Machine performance and stability of the first clinical self-shielded stereotactic radiosurgery system: Initial 2-year experience.

This study provides insight into the overall system performance, stability, and delivery accuracy of the first clinical self-shielded stereotactic radiosurgery (SRS) system. Quality assurance procedures specifically developed for this unit are discussed, and trends and variations over the course of 2-years for beam constancy, targeting and dose delivery are presented. Absolute dose calibration for this 2.7 MV unit is performed to deliver 1 cGy/MU at dmax  = 7 mm at a source-to-axis-distance (SAD) of 450 mm for a 25 mm collimator. Output measurements were made with 2-setups: a device that attaches to a fixed position on the couch (daily) and a spherical phantom that attaches to the collimating wheel (monthly). Beam energy was measured using a cylindrical acrylic phantom at depths of 100 (D10 ) and 200 (D20 ) mm. Beam profiles were evaluated using Gafchromic film and compared with TPS beam data. Accuracy in beam targeting was quantified with the Winston-Lutz (WL) and end-to-end (E2E) tests. Delivery quality assurance (DQA) was performed prior to clinical treatments using Gafchromic EBT3/XD film. Net cumulative output adjustments of 15% (pre-clinical), 9% (1st year) and 3% (2nd year) were made. The mean output was 0.997 ± 0.010 cGy/MU (range: 0.960-1.046 cGy/MU) and 0.993 ± 0.029 cGy/MU (range: 0.884-1.065 cGy/MU) for measurements with the daily and monthly setups, respectively. The mean relative beam energy (D10 /D20 ) was 0.998 ± 0.004 (range: 0.991-1.006). The mean total targeting error was 0.46 ± 0.17 mm (range: 0.06-0.98 mm) for the WL and 0.52 ± 0.28 mm (range: 0.11-1.27 mm) for the E2E tests. The average gamma pass rates for DQA measurements were 99.0% and 90.5% for 2%/2 mm and 2%/1 mm gamma criteria, respectively. This SRS unit meets tolerance limits recommended by TG-135, MPPG 9a., and TG-142 with a treatment delivery accuracy similar to what is achieved by other SRS systems.

Technical note: Absolute dose measurements of a vault-free radiosurgery system.

BACKGROUND: Methods for accurate absolute dose (AD) calibration are essential for the proper functioning of radiotherapy treatment machines. Many systems do not conform to TG-51 calibration standards, and modifications are required. TG-21 calibration is also a viable methodology for these situations with the appropriate setup, equipment, and factors. It has been shown that both these methods result in minimal errors. A similar approach has been taken in calibrating the dose for a recent vault-free radiosurgery system. PURPOSE: To evaluate modified TG-21 and TG-51 protocols for AD calibrations of the ZAP-X radiosurgery system using ion chambers, film, and thermoluminescent dosimeters (TLDs). METHODS: The current treatment planning system for ZAP-X requires AD calibration at dmax (7 mm) and 450 mm source-to-axis distance. Both N D , w 60 C o [ G y / C ] $N_{D,w}^{{60}Co}[ {Gy/C} ]$ and Nx [R/C] calibration coefficients were provided by an accredited dosimetry calibration laboratory for a physikalisch technische werkstatten (PTW) 31010 chamber (0.125 cc). The vendor provides an f-bracket that can be mounted on the collimator. Various phantoms can then be attached to the f-bracket. A custom acrylic phantom was designed based on recommendations from TG-21 and technical report series-398 that places the chamber at 500 mm from the source with a depth of 44-mm acrylic and 456-mm SSD. Nx along with other TG-21 parameters was used to calculate the AD. Measurements using a PTW MP3-XS water tank and the same chamber were used to calculate AD using N D , w 60 C o $N_{D,w}^{{60}Co}$ and TG-51 factors. Dose verification was performed using Gafchromic film and 3rd party TLDs. RESULTS: Measurements from TG-51, TG-21 (utilizing the custom acrylic phantom), film, and TLDs agreed to within ± 2%. CONCLUSIONS: A modified TG-51 AD calculation in water is preferred but may not be practical due to the difficulty in tank setup. The TG-21 modified protocol using a custom acrylic phantom is an accurate alternative option for dose calibration. Both of these methods are within acceptable agreement and provide confidence in the system's AD calibration.

Dosimetric Impacts of Source Migration, Radioisotope Type, and Decay with Permanent Implantable Collagen Tile Brachytherapy for Brain Tumors.

Introduction: Brachytherapy using permanently implantable collagen tiles containing cesium-131 (Cs-131) is indicated for treatment of malignant intracranial neoplasms. We quantified Cs-131 source migration and modeled the resulting dosimetric impact for Cs-131, iodine-125 (I-125), and palladium-103 (Pd-103). Methods and Materials: This was a retrospective analysis of a subgroup of patients enrolled in a prospective, single-center, nonrandomized, clinical trial (NCT03088579) of Cs-131 collagen tile brachytherapy. Postimplant Cs-131 plans and hypothetical I-125 and Pd-103 calculations were compared for 20 glioblastoma patients for a set seed geometry. Dosimetric impact of decay and seed migration was calculated for 2 hypothetical scenarios: Scenario 1, assuming seed positions on a given image set were unchanged until acquisition of the subsequent set; Scenario 2, assuming any change in seed positions occurred the day following acquisition of the prior images. Seed migration over time was quantified for a subset of 7 patients who underwent subsequent image-guided radiotherapy. Results: Mean seed migration was 1.7 mm (range: 0.7-3.1); maximum seed migration was 4.3 mm. Mean dose to the 60 Gy volume differed by 0.4 Gy (0.6%, range 0.1-1.0) and 0.9 Gy (1.5%, range 0.2-1.7) for Cs-131, 1.2 Gy (2.0%, range 0.1-2.1) and 1.6 Gy (2.6%, range 1.2-2.6) for I-125, and 0.8 Gy (1.3%, range 0.2-1.5) and 1.4 Gy (2.3%, range 0.3-1.9) for Pd-103, for Scenarios 1 and 2, respectively, compared with the postimplant plan. For a set seed geometry mean implant dose was higher for Pd-103 (1.3 times) and I-125 (1.1 times) versus Cs-131. Dose fall-off was steepest for Pd-103: gradient index 1.88 versus 2.23 (I-125) and 2.40 (Cs-131). Conclusions: Dose differences due to source migration were relatively small, suggesting robust prevention of seed migration from Cs-131-containing collagen tiles. Intratarget heterogeneity was greater with Pd-103 and I-125 than Cs-131. Dose fall-off was fastest with Pd-103 followed by I-125 and then Cs-131.

Image-guided radiotherapy using a mobile kilovoltage x-ray device.

A mobile isocentric C-arm kilovoltage imager has been evaluated as a potential tool for image-guided radiotherapy. The C-arm is equipped with an amorphous silicon flat panel for high-quality image acquisition. Additionally, the device is capable of cone beam computed tomography (CT) and volumetric reconstruction. This is achieved through the application of a modified Feldkamp algorithm with acquisition over a 180 degrees scan arc. The number of projections can be varied from 100 to 1000, resulting in a reconstructed volume 20 cm in diameter by 15-cm long. While acquisition time depends upon number of projections, acceptable quality images can be obtained in less than 60 seconds. Image resolution and contrast of cone-beam phantom images have been compared with images from a conventional CT scanner. The system has a spatial resolution of > or = 10 lp/cm and resolution is approximately equal in all 3 dimensions. Conversely, subject contrast is poorer than conventional CT, compromised by the increased scatter and underlying noise inherent in cone beam reconstruction, as well as the absence of filtering prior to reconstruction. The mobility of the C-arm makes it necessary to determine the C-arm position relative to the linear accelerator isocenter. Two solutions have been investigated: (1) the use of fiducial markers, embedded in the linac couch, that can subsequently be registered in the image sets; and (2), a navigation approach for infrared tracking of the C-arm relative to the linac isocenter. Observed accuracy in phantom positioning ranged from 1.0 to 1.5 mm using the navigation approach and 1.5 to 2.5 mm using the fiducial-based approach. As part of this work, the impact of respiratory motion on cone-beam image quality was evaluated, and a scheme for retrospective gating was devised. Results demonstrated that kilovoltage cone beam CT provides spatial integrity and resolution comparable to conventional CT. Cone-beam CT studies of patients undergoing radiotherapy have demonstrated acceptable soft tissue contrast, allowing assessment of daily changes in target anatomy. Of the 2 approaches developed to register images to the linac isocenter, the navigation method demonstrated superior accuracy for daily patient positioning relative to the fiducial-based method. Finally, significant image degradation due to respiratory motion was observed. It was demonstrated that this could be improved by correlating the acquisition of individual 2D projections with respiration for retrospective reconstruction of phase-based volumetric datasets.