/tilting motion is modeled in Monte Carlo (MC) for quality assurance (QA) of four-dimensional (4D) dose distributions created within the treatment planning system (TPS). Step-and-shoot intensity-modulated radiation therapy plans were optimized for 10 previously treated liver patients. These plans were recalculated on multiple phases of a 4D computed tomography (4DCT) scan using MC while modeling panning/tilting. The dose distributions on each phase were accumulated to create a respiratory-weighted 4D dose distribution. Differences between the TPS and MC modeled doses were examined. On average, 4D dose calculations in MC showed the maximum dose of an organ at risk (OAR) to be 10% greater than the TPS' three-dimensional dose calculation (collapsed cone [CC] convolution algorithm) predicted. MC's 4D dose calculations showed that 6 out of 24 OARs could exceed their specified dose limits, and calculated their maximum dose to be 4% higher on average (up to 13%) than the TPS' 4D dose calculations. Dose differences between MC and the TPS were greatest in the beam penumbra region. Modeling panning/tilting for DTT has been successfully modeled with MC and is a useful tool to QA respiratory-correlated 4D dose distributions. The dose differences between the TPS and MC calculations highlight the importance of using 4D MC to confirm the safety of OAR doses before DTT treatments.

End-to-end testing for stereotactic radiotherapy including the development of a Multi-Modality phantom

PurposeA new insert for a commercially available end-to-end test phantom was designed and in-house manufactured by 3D printing. Subsequently, the insert was tested for different stereotactic radiation therapy workflows (SRS, SBRT, FSRT, and Multimet) also in comparison to the original insert. Material and methodsWorkflows contained imaging (MR, CT), treatment planning, positioning, and irradiation. Positioning accuracy was evaluated for non-coplanar x-ray, kV- and MV-CBCT systems, as well as surface guided radiation therapy. Dosimetric accuracy of the irradiation was measured with an ionization chamber at four different linear accelerators including dynamic tumor tracking for SBRT. ResultsCT parameters of the insert were within the specification. For MR images, the new insert allowed quantitative analysis of the MR distortion. Positioning accuracy of the phantom with the new insert using the imaging systems of the different linacs was < 1 mm/degree also for MV-CBCT and a non-coplanar imaging system which caused > 3 mm deviation with the original insert. Deviation of point dose values was <3% for SRS, FSRT, and SBRT for both inserts. For the Multimet plans deviations exceeded 10% because the ionization chamber was not positioned in each metastasis, but in the center of phantom and treatment plan. ConclusionThe in-house manufactured insert performed well in all steps of four stereotactic treatment end-to-end tests. Advantages over the commercially available alternative were seen for quantitative analysis of deformation correction in MR images, applicability for non-coplanar x-ray imaging, and dynamic tumor tracking.

Clinical results of dynamic tumor-tracking stereotactic body radiotherapy with real-time monitoring for liver tumors using a gimbal-mounted linac: A multi-institutional phase II study.

441 Background: Dynamic tumor-tracking (DTT) stereotactic body radiotherapy (SBRT) with real-time monitoring for liver tumors using a gimbal-mounted linac can decrease the normal liver dose and planning target volume (PTV) without sacrificing the target dose. We hypothesized that this method would yield excellent tumor control comparable to that by the conventional SBRT. We conducted a multi-institutional prospective phase II study to evaluate the safety and efficacy of DTT-SBRT for liver tumors (UMIN-CTR, UMIN000017886). Methods: The main eligibility criteria were as follows: (1) &lt; 4 liver tumors; (2) technical difficulties with percutaneous ablation therapies, inoperable lesions, or patient refusal to undergo surgery; (3) Eastern Cooperative Oncology Group performance status of 0–2; (4) Child-Pugh score of ≤8; and (5) respiratory motion of ≥10 mm. The primary endpoint was the local control rate (LC) at 2 years (expected endpoint: &gt; 90%), and the secondary endpoints were overall survival rate (OS), progression-free survival rate (PFS), and treatment-related toxicity. We performed DTT-SBRT with 40 Gy in 5 fractions prescribed to the PTV D95, and a maximum dose of 55−58 Gy using 7–9 static non-coplanar ports of the 6-MV beam was applied. The Kaplan-Meier analysis was performed to estimate the OS and LC. Treatment-related toxicity was scored according to the Common Terminology Criteria for Adverse Events v.4.0. Results: Forty-eight patients (48 tumors) were registered from four institutes and successfully treated between September 2015 and March 2019. Their median age was 74 years (range, 52–97). Thirty-nine tumors were hepatocellular carcinomas, and nine were metastatic liver tumors. The median tumor size was 5.8 ml (range, 0.7–64.1). At the time of data cutoff on March 31, 2021, the median follow-up period was 36.5 months (range, 3.0–62.4). The 2-year LC was 98.0%. Two-year OS and PFS were 88.8% and 55.1%, respectively. Recurrences were observed in 33 patients (68.5%). One patient (0.2%) had local recurrence, 30 (62.5%) developed new hepatic lesions outside of the irradiation field, and 9 (18.8%) had distant metastases. Grade 3 adverse events were observed in seven patients, including elevation of liver or biliary enzyme levels and hematopoietic adverse events. No treatment-related toxicity of grade 4 or higher was observed. Conclusions: Employing DTT-SBRT to treat liver tumors resulted in excellent LC with acceptable incidence of adverse events. This method will be a promising alternative therapy for patients who cannot receive surgery or percutaneous ablation therapy. To the best of our knowledge, this is the first phase II trial evaluating DTT-SBRT for liver tumors. Clinical trial information: UMIN000017886.

Local control of stereotactic body radiotherapy with dynamic tumor tracking for lung tumors: a propensity score-matched analysis.

Dynamic tumor tracking (DTT) is a method of respiratory motion management in radiotherapy. It reduces the radiation field but risks delivering an insufficient radiation dose to the tumor. We investigated the local control of DTT-stereotactic body radiotherapy (SBRT) for lung tumors. Patients treated with SBRT for early-stage, non-small-cell lung cancer and lung metastases (2013-18) were retrospectively reviewed. Patients with tumor motion >1cm were treated with DTT-SBRT (DTT group); those with tumor motion ≤1cm were treated with static-SBRT (static group). A static planning target volume for the static-SBRT plan was also created for patients in the DTT group, and planning target volume reduction relative to the planning target volume for the DTT-SBRT plan was assessed. Patients were matched in a 1:1 ratio using a propensity score predictive of the SBRT technique. Of the 245 lesions in 218 patients (median follow-up, 25.4months), 69 were treated with DTT-SBRT and 176 with static-SBRT. The median planning target volume reduction in the DTT group was 30.3%. After propensity score matching, 124 lesions were included (62 per group). Two-year local control rates for the DTT and static groups were 94.2 and 95.9%, respectively, for all lesions (P=0.19) and 96.3 and 94.5%, respectively, for matched lesions (P=0.79). In univariate analysis, DTT-SBRT was not associated with local control for all lesions (hazard ratio, 2.06; P=0.20) or matched lesions (hazard ratio, 1.22; P=0.79). No grade 4/5 toxicities were observed. DTT-SBRT for lung tumors reduced the planning target volume, but not local control rates. DTT was useful for respiratory motion management.

Four-dimensional dose calculations for dynamic tumour tracking with a gimbal-mounted linear accelerator.

PurposeIn this study we present a novel method for re‐calculating a treatment plan on different respiratory phases by accurately modeling the panning and tilting beam motion during DTT (the “rotation method”). This method is used to re‐calculate the dose distribution of a plan on multiple breathing phases to accurately assess the dosimetry.MethodssIMRT plans were optimized on a breath hold computed tomography (CT) image taken at exhale (BHexhale) for 10 previous liver stereotactic ablative radiotherapy patients. Our method was used to re‐calculate the plan on the inhale (0%) and exhale (50%) phases of the four‐dimensional CT (4DCT) image set. The dose distributions were deformed to the BHexhale CT and summed together with proper weighting calculated from the patient’s breathing trace. Subsequently, the plan was re‐calculated on all ten phases using our method and the dose distributions were deformed to the BHexhale CT and accumulated together. The maximum dose for certain organs at risk (OARs) was compared between calculating on two phases and all ten phases.ResultsIn total, 26 OARs were examined from 10 patients. When the dose was calculated on the inhale and exhale phases six OARs exceeded their dose limit, and when all 10 phases were used five OARs exceeded their limit.ConclusionDynamic tumor tracking plans optimized for a single respiratory phase leave an OAR vulnerable to exceeding its dose constraint during other respiratory phases. The rotation method accurately models the beam’s geometry. Using deformable image registration to accumulate dose from all 10 breathing phases provides the most accurate results, however it is a time consuming procedure. Accumulating the dose from two extreme breathing phases (exhale and inhale) and weighting them properly provides accurate results while requiring less time. This approach should be used to confirm the safety of a DTT treatment plan prior to delivery.

Quality assurance for dynamic tumor tracking

The purpose of this work was to develop a treatment plan verification routine for a linear accelerator dedicated to SBRT treatments with gimbal based dynamic tumor tracking using three commercially available phantoms.The accelerator system has two special features: It operates with a rotation of the ring shaped gantry instead of a couch rotation and target motion can be compensated for via a gimbal system (dynamic tumor tracking, DTT). DTT plans were each measured with the three different phantoms. Afterwards the measured dose distribution was compared with the calculated dose distribution via global Gamma Index analysis (3mm / 3%, threshold: 10%).The global gamma pass rates were on average (93.5±7.2) % for ArcCHECK, (98.0±2.6) % for OCTAVIUS® 4D and (98.4±4.2) % for MatriXX Evolution.All three systems could be used for quality assurance with ring rotations and DTT, however, each with limitations.

Dense feature-based motion estimation in MV fluoroscopy during dynamic tumor tracking treatment: preliminary study on reduced aperture and partial occlusion handling

Quality assurance solutions to complement available motion compensation technologies are central for their safe routine implementation and success of treatment. This work presents a dense feature-based method for soft-tissue tumor motion estimation in megavoltage (MV) beam’s-eye-view (BEV) projections for potential intra-treatment monitoring during dynamic tumor tracking (DTT). Dense sampling and matching principles were employed to track a gridded set of features landmarks (FLs) in MV-BEV projections and estimate tumor motion, capable to overcome reduced field aperture and partial occlusion challenges. The algorithm’s performance was evaluated by retrospectively applying it to fluoroscopic sequences acquired at ∼2 frames s−1 (fps) for a dynamic phantom and two lung stereotactic body radiation therapy (SBRT) patients treated with DTT on the Vero SBRT system. First, a field-specific train image is initialized by sampling the tumor region at, S, pixel intervals on a grid using a representative frame from a stream of query frames. Sampled FLs are locally characterized in the form of descriptor vectors and geometric attributes representing the target. For motion tracking, subsequent query frames are likewise sampled, corresponding feature descriptors determined, and then patch-wise matched to the training set based on their descriptors and geometric relationships. FLs with high correspondence are pruned and used to estimate tumor displacement. In scenarios of partial occlusions, position is estimated from the set of correctly (visible) FLs on past observations. Reconstructed trajectories were benchmarked against ground-truth manual tracking using the root-mean-square (RMS) as a metric of positional accuracy. A total of 19 fluoroscopy sequences were analyzed. This included scenarios of field aperture obstruction during three-dimensional conformal, as well as step-and-shoot intensity modulated radiotherapy (IMRT) delivery assisted with DTT. The algorithm resolved target motion satisfactorily. The RMS was <1.2 mm and <1.8 mm for the phantom and the clinical dataset, respectively. Dense tracking showed promising results to overcome localization challenges at the field penumbra and partial obstruction by multi-leaf collimator (MLC). Motion retrieval was possible in ∼66% of the control points studied. In addition to MLC obstruction, changes in the external/internal breathing dynamics and baseline drifts were a major source of estimation bias. Dense feature-based tracking is a viable alternative. The algorithm is rotation-/scale-invariant and robust to photometric changes. Tracking multiple features may help overcome partial occlusion challenges by the MLC. This in turn opens up new possibilities for motion detection and intra-treatment monitoring during IMRT and potentially VMAT.