Karl Farrey

The utility of FDG-PET for assessing outcomes in oligometastatic cancer patients treated with stereotactic body radiotherapy: a cohort study

BackgroundStudies suggest that patients with metastases limited in number and destination organ benefit from metastasis-directed therapy. Stereotactic body radiotherapy (SBRT) is commonly used for metastasis directed therapy in this group. However, the characterization of PET response following SBRT is unknown in this population. We analyzed our cohort of patients to describe the PET response following SBRT.MethodsPatients enrolled on a prospective dose escalation trial of SBRT to all known sites of metastatic disease were reviewed to select patients with pre- and post-therapy PET scans. Response to SBRT was characterized on PET imaging based on standard PET response criteria and compared to CT based RECIST criteria for each treated lesion.Results31 patients had PET and CT data available before and after treatment for analysis in this study. In total, 58 lesions were treated (19 lung, 11 osseous, 11 nodal, 9 liver, 6 adrenal and 2 soft tissue metastases). Median follow-up was 14 months (range: 3–41). Median time to first post-therapy PET was 1.2 months (range; 0.5-4.1). On initial post-therapy PET evaluation, 96% (56/58) of treated metastases responded to therapy. 60% (35/58) had a complete response (CR) on PET and 36% (21/58) had a partial response (PR). Of 22 patients with stable disease (SD) on initial CT scan, 13 had CR on PET, 8 had PR, and one had SD. Of 21 metastases with PET PR, 38% became CR, 52% remained PR, and 10% had progressive disease on follow-up PET. 10/35 lesions (29%) with an initial PET CR progressed on follow-up PET scan with median time to progression of 4.11 months (range: 2.75-9.56). Higher radiation dose correlated with long-term PET response.ConclusionsPET response to SBRT enables characterization of metastatic response in tumors non-measurable by CT. Increasing radiation dose is associated with prolonged complete response on PET.

Development of a frameless stereotactic radiosurgery system based on real-time 6D position monitoring and adaptive head motion compensation

Stereotactic radiosurgery delivers radiation with great spatial accuracy. To achieve sub-millimeter accuracy for intracranial SRS, a head ring is rigidly fixated to the skull to create a fixed reference. For some patients, the invasiveness of the ring can be highly uncomfortable and not well tolerated. In addition, placing and removing the ring requires special expertise from a neurosurgeon, and patient setup time for SRS can often be long. To reduce the invasiveness, hardware limitations and setup time, we are developing a system for performing accurate head positioning without the use of a head ring. The proposed method uses real-time 6D optical position feedback for turning on and off the treatment beam (gating) and guiding a motor-controlled 3D head motion compensation stage. The setup consists of a central control computer, an optical patient motion tracking system and a 3D motion compensation stage attached to the front of the LINAC couch. A styrofoam head cast was custom-built for patient support and was mounted on the compensation stage. The motion feedback of the markers was processed by the control computer, and the resulting motion of the target was calculated using a rigid body model. If the target deviated beyond a preset position of 0.2 mm, an automatic position correction was performed with stepper motors to adjust the head position via the couch mount motion platform. In the event the target deviated more than 1 mm, a safety relay switch was activated and the treatment beam was turned off. The feasibility of the concept was tested using five healthy volunteers. Head motion data were acquired with and without the use of motion compensation over treatment times of 15 min. On average, test subjects exceeded the 0.5 mm tolerance 86% of the time and the 1.0 mm tolerance 45% of the time without motion correction. With correction, this percentage was reduced to 5% and 2% for the 0.5 mm and 1.0 mm tolerances, respectively.

Stereotactic Body Radiation Therapy for Curative Treatment of Adrenal Metastases

The detection of oligometastatic adrenal metastases is increasing and there are limited data supporting the use of curative intent stereotactic body radiation therapy (SBRT) to treat patients with limited metastatic disease with adrenal involvement. Therefore, we utilized a prospectively maintained database of consecutive patients treated with SBRT for limited metastatic disease (<5 sites) to identify patients with adrenal metastases. Patients were either treated on a three-fraction dose escalation protocol or a ten fraction off-protocol regimen. Outcomes including treated-metastasis control (TMC), distant control (DC), and overall survival (OS) were calculated by the Kaplan-Meier method. Ten patients with 13 adrenal metastases were identified for this case series. The median follow-up was 14.9 months. No patient experienced grade 3 toxicity. The most common grade 1–2 acute toxicities were fatigue (80%) and GI toxicity (40%). One patient experienced late grade 2 adrenal insufficiency. Overall, the 1-year TMC rate was 73%, DC was 30%, and OS was 90%. Three treated adrenal metastases progressed, all receiving the lowest BED10 (43.2 Gy), corresponding to 24 Gy in 3 fractions. After treatment of adrenal metastases with SBRT, the median time to salvage chemotherapy was 5.3 months (range 1.0–38.8 months) and 1-year freedom from salvage chemotherapy was 44%. These results suggest that SBRT to adrenal metastases was tolerated with low toxicity in limited metastatic patients and control rates are promising. This study supports the growing body of literature treating patients with adrenal metastases with SBRT.

Improvements in dose accuracy delivered with static-MLC IMRT on an integrated linear accelerator control system.

PURPOSE Dose accuracy has been shown to vary with dose per segment and dose rate when delivered with static multileaf collimator (SMLC) intensity modulated radiation therapy (IMRT) by Varian C-series MLC controllers. The authors investigated the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of SMLC-IMRT fields on a Varian TrueBeam linear accelerator (LINAC), which delivers dose and manages motion of all components using a single integrated controller. METHODS An SMLC sequence was created consisting of ten identical 10 × 10 cm(2) segments with identical MUs. Beam holding between segments was achieved by moving one out-of-field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 1 to 40 and dose rates of 100-600 MU/min for a 6 MV photon beam (6X) and dose rates of 800-2400 MU/min for a 10 MV flattening-filter free photon (10XFFF) beam. All measurements were made with a Farmer (0.6 cm(3)) ionization chamber placed at the isocenter in a solid-water phantom at 10 cm depth. The measurements were performed on two Varian LINACs: C-series Trilogy and TrueBeam. Each sequence was delivered three times and the dose readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Δ(i)) were compared using F-tests (α = 0.05). RESULTS On the Trilogy, large Δ(i) was observed in small MU segments: at 1 MU/segment, the maximum Δ(i) was 10.1% and 57.9% at 100 MU/min and 600 MU/min, respectively. Also, the first segment of each sequence consistently overshot (Δ(i) > 0), while the last segment consistently undershot (Δ(i) < 0). On the TrueBeam, at 1 MU/segment, Δ(i) ranged from 3.0% to 4.5% at 100 and 600 MU/min; no obvious overshoot/undershoot trend was observed. F-tests showed statistically significant difference [(1 - β) =1.0000] between the Trilogy and the TrueBeam up to 10 MU/segment, at all dose rates greater than 100 MU/min. The linear trend of decreasing dose accuracy as a function of increasing dose rate on the Trilogy is no longer apparent on TrueBeam, even for dose rates as high as 2400 MU/min. Dose inaccuracy averaged over all ten segments in each beam delivery sequence was larger for Trilogy than TrueBeam, with the largest discrepancy (0.2% vs 3%) occurring for 1 MU/segment beams at both 300 and 600 MU/min. CONCLUSIONS Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SMLC-IMRT delivery. Our results confirmed these findings. The dose delivery accuracy for SMLC-IMRT is significantly improved on TrueBeam compared to Trilogy for every combination of low MU/segment (1-10) and high dose rate (200-600 MU/min), in part due to the faster sampling rate (100 vs 20 Hz) and enhanced electronic integration of the MLC controller with the LINAC. SMLC-IMRT can be implemented on TrueBeam with higher dose accuracy per beam (±0.2% vs ±3%) than previous generations of Varian C-series LINACs for 1 MU/segment delivered at 600 MU/min).

Automating linear accelerator quality assurance.

PURPOSE The purpose of this study was 2-fold. One purpose was to develop an automated, streamlined quality assurance (QA) program for use by multiple centers. The second purpose was to evaluate machine performance over time for multiple centers using linear accelerator (Linac) log files and electronic portal images. The authors sought to evaluate variations in Linac performance to establish as a reference for other centers. METHODS The authors developed analytical software tools for a QA program using both log files and electronic portal imaging device (EPID) measurements. The first tool is a general analysis tool which can read and visually represent data in the log file. This tool, which can be used to automatically analyze patient treatment or QA log files, examines the files for Linac deviations which exceed thresholds. The second set of tools consists of a test suite of QA fields, a standard phantom, and software to collect information from the log files on deviations from the expected values. The test suite was designed to focus on the mechanical tests of the Linac to include jaw, MLC, and collimator positions during static, IMRT, and volumetric modulated arc therapy delivery. A consortium of eight institutions delivered the test suite at monthly or weekly intervals on each Linac using a standard phantom. The behavior of various components was analyzed for eight TrueBeam Linacs. RESULTS For the EPID and trajectory log file analysis, all observed deviations which exceeded established thresholds for Linac behavior resulted in a beam hold off. In the absence of an interlock-triggering event, the maximum observed log file deviations between the expected and actual component positions (such as MLC leaves) varied from less than 1% to 26% of published tolerance thresholds. The maximum and standard deviations of the variations due to gantry sag, collimator angle, jaw position, and MLC positions are presented. Gantry sag among Linacs was 0.336 ± 0.072 mm. The standard deviation in MLC position, as determined by EPID measurements, across the consortium was 0.33 mm for IMRT fields. With respect to the log files, the deviations between expected and actual positions for parameters were small (<0.12 mm) for all Linacs. Considering both log files and EPID measurements, all parameters were well within published tolerance values. Variations in collimator angle, MLC position, and gantry sag were also evaluated for all Linacs. CONCLUSIONS The performance of the TrueBeam Linac model was shown to be consistent based on automated analysis of trajectory log files and EPID images acquired during delivery of a standardized test suite. The results can be compared directly to tolerance thresholds. In addition, sharing of results from standard tests across institutions can facilitate the identification of QA process and Linac changes. These reference values are presented along with the standard deviation for common tests so that the test suite can be used by other centers to evaluate their Linac performance against those in this consortium.

SU-G-TeP4-07: Automatic EPID-Based 2D Measurement of MLC Leaf Offset as a Quality Control Tool

PURPOSE The MLC dosimetric leaf gap (DLG) and transmission are measured parameters which impact the dosimetric accuracy of IMRT and VMAT plans. This investigation aims to develop an efficient and accurate routine constancy check of the physical DLG in two dimensions. METHODS The manufacturers recommended DLG measurement method was modified by using 5 fields instead of 11 and by utilizing the Electronic Portal Imaging Device (EPID). Validations were accomplished using an ion chamber (IC) in solid water and a 2D IC array. EPID data was collected for 6 months on multiple TrueBeam linacs using both Millennium and HD MLCs at 5 different clinics in an international consortium. Matlab code was written to automatically analyze the images and calculate the 2D results. Sensitivity was investigated by introducing deliberate leaf position errors. MLC calibration and initialization history was recorded to allow quantification of their impact. Results were analyzed using statistical process control (SPC). RESULTS The EPID method took approximately 5 minutes. Due to detector response, the EPID measured DLG and transmission differed from the IC values but were reproducible and consistent with changes measured using the ICs. For the Millennium MLC, the EPID measured DLG and transmission were both consistently lower than IC results. The EPID method was implemented as leaf offset and transmission constancy tests (LOC and TC). Based on 6 months of measurements, the initial leaf-specific action thresholds for changes from baseline were set to 0.1 mm. Upper and lower control limits for variation were developed for each machine. CONCLUSION Leaf offset and transmission constancy tests were implemented on Varian HD and Millennium MLCs using an EPID and found to be efficient and accurate. The test is effective for monitoring MLC performance using dynamic delivery and performing process control on the DLG in 2D, thus enhancing dosimetric accuracy. This work was supported by a grant from Varian Medical Systems.

WE‐E‐141‐07: Automating Linac QA for Delivery and Analysis

PURPOSE To assess the quantitative performance and reproducibility of a new generation linear accelerator using images and trajectory log files across institutions. METHODS A test suite was created to include tests recommended by TG142 (e.g, picket fence at cardinal static gantry angles and during VMAT) and TG179 (e.g. image quality). The test suite, distributed to a consortium of 7 institutions, consisted of DICOM-RT files and a phantom with BBs at known locations. During each delivery, EPID images were acquired along with trajectory log files. Baseline for each irradiation was set with a flood field without the table or phantom. The phantom was then placed in position and the remaining tests were performed. An analysis program, created in Matlab, assessed the accuracy of leaf, jaw, and collimator positions utilizing the EPID images. Trajectory log files were analyzed as well to assess dynamic parameters such as the reproducibility of gantry motion during arc delivery. RESULTS Fifteen irradiations were performed on 5 accelerators. Leaf position reproducibility was 0.095 mm for a standard MLC and 0.110 mm for an HDMLC, with maximum standard deviations of 0.019, 0.053, and 0.002 mm for static, IMRT, and arc fields over all linacs. Trajectory logs were consistent with measurements. The maximum gantry deviation was 0.247 ± 0.0160 degrees. Using two different materials, the contrast-to-noise ratio was 1.43 ±0.740 and 7.41 ±0.24 for kV and MV images with kV CNR varying by more than a factor of 2 between different machines. CONCLUSION EPID and trajectory logs demonstrated thresholds for detection of leaf position errors that were an order of magnitude less than TG142 requirements for different delivery types across institutions. Trajectory log files provided more detailed information regarding stability of gantry position. When tracked over time, these data can be used to reassess the frequency of different test types. This work is supported by Varian Medical Systems.

SU‐E‐T‐434: Improvements in Step‐And‐Shoot Dose Delivery Accuracy on Varian TrueBeam

Purpose: To investigate the impact of monitor units (MUs) per segment and dose rate on the dose delivery accuracy of step‐and‐shoot intensity modulated radiation (SS‐IMRT) fields on a TrueBeam LINAC.Methods: A step‐and‐shoot multi‐leaf collimator(MLC) sequence was created consisting of 10 identical 10×10cm segments with identical MUs. Beam holding between segments was achieved by moving one out‐of‐field MLC leaf pair. Measurements were repeated for various combinations of MU/segment ranging from 2–40 and dose rates of either 300 or 600MU/min. All measurements were made with a Farmer (0.6cc) ionization chamber placed at the isocenter in a SolidWater phantom at 10cm depth. The measurements were performed on two Varian LINACs: Trilogy and TrueBeam. Each sequence was delivered three times and the charge readings for the corresponding segments were averaged. The effects of MU/segment, dose rate, and LINAC type on the relative dose variation (Delta_i) was compared using F‐tests.Results: On the Trilogy, large Delta_i was observed in small MU segments: at 2MU/segment, the maximum Delta_i was 20.0%/26.2% at 300/600MU/min, respectively. Also, the first segment of each sequence consistently over‐shot(Delta_i>0), while the last segment consistently under‐shot(Delta_i 10 MU/segment) MU segments Conclusions: Earlier generations of Varian LINACs exhibited large dose variations for small MU segments in SS‐IMRT delivery. Our results appeared to confirm these findings. The dose delivery accuracy in small MU segments and at high dose rate for SS‐IMRT is significantly improved on TrueBeam compared to Trilogy, likely due to the faster sampling rate (100Hz vs. 20Hz).

SU‐E‐T‐228: Correlation of 3D Surface Matching with AlignRT and MV Imaging for Whole‐Breast Radiotherapy (WBRT)

Purpose: Surface matching provides quantitative shifts for patient positioning of superficial treatment sites such as breast. We investigated the reliability of 3D surface matching using AlignRT compared to positioning using skin marks followed by MV portal imaging for WBRT. Methods: Five patients receiving two‐field WBRT without respiratory gating were positioned daily on a breastboard (n=2) or custom alphacradle (n=3). For each treatment fraction guided by MV (n=23), the 3D surface captured using AlignRT (v4.5) was compared to the surface generated from CT simulation data. The AlignRT registration algorithm outputs 3D rotations plus translations to optimize matching between surfaces in user‐defined ROIs. The correlation of AlignRT and MV shifts in 4 degree‐of‐freedom (3D translations and table rotation) was studied for two ROIs: the entire surface (‘all’) and the treated breast (‘Breast’). Two surfaces were re‐captured in the treatment position to compare residual registration errors. Parametric statistical tests were considered significant at p 0.9) although residual 3D distances according to AlignRT remained high for ‘all’ (5.0±4.7mm) and ‘Breast’ (4.7±4.2mm). Furthermore, absolute table rotations calculated by AlignRT for consecutively acquired surfaces exhibited fluctuations that were significantly larger for registrations of ‘Breast’ (0.62±0.58 degrees) than ‘all’ (0.27±0.18 degrees). Conclusion: Breast surface matching using AlignRT depends upon the registered ROI. A large ROI showed higher correlation with MV shifts and increased stability when calculating table rotations. Registrations exhibited a large baseline offset for all ROIs, indicating that quantitative table shifts from AlignRT overestimate those determined from MV imaging by expert physicians for WBRT.

SU‐FF‐T‐529: A Feasibility Study On Frameless Gated Head Stereotactic Radiosurgery/Radiotherapy Via Real‐Time Optical Position Monitoring and Adaptive Head Motion Compensation

Purpose: To reduce patient setup time and to perform accurate non‐invasive frameless head radiosurgery/radiotherapy by use of real‐time position feedback for treatment beam gating and head motion stage guiding. Method and Materials: A Polaris 4D tracker (NDI) was used to monitor four optical reflective markers rigidly fixated to a biteblock at 30fps with an RMS accuracy of 0.25mm. Head motion monitoring was performed on healthy volunteers using a styrofoam head cast for support. Simulation and prototyping were investigated using the head motion as the feedback input for beam gating and head motion compensation. Design specifications include using Labview to import the real‐time biofeedback information and to monitor whether the center of the PTV is within a 3D motion tolerance of 1.0mm. If the PTV exceeds the tolerance, a relay switch is activated and the MV beam is turned off. After a 5s stabilization period, provided that the PTV has reentered the set tolerance, the beam is automatically turned on. However, if the patient stabilizes to a new position outside the tolerance, an automatic position correction is performed using a stepper motor controlled head stage. Results: With only a head cast cradle, healthy volunteers mimicked natural sudden sporadic motions on the treatment couch. Data showed that the simulated PTV stayed within the 1mm tolerance for extended periods (∼50s) between large motion excursions. Simulations using recorded head motion data as an input for gating, suggested a total beam‐on time, or duty cycle of 75%. With active head position correction, the duty cycle and spatial accuracy can be further improved. Conclusion: With an optical tracking system and minimal back‐of‐the‐head support, healthy volunteers demonstrated the ability to maintain submillimeter head position with sufficient time windows for treatment. Adaptive motion compensation can be helpful in implementing a high‐efficient and accurate frameless SRS/SRT system.