Justus Adamson

Commissioning and dosimetric characteristics of TrueBeam system: Composite data of three TrueBeam machines

PURPOSE A TrueBeam linear accelerator (TB-LINAC) is designed to deliver traditionally flattened and flattening-filter-free (FFF) beams. Although it has been widely adopted in many clinics for patient treatment, limited information is available related to commissioning of this type of machine. In this work, commissioning data of three units were measured, and multiunit comparison was presented to provide valuable insights and reliable evaluations on the characteristics of the new treatment system. METHODS The TB-LINAC is equipped with newly designed waveguide, carousel assembly, monitoring control, and integrated imaging systems. Each machine in this study has 4, 6, 8, 10, 15 MV flattened photon beams, and 6 MV and 10 MV FFF photon beams as well as 6, 9, 12, 16, 20, and 22 MeV electron beams. Dosimetric characteristics of the three new TB-LINAC treatment units are systematically measured for commissioning. High-resolution diode detectors and ion chambers were used to measure dosimetric data for a range of field sizes from 10 × 10 to 400 × 400 mm(2). The composite dosimetric data of the three units are presented in this work. The commissioning of intensity modulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), image-guided radiation therapy, and gating systems are also illustrated. Critical considerations of P(ion) of FFF photon beams and small field dosimetric measurements were investigated. RESULTS The authors found all PDDs and profiles matched well among the three machines. Beam data were quantitatively compared and combined through average to yield composite beam data. The discrepancies among the machines were quantified using standard deviation (SD). The mean SD of the PDDs among the three units is 0.12%, and the mean SD of the profiles is 0.40% for 10 MV FFF open fields. The variations of P(ion) of the chamber CC13 is 1.2 ± 0.1% under 6 MV FFF and 2.0 ± 0.5% under 10 MV FFF from dmax to the 18 cm-off-axis point at 35 cm depth under 40 × 40 cm(2). The mean penumbra of crossplane flattened photon beams at collimator angle of 0° is measured from 5.88 ± 0.09 to 5.99 ± 0.13 mm from 4 to 15 MV at 10 cm depth of 100 × 100 mm(2). The mean penumbra of crossplane beams at collimator angle of 0° is measured as 3.70 ± 0.21 and 4.83 ± 0.04 mm for 6 MV FFF and 10 MV FFF, respectively, at 10 cm depth with a field size of 5 × 5 cm(2). The end-to-end test procedures of both IMRT and VMAT were performed for various energy modes. The mean ion chamber measurements of three units showed less than 2% between measurement and calculation; the mean MultiCube ICA measurements demonstrated over 90% pixels passing gamma analysis (3%, 3 mm, 5% threshold). The imaging dosimetric data of KV planar imaging and CBCT demonstrated improved consistency with vendor specifications and dose reduction for certain imaging protocols. The gated output verification showed a discrepancy of 0.05% or less between gating radiation delivery and nongating radiation delivery. CONCLUSIONS The commissioning data indicated good consistency among the three TB-LINAC units. The commissioning data provided us valuable insights and reliable evaluations on the characteristics of the new treatment system. The systematically measured data might be useful for future reference.

Prostate intrafraction motion evaluation using kV fluoroscopy during treatment delivery: a feasibility and accuracy study.

Margin reduction for prostate radiotherapy is limited by uncertainty in prostate localization during treatment. We investigated the feasibility and accuracy of measuring prostate intrafraction motion using kV fluoroscopy performed simultaneously with radiotherapy. Three gold coils used for target localization were implanted into the patients prostate gland before undergoing hypofractionated online image-guided step-and-shoot intensity modulated radiation therapy (IMRT) on an Elekta Synergy linear accelerator. At each fraction, the patient was aligned using a cone-beam computed tomography (CBCT), after which the IMRT treatment delivery and fluoroscopy were performed simultaneously. In addition, a post-treatment CBCT was acquired with the patient still on the table. To measure the intrafraction motion, we developed an algorithm to register the fluoroscopy images to a reference image derived from the post-treatment CBCT, and we estimated coil motion in three-dimensional (3D) space by combining information from registrations at different gantry angles. We also detected the MV beam turning on and off using MV scatter incident in the same fluoroscopy images, and used this information to synchronize our intrafraction evaluation with the treatment delivery. In addition, we assessed the following: the method to synchronize with treatment delivery, the dose from kV imaging, the accuracy of the localization, and the error propagated into the 3D localization from motion between fluoroscopy acquisitions. With 0.16 mAs/frame and a bowtie filter implemented, the coils could be localized with the gantry at both 0 degrees and 270 degrees with the MV beam off, and at 270 degrees with the MV beam on when multiple fluoroscopy frames were averaged. The localization in two-dimensions for phantom and patient measurements was performed with submillimeter accuracy. After backprojection into 3D the patient localization error was (-0.04 +/- 0.30) mm, (0.09 +/- 0.36)mm, and (0.03 +/- 0.68)mm in the right-left (RL), anterior-posterior (AP), and superior-inferior (SI) axes, respectively. Simulations showed that while oscillating (stationary) motion cannot be effectively represented in 3D, linearly drifting (nonstationary) motion is detectable with good accuracy. These results show that measuring prostate intrafraction motion using a single kV imager during radiotherapy is feasible and can be performed with acceptable accuracy.

Inferences About Prostate Intrafraction Motion From Pre- and Posttreatment Volumetric Imaging

PURPOSE To evaluate the ability of rectal filling, bladder filling, and prostate localization from pre- and posttreatment volumetric imaging to predict prostate intrafraction motion. METHODS AND MATERIALS Pre- and posttreatment cone beam computed tomography images (CBCTs) and intrafractional kV fluoroscopy were acquired at each fraction for 20 prostate patients in supine position, totaling 374 fractions available for analysis. Rectal and bladder filling status were evaluated for each CBCT, and correlation with prostate intrafraction motion measured from kV fluoroscopy was performed. The accuracy of pre and posttreatment prostate localization to predict intrafraction motion was evaluated. RESULTS Rectal filling status was a significant predictor of prostate intrafraction motion (p <0.001), and gas volume was correlated with the maximum vector displacement at MV delivery with a correlation coefficient (cc) of 0.37 and p <0.001. Prostate motion was greater for patients who consistently had gas volume >0.5 cm(3) within the imaged region (cc = 0.52, p = 0.028). A weak relationship was found between bladder filling and posterior prostate drift for fractions with gas volume <or=0.5 cm(3) (cc = -0.17, p = 0.046). The sensitivity of detecting a 3-, 5-, and 7-mm excursion at MV delivery using posttreatment imaging was 76%, 75%, and 81% respectively. CONCLUSIONS Rectal filling is a significant predictor of prostate intrafraction motion, whereas bladder filling is of limited usefulness. Pre- and posttreatment localization can provide a reasonable estimate of prostate motion during MV delivery when intrafraction localization is not available, with an error of 95% within 3.1 mm.

Prostate Intrafraction Motion Assessed by Simultaneous Kilovoltage Fluoroscopy at Megavoltage Delivery I: Clinical Observations and Pattern Analysis

PURPOSE To describe prostate intrafraction motion using kilovoltage fluoroscopy at treatment delivery for a hypofractionated radiotherapy protocol. METHODS AND MATERIALS Kilovoltage images were acquired during treatment delivery, as well as pre- and posttreatment cone-beam computed tomography (CBCT) for each fraction of 30 patients, totaling 571 fractions for analysis. We calculated population statistics, evaluated correlation between interfraction and intrafraction motion, evaluated effect of treatment duration, classified whether motion resolved by posttreatment CBCT, and compared motion magnitude on a per-patient basis. RESULTS The elapsed time between pre- and post-CBCTs was (18.6 ± 4.5) min. The population mean of motion measured by kilovoltage fluoroscopy was (-0.1, 0.5, -0.6) mm, the systematic was (0.5, 1.3, 1.2) mm, and random was (0.9, 1.9, 2.0) mm in the right-left, anterior-posterior, and superior-inferior axes, respectively. The probability of motion increased with treatment duration, with the mean increasing to (0.0, 1.0, -0.9) mm and the systematic to (0.6, 1.7, 1.5) mm when measured using posttreatment CBCT. For any motion ≥2 mm, approximately 75% did not resolve by posttreatment CBCT. Motion magnitude varied considerably among patients, with the probability of a 5-mm displacement ranging from 0.0% to 58.8%. CONCLUSIONS Time dependency of intrafraction motion should be considered to avoid bias in margin assessment, with posttreatment CBCT slightly exaggerating the true motion. The patient-specific nature of the intrafraction motion suggests that a patient-specific management approach may be beneficial.

Dosimetric Effect of Intrafraction Motion and Residual Setup Error for Hypofractionated Prostate Intensity-Modulated Radiotherapy With Online Cone Beam Computed Tomography Image Guidance

PURPOSE To quantify the dosimetric effect and margins required to account for prostate intrafractional translation and residual setup error in a cone beam computed tomography (CBCT)-guided hypofractionated radiotherapy protocol. METHODS AND MATERIALS Prostate position after online correction was measured during dose delivery using simultaneous kV fluoroscopy and posttreatment CBCT in 572 fractions to 30 patients. We reconstructed the dose distribution to the clinical tumor volume (CTV) using a convolution of the static dose with a probability density function (PDF) based on the kV fluoroscopy, and we calculated the minimum dose received by 99% of the CTV (D(99)). We compared reconstructed doses when the convolution was performed per beam, per patient, and when the PDF was created using posttreatment CBCT. We determined the minimum axis-specific margins to limit CTV D(99) reduction to 1%. RESULTS For 3-mm margins, D(99) reduction was ≤5% for 29/30 patients. Using post-CBCT rather than localizations at treatment delivery exaggerated dosimetric effects by ~47%, while there was no such bias between the dose convolved with a beam-specific and patient-specific PDF. After eight fractions, final cumulative D(99) could be predicted with a root mean square error of <1%. For 90% of patients, the required margins were ≤2, 4, and 3 mm, with 70%, 40%, and 33% of patients requiring no right-left (RL), anteroposterior (AP), and superoinferior margins, respectively. CONCLUSIONS For protocols with CBCT guidance, RL, AP, and SI margins of 2, 4, and 3 mm are sufficient to account for translational errors; however, the large variation in patient-specific margins suggests that adaptive management may be beneficial.

Prostate intrafraction motion assessed by simultaneous kV fluoroscopy at MV delivery II: adaptive strategies.

PURPOSE To investigate potential benefits of adaptive strategies for managing prostate intrafractional uncertainties when interfraction motion is corrected online. METHODS AND MATERIALS Prostate intrafraction motion was measured using kV fluoroscopy during MV delivery for 571 fractions from 30 hypofractionated radiotherapy patients. We evaluated trending over treatment course using analysis of variance statistics, and we evaluated the ability to correct patient-specific systematic error and apply patient-specific statistical margins after 2 to 15 fractions to compensate 90% of motion. We also evaluated the ability to classify patients into small- and large-motion subgroups based on the first 2 to 20 fractions using discriminant analysis. RESULTS No time trend was observed over treatment course, and intrafraction motion was patient specific (p < 0.0001). Systematic error in the first week correlated well with that in subsequent weeks, with correlation coefficients of 0.53, 0.50, and 0.41 in right-left (RL), anterior-posterior (AP), and superior-inferior (SI), respectively. After 5 fractions, the adaptive strategy resulted in average margin reductions of 0.3, 0.7, and 0.7 mm in RL, AP, and SI, respectively, with margins ranging from 1 to 3.2 mm in RL, 2 to 7.0 mm in AP, and 2 to 6.6 mm in SI. By contrast, population margins to include the same percentage of motion were 1.7, 4.0, and 4.1 mm. After 2 and 5 fractions, patients were classified into small- and large-motion groups with ~77% and ~83% accuracy. CONCLUSIONS Adaptive strategies are feasible and beneficial for intrafraction motion management in prostate cancer online image guidance. Patients may be classified into large- and small-motion groups in early fractions using discriminant analysis.

Independent verification of gantry angle for pre-treatment VMAT QA using EPID.

We propose a method to incorporate independent verification of gantry angle for electronic portal imaging device (EPID)-based pre-treatment quality assurance (QA) of clinical volumetric modulated arc therapy (VMAT) plans. Gantry angle is measured using projections in the EPID of a custom phantom placed on the couch and the treatment plan is modified so as to be incident on the phantom with a portion of the beam that is collimated in the clinical plan. For our implementation, collimator and couch angles were set to zero and the inferior jaw and two most inferior multi-leaf collimator pairs were opened for the entire QA delivery. A phantom containing five gold coils was used to measure the gantry rotation through which each portal image was acquired. We performed the EPID QA for ten clinical plans and evaluated accuracy of gantry angle measurement, scatter incident on the imager due to the phantom, inter-image pixel linearity and inter- and intra-image noise. The gantry angle could be measured to within 0.0 ± 0.3° for static gantry and 0.2 ± 0.2° for arc acquisitions. Scatter due to the presence of the phantom was negligible. The procedure was shown to be feasible and adds gantry angle to the treatment planning parameters that can be verified by EPID-based pre-treatment VMAT QA.

Optimizing monoscopic kV fluoro acquisition for prostate intrafraction motion evaluation

Monoscopic kV imaging during radiotherapy has been recently implemented for prostate intrafraction motion evaluation. However, the accuracy of 3D localization techniques from monoscopic imaging of prostate and the effect of acquisition parameters on the 3D accuracy have not been studied in detail, with imaging dose remaining a concern. In this paper, we investigate methods to optimize the kV acquisition parameters and imaging protocol to achieve improved 3D localization and 2D image registration accuracy for minimal imaging dose. Prostate motion during radiotherapy was simulated using existing cine-MRI measurements, and was used to investigate the accuracy of various 3D localization techniques and the effect of the kV acquisition protocol. We also investigated the relationship between mAs and the accuracy of the 2D image registration for localization of fiducial markers and we measured imaging dose for a 30 cm diameter phantom to evaluate the necessary dose to achieve acceptable image registration accuracy. Simulations showed that the error in assuming the shortest path to localize the prostate in 3D using monoscopic imaging during a typical IMRT fraction will be less than approximately 1.5 mm for 95% of localizations, and will also depend on prostate motion distribution, treatment duration and image acquisition and treatment protocol. Most uncertainty cannot be reduced from higher imaging frequency or acquiring during gantry rotation between beams. Measured maximum surface dose to the cylindrical phantom from monoscopic kV intrafraction acquisitions varied between 0.4 and 5.5 mGy, depending on the acquisition protocol, and was lower than the required dose for CBCT (21.1 mGy). Imaging dose can be lowered by approximately 15-40% when mAs is optimized with acquisition angle. Images acquired during MV beam delivery require increased mAs to obtain the same level of registration accuracy, with mAs/registration increasing roughly linearly with field size and dose rate.

Physics considerations for single-isocenter, volumetric modulated arc radiosurgery for treatment of multiple intracranial targets

OBJECTIVE Our purpose was to address challenges associated with single-isocenter radiosurgery for multiple intracranial targets (SIRMIT) including increased sensitivity to rotational uncertainties (resulting from distance of the targets from isocenter) as well as potential for decreased plan quality from larger multileaf collimator width >4 cm from isocenter. METHODS AND MATERIALS We evaluated the effect that a 6 degrees-of-freedom couch correction had on localization uncertainty for SIRMIT using thermoplastic mask immobilization. Required setup margin was determined from rotation of the skull and mask (setup kV cone beam computed tomography relative to planning computed tomography). Intraoperational margin was determined from skull rotation within the mask (difference between pre- and posttreatment cone beam computed tomography). We also investigated 4 isocenter placement strategies: volume centroid, centroid of equally weighted points (1 per target), centroid of points weighted by inverse of volume, and Eclipses built-in method. RESULTS When no 6 degrees-of-freedom couch correction is performed after initial setup, a 0.35-mm margin is required per centimeter of target-isocenter separation to account for 95% of rotational uncertainties at initial setup. This margin is reduced to 0.10 mm/cm of target-isocenter separation to account for intraoperative rotational uncertainties when the initial setup uncertainty is eliminated via image guided 6 degrees-of-freedom couch correction. Analysis of 11 multitarget plans (37 targets) showed that conformity index and gradient index improved with decreasing distance from isocenter, this trend being more pronounced for targets <1 mL. Alternative isocenters aimed at decreasing distance of small targets improved their gradient index, but resulted in poorer dose indices for large targets. Mean distance from isocenter was smallest for the centroid of equally weighted points (4.1 ± 1.6cm vs 4.2-4.5cm). CONCLUSIONS Rotational corrections via image guidance are necessary for SIRMIT with a thermoplastic mask for immobilization. There is a clear tradeoff between dosimetric quality of small and large targets that should be considered carefully when placing the isocenter.

Treatment Planning and Delivery of Whole Brain Irradiation with Hippocampal Avoidance in Rats.

Background Despite the clinical benefit of whole brain radiotherapy (WBRT), patients and physicians are concerned by the long-term impact on cognitive functioning. Many studies investigating the molecular and cellular impact of WBRT have used rodent models. However, there has not been a rodent protocol comparable to the recently reported Radiation Therapy Oncology Group (RTOG) protocol for WBRT with hippocampal avoidance (HA) which is intended to spare cognitive function. The aim of this study was to develop a hippocampal-sparing WBRT protocol in Wistar rats. Methods The technical and clinical challenges encountered in hippocampal sparing during rat WBRT are substantial. Three key challenges were identified: hippocampal localization, treatment planning, and treatment localization. Hippocampal localization was achieved with sophisticated imaging techniques requiring deformable registration of a rat MRI atlas with a high resolution MRI followed by fusion via rigid registration to a CBCT. Treatment planning employed a Monte Carlo dose calculation in SmART-Plan and creation of 0.5cm thick lead blocks custom-shaped to match DRR projections. Treatment localization necessitated the on-board image-guidance capability of the XRAD C225Cx micro-CT/micro-irradiator (Precision X-Ray). Treatment was accomplished with opposed lateral fields with 225 KVp X-rays at a current of 13mA filtered through 0.3mm of copper using a 40x40mm square collimator and the lead blocks. A single fraction of 4Gy was delivered (2Gy per lateral field) with a 41 second beam on time per field at a dose rate of 304.5 cGy/min. Dosimetric verification of hippocampal sparing was performed using radiochromic film. In vivo verification of HA was performed after delivery of a single 4Gy fraction either with or without HA using γ-H2Ax staining of tissue sections from the brain to quantify the amount of DNA damage in rats treated with HA, WBRT, or sham-irradiated (negative controls). Results The mean dose delivered to radiochromic film beneath the hippocampal block was 0.52Gy compared to 3.93Gy without the block, indicating an 87% reduction in the dose delivered to the hippocampus. This difference was consistent with doses predicted by Monte Carlo dose calculation. The Dose Volume Histogram (DVH) generated via Monte Carlo simulation showed an underdose of the target volume (brain minus hippocampus) with 50% of the target volume receiving 100% of the prescription isodose as a result of the lateral blocking techniques sparing some midline thalamic and subcortical tissue. Staining of brain sections with anti-phospho-Histone H2A.X (reflecting double-strand DNA breaks) demonstrated that this treatment protocol limited radiation dose to the hippocampus in vivo. The mean signal intensity from γ-H2Ax staining in the cortex was not significantly different from the signal intensity in the cortex of rats treated with WBRT (5.40 v. 5.75, P = 0.32). In contrast, the signal intensity in the hippocampus of rats treated with HA was significantly lower than rats treated with WBRT (4.55 v. 6.93, P = 0.012). Conclusion Despite the challenges of planning conformal treatments for small volumes in rodents, our dosimetric and in vivo data show that WBRT with HA is feasible in rats. This study provides a useful platform for further application and refinement of the technique.