Prediction of conical collimator collision for stereotactic radiosurgery.

The purpose of this study is to predict the collision clearance distance of stereotactic cones with treatment setup devices in cone-based stereotactic radiosurgery (SRS). The BrainLAB radiosurgery system with a Frameless Radiosurgery Positioning Array and dedicated couch top was targeted in this study. The positioning array and couch top were scanned with CT simulators, and their outer contours of were detected. The minimum clearance distance was estimated by calculating the Euclidian distances between the surface of the SRS cones and the nearest surface of the outer contours. The coordinate transformation of the outer contour was performed by incorporating the Beam's Eye View at a planned arc range and couch angle. From the minimum clearance distance, the collision-free gantry ranges for each couch angle were sequentially determined. An in-house software was developed to calculate the clearance distance between the cone surface and the outer contours, and thus determine the occurrence of a collision. The software was extensively tested for various combinations of couch and arc angles at multiple isocenter locations for two combinations of cone-couch systems. A total of 50 arcs were used to validate the calculation accuracies of the software for each system. The calculated minimum distances and collision-free angles from the software were verified by physical measurements. The calculated minimum distances were found to agree with the measurements to within 0.3 ± 0.9 mm. The collision-free arc angles from the software also agreed with the measurements to within 1.1 ± 1.1° with a 5-mm safety margin for 20 arcs. In conclusion, the in-house software was able to calculate the minimum clearance distance with <1.0 mm accuracy and to determine the collision-free arc range for the cone-based BrainLab SRS system.

Single fraction spine stereotactic ablative body radiotherapy with volumetric modulated arc therapy.

This study investigated a single institution's experience with volumetric modulated arc therapy (VMAT) directed stereotactic ablative body radiotherapy (SABR) for vertebral metastases. From 2010 to 2014, 95 lesions of spinal metastases in 73 patients were treated with SABR using VMAT. Clinical local control, pain level, and use of steroid medication were employed to evaluate treatment responses. The majority (79%) of patients were treated with a radiation dose of 20 Gy in a single fraction. However, when normal tissue constraints could not be achieved, the dose was reduced to 18 Gy (11%) or 16 Gy (8%) in 1 fraction. At the median follow up of 12.7 months (mean 18.0, range 1-56 months), clinical local control was 97% (92 out of 95). There was a mean 81% (median 100%, range 28-100%) decrease in subjective pain score. Seventy-seven percent of patients had a decrease in narcotic pain medication use. Pain was completely resolved at the treatment site for 69% (66/95) of patients. Prior to the SABR treatment, 33% (31/95) of patients had epidural extension of tumor. Among patients with epidural involvement, 45% (14/31) exhibited neurologic impairment prior to treatment. Twenty-three percent (7/31) experienced spinal cord compression. Prior to treatment, 34 patients experienced some form of neurologic impairment. Of these patients, 24% (8/34) experienced improved motor functioning; the remaining 76% (26/34) of patients' neurological dysfunction were stable. Our results indicate the SABR regimen using VMAT technique is clinically effective in achieving clinical local control and palliation. This is the first publication reporting clinical outcomes of VMAT directed SABR.

Implementation of full/half bowtie filter models in a commercial treatment planning system for kilovoltage cone-beam CT dose estimations.

The purpose of this study was to implement full/half bowtie filter models in a com-mercial treatment planning system (TPS) to calculate kilovoltage (kV) cone-beam CT (CBCT) doses of Varian On-Board Imager (OBI) kV X-ray imaging system. The full/half bowtie filter models were created as compensators in Pinnacle TPS using MATLAB software. The physical profiles of both bowtie filters were imported and hard-coded in the MATLAB system. Pinnacle scripts were written to import bowtie filter models into Pinnacle treatment plans. Bowtie filter-free kV X-ray beam models were commissioned and the bowtie filter models were validated by analyzing the lateral and percent-depth-dose (PDD) profiles of anterior/posterior X-ray beams in water phantoms. A CT dose index (CTDI) phantom was employed to calculate CTDI and weighted CTDI values for pelvis and pelvis-spotlight CBCT protocols. A five-year-old pediatric anthropomorphic phantom was utilized to evaluate absorbed and effective doses (ED) for standard and low-dose head CBCT protocols. The CBCT dose calculation results were compared to ion chamber (IC) and Monte Carlo (MC) data for the CTDI phantom and MOSFET and MC results for the pediatric phantom, respectively. The differences of lateral and PDD profiles between TPS calculations and IC measurements were within 6%. The CTDI and weighted CTDI values of the TPS were respectively within 0.25 cGy and 0.08 cGy compared to IC measurements. The absorbed doses ranged from 0 to 7.22 cGy for the standard dose CBCT and 0 to 1.56 cGy for the low-dose CBCT. The ED values were found to be 36-38 mSv and 7-8 mSv for the standard and low-dose CBCT protocols, respectively. This study demonstrated that the established full/half bowtie filter beam models can produce reasonable dose calculation results. Further study is to be performed to evaluate the models in clinical situations.

Off-target-isocentric approach in non-coplanar Volumetric Modulated Arc Therapy (VMAT) planning for lung SBRT treatments.

PURPOSE: Volumetric Modulated Arc Therapy (VMAT) has emerged as an efficient alternative to traditional three-dimensional (3D) non-coplanar conformal (3D-NC-C) beams for lung cancer stereotactic body radiotherapy (SBRT) because of its superior dosimetric properties and native ease in planning and treatment delivery. However, patient immobilization in lung SBRT often presents challenging geometrical clearance issues in the execution of large (in excess of 180°) non-coplanar arcs. In this study, we present an off-target-isocentric, non-coplanar VMAT (OTI-NC-VMAT) technique that appears to be simple, dosimetrically robust and allows for ample patient/couch-gantry clearance. We compared this technique to a target-isocentric, non-coplanar VMAT (TI-NC-VMAT) technique and the 3D-NC-C beams for dosimetric evaluations. METHODS: Nineteen lung cancer patients previously treated with 3D-NC-C SBRT technique at our institution were selected. For each patient, an OTI-NC-VMAT plan and TI-NC-VMAT plan were created and compared to the original 3D-NC-C treatment plan. All of the plans were created for the same prescription dose of 54 Gy total in 3 fractions, covering 95% of the planning target volume (PTV). Nine to ten non-coplanar beams were used for the 3D technique and three non-coplanar arcs were used in both the TI-NC-VMAT and OTI-NC-VMAT plans, with the couch set at ± 20° and 0°, with each arc rotation in excess of 180°. Progressive Resolution Optimizer (PRO) in Varian Eclipse version 11 was used for all of the treatment planning. Conformity Index (CI), conformity number (CN), gradient index (GI), maximum dose at 2 cm away from the PTV (D2cm), mean lung dose (MLD), V20, V5 and mean target dose (MTD) were analyzed for all of the plans. We also performed statistical analysis to examine differences in the dosimetric indices between 3D and VMAT techniques. RESULTS: Dosimetric indices CI, CN, GI, V20 and MTD values were similar, within 5%, for all three plans: 3D-NC-C, TI-NC-VMAT and OTI-NC-VMAT. However, both types of VMAT plans were dosimetrically superior to 3D conformal plans in organ-at-risk (OAR) sparing; D2cm, MLD, and V5 values were significantly lower at 6-8%, 9-12% and 26-30% in VMAT plans, respectively. The OTI-NC-VMAT plans showed equivalent plan quality to the TI-NC-VMAT plans and exhibited robust freedom from limiting arc rotation due to potential patient/couch-gantry collision. CONCLUSIONS: The OTI-NC-VMAT plans appear dosimetrically equivalent to TI-NC-VMAT plans for lung SBRT, while permitting large angle arc selection, free from obstructional limitations. Both OTI-NC-VMAT and TI-NC-VMAT plans were dosimetrically superior to 3D-NC-C plans in terms of organ-at-risk (OAR) sparing.

Spatial variations of multiple off-axial targets for a single isocenter SRS treatment in Novalis Tx linac system.

PURPOSE: To develop a method to evaluate the positional variations of multiple off-axial targets for a single isocenter stereotactic radiosurgery (SRS) treatment in Novalis Tx linac system.Method and Materials: Five metallic ball bearing (BB) markers were placed sparsely in 3D off-axial locations (non-coplanar) inside a skull phantom as the representatives of multiple targets mimicking multiple brain metastases. The locations of the BB markers were carefully chosen to minimize overlapping of each other in a portal imaging detector plane. The skull phantom was immobilized by a frameless mask and CT scanned with a BrainLab Head&Neck Localizer using a GE Optima multi-detector CT (MDCT) scanner. The CT images were exported to iPlan treatment planning software and a multiple target PTV was drawn by combining all the contours of the BB markers. The margin of the MLC opening was selected as 3 mm expansion outward. Four non-coplanar arc beams were placed to generate a single isocenter SRS plan to treat the PTV. The skull phantom was localized to the treatment position using ExacTrac 6D Patient Positioning system. The four dynamic conformal arc beams were delivered using Novalis Tx system with portal imaging acquisition mode per 10% temporal resolution. The locations of the BB markers were visualized and analyzed with respect to the MLC aperture in the treatment plan similar to the Winston-Lutz (WL) test. RESULTS: All the BB markers were clearly identified inside the MLC openings. The total positional errors for the MLC aperture were 0.61 ± 0.2 mm along the rotational path of the four arcs. CONCLUSION: This study verified that the spatial deviations of multiple off-axial targets for a single isocenter SRS treatment plan is within sub-millimeter range in Novalis Tx linac system. Accompanied with the WL test, this simple test will quality-assure the spatial accuracies of the isocenter as well as the positions of multiple off-axial targets for the SRS treatment using a single isocenter multiple target treatment plan.

Transient activation of hedgehog pathway rescued irradiation-induced hyposalivation by preserving salivary stem/progenitor cells and parasympathetic innervation.

PURPOSE: To examine the effects and mechanisms of transient activation of the Hedgehog pathway on rescuing radiotherapy-induced hyposalivation in survivors of head and neck cancer. EXPERIMENTAL DESIGN: Mouse salivary glands and cultured human salivary epithelial cells were irradiated by a single 15-Gy dose. The Hedgehog pathway was transiently activated in mouse salivary glands, by briefly overexpressing the Sonic hedgehog (Shh) transgene or administrating smoothened agonist, and in human salivary epithelial cells, by infecting with adenovirus encoding Gli1. The activity of Hedgehog signaling was examined by the expression of the Ptch1-lacZ reporter and endogenous Hedgehog target genes. The salivary flow rate was measured following pilocarpine stimulation. Salivary stem/progenitor cells (SSPC), parasympathetic innervation, and expression of related genes were examined by flow cytometry, salisphere assay, immunohistochemistry, quantitative reverse transcription PCR, Western blotting, and ELISA. RESULTS: Irradiation does not activate Hedgehog signaling in mouse salivary glands. Transient Shh overexpression activated the Hedgehog pathway in ductal epithelia and, after irradiation, rescued salivary function in male mice, which is related with preservation of functional SSPCs and parasympathetic innervation. The preservation of SSPCs was likely mediated by the rescue of signaling activities of the Bmi1 and Chrm1-HB-EGF pathways. The preservation of parasympathetic innervation was associated with the rescue of the expression of neurotrophic factors such as Bdnf and Nrtn. The expression of genes related with maintenance of SSPCs and parasympathetic innervation in female salivary glands and cultured human salivary epithelial cells was similarly affected by irradiation and transient Hedgehog activation. CONCLUSIONS: These findings suggest that transient activation of the Hedgehog pathway has the potential to restore salivary gland function after irradiation-induced dysfunction.

Spiral computed tomography phase-space source model in the BEAMnrc/EGSnrc Monte Carlo system: implementation and validation.

Currently, the BEAMnrc/EGSnrc Monte Carlo (MC) system does not provide a spiral CT source model for the simulation of spiral CT scanning. We developed and validated a spiral CT phase-space source model in the BEAMnrc/EGSnrc system. The spiral phase-space source model was implemented in the DOSXYZnrc user code of the BEAMnrc/EGSnrc system by analyzing the geometry of spiral CT scan-scan range, initial angle, rotational direction, pitch, slice thickness, etc. Table movement was simulated by changing the coordinates of the isocenter as a function of beam angles. Some parameters such as pitch, slice thickness and translation per rotation were also incorporated into the model to make the new phase-space source model, designed specifically for spiral CT scan simulations. The source model was hard-coded by modifying the 'ISource = 8: Phase-Space Source Incident from Multiple Directions' in the srcxyznrc.mortran and dosxyznrc.mortran files in the DOSXYZnrc user code. In order to verify the implementation, spiral CT scans were simulated in a CT dose index phantom using the validated x-ray tube model of a commercial CT simulator for both the original multi-direction source (ISOURCE = 8) and the new phase-space source model in the DOSXYZnrc system. Then the acquired 2D and 3D dose distributions were analyzed with respect to the input parameters for various pitch values. In addition, surface-dose profiles were also measured for a patient CT scan protocol using radiochromic film and were compared with the MC simulations. The new phase-space source model was found to simulate the spiral CT scanning in a single simulation run accurately. It also produced the equivalent dose distribution of the ISOURCE = 8 model for the same CT scan parameters. The MC-simulated surface profiles were well matched to the film measurement overall within 10%. The new spiral CT phase-space source model was implemented in the BEAMnrc/EGSnrc system. This work will be beneficial in estimating the spiral CT scan dose in the BEAMnrc/EGSnrc system.

Practical methods for improving dose distributions in Monte Carlo-based IMRT planning of lung wall-seated tumors treated with SBRT.

Current commercially available planning systems with Monte Carlo (MC)-based final dose calculation in IMRT planning employ pencil-beam (PB) algorithms in the optimization process. Consequently, dose coverage for SBRT lung plans can feature cold-spots at the interface between lung and tumor tissue. For lung wall (LW)-seated tumors, there can also be hot spots within nearby normal organs (example: ribs). This study evaluated two different practical approaches to limiting cold spots within the target and reducing high doses to surrounding normal organs in MC-based IMRT planning of LW-seated tumors. First, "iterative reoptimization", where the MC calculation (with PB-based optimization) is initially performed. The resultant cold spot is then contoured and used as a simultaneous boost volume. The MC-based dose is then recomputed. The second technique uses noncoplanar beam angles with limited path through lung tissue. Both techniques were evaluated against a conventional coplanar beam approach with a single MC calculation. In all techniques the prescription dose was normalized to cover 95% of the PTV. Fifteen SBRT lung cases with LW-seated tumors were planned. The results from iterative reoptimization showed that conformity index (CI) and/or PTV dose uniformity (UPTV) improved in 12/15 plans. Average improvement was 13%, and 24%, respectively. Nonimproved plans had PTVs near the skin, trachea, and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in 14/15 plans (average 10%). Using noncoplanar beams showed an average improvement of 7% in 10/15 cases and 11% in 5/15 cases for CI and UPTV, respectively. The D1cc was reduced by an average of 6% in 10/15 cases to surrounding OARs. Choice of treatment planning technique did not statistically significantly change lung V5. The results showed that the proposed practical approaches enhance dose conformity in MC-based IMRT planning of lung tumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs.

Clinical commissioning and use of the Novalis Tx linear accelerator for SRS and SBRT.

The purpose of this study was to perform comprehensive measurements and testing of a Novalis Tx linear accelerator, and to develop technical guidelines for com-missioning from the time of acceptance testing to the first clinical treatment. The Novalis Tx (NTX) linear accelerator is equipped with, among other features, a high-definition MLC (HD120 MLC) with 2.5 mm central leaves, a 6D robotic couch, an optical guidance positioning system, as well as X-ray-based image guidance tools to provide high accuracy radiation delivery for stereotactic radiosurgery and stereotactic body radiation therapy procedures. We have performed extensive tests for each of the components, and analyzed the clinical data collected in our clinic. We present technical guidelines in this report focusing on methods for: (1) efficient and accurate beam data collection for commissioning treatment planning systems, including small field output measurements conducted using a wide range of detectors; (2) commissioning tests for the HD120 MLC; (3) data collection for the baseline characteristics of the on-board imager (OBI) and ExacTrac X-ray (ETX) image guidance systems in conjunction with the 6D robotic couch; and (4) end-to-end testing of the entire clinical process. Established from our clinical experience thus far, recommendations are provided for accurate and efficient use of the OBI and ETX localization systems for intra- and extracranial treatment sites. Four results are presented. (1) Basic beam data measurements: Our measurements confirmed the necessity of using small detectors for small fields. Total scatter factors varied significantly (30% to approximately 62%) for small field measurements among detectors. Unshielded stereotactic field diode (SFD) overestimated dose by ~ 2% for large field sizes. Ion chambers with active diameters of 6 mm suffered from significant volume averaging. The sharpest profile penumbra was observed for the SFD because of its small active diameter (0.6 mm). (2) MLC commissioning: Winston Lutz test, light/radiation field congruence, and Picket Fence tests were performed and were within criteria established by the relevant task group reports. The measured mean MLC transmission and dynamic leaf gap of 6 MV SRS beam were 1.17% and 0.36 mm, respectively. (3) Baseline characteristics of OBI and ETX: The isocenter localization errors in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ETX, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI cone-beam computed tomography. The registration angular discrepancy was 0.1 ± 0.2°, and the maximum robotic couch error was 0.2°. (4) End-to-end tests: The measured isocenter dose differences from the planned values were 0.8% and 0.4%, measured respectively by an ion chamber and film. The gamma pass rate, measured by EBT2 film, was 95% (3% DD and 1 mm DTA). Through a systematic series of quantitative commissioning experiments and end-to-end tests and our initial clinical experience, described in this report, we demonstrate that the NTX is a robust system, with the image guidance and MLC requirements to treat a wide variety of sites - in particular for highly accurate delivery of SRS and SBRT-based treatments.

Clinical use of dual image-guided localization system for spine radiosurgery.

The recently released Novalis TX linac platform provides various image guided localization methods including a stereoscopic X-ray imaging technique (ExacTrac) and a volumetric cone beam computed tomography (CBCT) imaging technique. The ExacTrac combined with the robotic six dimensional (6D) couch provides fast and accurate patient setup based on bony structures and offers "snap shot" imaging at any point during the treatment to detect patient motion. The CBCT offers a three dimensional (3D), volumetric image of the patient's setup with visualization of anatomic structures. However, each imaging system has a separate isocenter, which may not coincide with each other or with the linac isocenter. The aim of this paper was to compare the localization accuracy between Exactrac and CBCT for single fraction spine radiosurgery treatments. The study was performed for both phantom and patients (96 clinical treatments of 57 patients). The discrepancies between the isocenter between the ExacTrac and CBCT in four dimensions (three translations and one rotation) were recorded and statistically analyzed using two-tailed t-test.

Characteristics of x-ray beams in two commercial multidetector computed tomography simulators: Monte Carlo simulations.

PURPOSE: As multidetector computed tomography (MDCT) scanning is routinely performed for treatment planning in radiation oncology, understanding the characteristics of the MDCT x-ray beam is essential to accurately estimate patient dose. The purpose of this study is to characterize the x-ray beams of two commercial MDCT simulators widely used in radiation oncology by Monte Carlo (MC) simulations. METHODS: X-ray tube systems of two wide bore MDCT scanners (GE LightSpeed RT 4 and Philips Brilliance Big Bore) were modeled in the BEAMNRC/EGSNRC MC system. All the tube components were modeled from targets to bowtie filters. To validate our MC models, the authors measured half-value layers (HVL) using aluminum sheets and multifunctional radiation detectors and compared them to those obtained from MC simulations for 120 kVp beams. The authors also compared x-ray spectra obtained from MC simulation to the data provided by manufacturers. Additionally, lateral/axial beam profiles were measured in-air using radiochromic films and compared to the MC results. To understand the scatter effect, the authors also derived the scatter-to-primary energy fluence ratio (SPR) profiles and calculated the total SPR for each CT system with the CT dose index (CTDI) head and body phantoms using the BEAMNRC system. RESULTS: The authors found that the HVL, x-ray spectrum and beam profiles of the MC simulations agreed well with the manufacturer-specified data within 1%-10% on average for both scanners. The total SPR were ranged from 7.8 to 13.7% for the head phantom and from 10.7 to 18.9% for the body phantom. CONCLUSIONS: The authors demonstrate the full MC simulations of two commercial MDCT simulators to characterize their x-ray beams. This study may be useful to establish a patient-specific dosimetry for the MDCT systems.

Radiation dose from 3D rotational X-ray imaging: organ and effective dose with conversion factors.

The purpose of this study was to measure organ doses and the effective dose (ED) using a three-dimensional rotational X-ray (3D-RX) system and to determine the ED conversion factor from the dose area product (DAP) for skull, spine and biliary protocols. A commercial 3D-RX imaging system was used to simulate the protocols with the adult female anthropomorphic phantom. Twenty MOSFET detectors were used to measure the absorbed doses at various organ locations. The ED was calculated for each protocol and the corresponding DAP was obtained. The skin dose was the highest for all the protocols. The second highest organ doses were those of the brain for the skull, the intestine for the spine and the kidney for the biliary protocol. The ED was 0.4-0.9, 4.2-8.4 and 3.2-4.6 mSv, and the ED conversion factor was 0.06-0.09, 0.18-0.31 and 0.13-0.23 mSv Gy(-1) cm(-2) for each protocol, respectively. This data may be used to estimate the patient ED for those protocols in the 3D-RX.

Computed tomography dose index and dose length product for cone-beam CT: Monte Carlo simulations.

Dosimetry in kilovoltage cone beam computed tomography (CBCT) is a challenge due to the limitation of physical measurements. To address this, we used a Monte Carlo (MC) method to estimate the CT dose index (CTDI) and the dose length product (DLP) for a commercial CBCT system. As Dixon and Boone showed that CTDI concept can be applicable to both CBCT and conventional CT, we evaluated weighted CT dose index (CTDI(w)) and DLP for a commercial CBCT system. Two extended CT phantoms were created in our BEAMnrc/EGSnrc MC system. Before the simulations, the beam collimation of a Varian On-Board Imager (OBI) system was measured with radiochromic films (model: XR-QA). The MC model of the OBI X-ray tube, validated in a previous study, was used to acquire the phase space files of the full-fan and half-fan cone beams. Then, DOSXYZnrc user code simulated a total of 20 CBCT scans for the nominal beam widths from 1 cm to 10 cm. After the simulations, CBCT dose profiles at center and peripheral locations were extracted and integrated (dose profile integral, DPI) to calculate the CTDI per each beam width. The weighted cone-beam CTDI (CTDI(w,l)) was calculated from DPI values and mean CTDI(w,l) (CTDI(w,l)) and DLP were derived. We also evaluated the differences of CTDI(w) values between MC simulations and point dose measurements using standard CT phantoms. In results, it was found that CTDI(w,600) was 8.74 ± 0.01 cGy for head and CTDI(w,900) was 4.26 ± 0.01 cGy for body scan. The DLP was found to be proportional to the beam collimation. We also found that the point dose measurements with standard CT phantoms can estimate the CTDI within 3% difference compared to the full integrated CTDI from the MC method. This study showed the usability of CTDI as a dose index and DLP as a total dose descriptor in CBCT scans.

Kilovoltage cone-beam CT: comparative dose and image quality evaluations in partial and full-angle scan protocols.

PURPOSE: To assess imaging dose of partial and full-angle kilovoltage CBCT scan protocols and to evaluate image quality for each protocol. METHODS: The authors obtained the CT dose index (CTDI) of the kilovoltage CBCT protocols in an on-board imager by ion chamber (IC) measurements and Monte Carlo (MC) simulations. A total of six new CBCT scan protocols were evaluated: Standard-dose head (100 kVp, 151 mA s, partial-angle), low-dose head (100 kVp, 75 mA s, partial-angle), high-quality head (100 kVp, 754 mA s, partial-angle), pelvis (125 kVp, 706 mA s, full-angle), pelvis spotlight (125 kVp, 752 mA s, partial-angle), and low-dose thorax (110 kVp, 271 mA s, full-angle). Using the point dose method, various CTDI values were calculated by (1) the conventional weighted CTDI (CTDIw) calculation and (2) Bakalyar's method (CTDIwb). The MC simulations were performed to obtain the CTDIw and CTDIwb, as well as from (3) central slice averaging (CTDI(2D)) and (4) volume averaging (CTDI(3D)) techniques. The CTDI values of the new protocols were compared to those of the old protocols (full-angle CBCT protocols). Image quality of the new protocols was evaluated following the CBCT image quality assurance (QA) protocol [S. Yoo et al., "A quality assurance program for the on-board image, "Med. Phys. 33(11), 4431-4447 (2006)] testing Hounsfield unit (HU) linearity, spatial linearity/resolution, contrast resolution, and HU uniformity. RESULTS: The CTDI, were found as 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the new protocols, respectively. The CTDI, and CTDIwb differed within +3% between IC measurements and MC simulations. Method (2) results were within +/- 12% of method (1). In MC simulations, the CTDIw and CTDIwb were comparable to the CTDI(2D) and CTDI(3D) with the differences ranging from -4.3% to 20.6%. The CTDI(3D) were smallest among all the CTDI values. CTDIw of the new protocols were found as approximately 14 times lower for standard head scan and 1.8 times lower for standard body scan than the old protocols, respectively. In the image quality QA tests, all the protocols except low-dose head and low-dose thorax protocols were within the tolerance in the HU verification test. The HU value for the two protocols was always higher than the nominal value. All the protocols passed the spatial linearity/resolution and HU uniformity tests. In the contrast resolution test, only high-quality head and pelvis scan protocols were within the tolerance. In addition, crescent effect was found in the partial-angle scan protocols. CONCLUSIONS: The authors found that CTDIw of the new CBCT protocols has been significantly reduced compared to the old protocols with acceptable image quality. The CTDIw values in the point dose method were close to the volume averaging method within 9%-21% for all the CBCT scan protocols. The Bakalyar's method produced more accurate dose estimation within 14%. The HU inaccuracy from low-dose head and low-dose thorax protocols can render incorrect dose results in the treatment planning system. When high soft-tissue contrast data are desired, high-quality head or pelvis scan protocol is recommended depending on the imaging area. The point dose method can be applicable to estimate CBCT dose with reasonable accuracy in the clinical environment.

Estimation of computed tomography dose index in cone beam computed tomography: MOSFET measurements and Monte Carlo simulations.

To address the lack of accurate dose estimation method in cone beam computed tomography (CBCT), we performed point dose metal oxide semiconductor field-effect transistor (MOSFET) measurements and Monte Carlo (MC) simulations. A Varian On-Board Imager (OBI) was employed to measure point doses in the polymethyl methacrylate (PMMA) CT phantoms with MOSFETs for standard and low dose modes. A MC model of the OBI x-ray tube was developed using BEAMnrc/EGSnrc MC system and validated by the half value layer, x-ray spectrum and lateral and depth dose profiles. We compared the weighted computed tomography dose index (CTDIw) between MOSFET measurements and MC simulations. The CTDIw was found to be 8.39 cGy for the head scan and 4.58 cGy for the body scan from the MOSFET measurements in standard dose mode, and 1.89 cGy for the head and 1.11 cGy for the body in low dose mode, respectively. The CTDIw from MC compared well to the MOSFET measurements within 5% differences. In conclusion, a MC model for Varian CBCT has been established and this approach may be easily extended from the CBCT geometry to multi-detector CT geometry.

Estimation of absorbed doses from paediatric cone-beam CT scans: MOSFET measurements and Monte Carlo simulations.

The purpose of this study was to establish a dose estimation tool with Monte Carlo (MC) simulations. A 5-y-old paediatric anthropomorphic phantom was computed tomography (CT) scanned to create a voxelised phantom and used as an input for the abdominal cone-beam CT in a BEAMnrc/EGSnrc MC system. An X-ray tube model of the Varian On-Board Imager((R)) was built in the MC system. To validate the model, the absorbed doses at each organ location for standard-dose and low-dose modes were measured in the physical phantom with MOSFET detectors; effective doses were also calculated. In the results, the MC simulations were comparable to the MOSFET measurements. This voxelised phantom approach could produce a more accurate dose estimation than the stylised phantom method. This model can be easily applied to multi-detector CT dosimetry.

Radiation dose from cone beam CT in a pediatric phantom: risk estimation of cancer incidence.

OBJECTIVE: The objective of our study was to measure absorbed doses and calculate effective dose (ED) from cone beam CT (CBCT) with metal oxide semiconductor field effect transistor (MOSFET) detectors in an anthropomorphic phantom and to estimate the risk of cancer incidence for CBCT. MATERIALS AND METHODS: Abdominal CBCT was performed in an anthropomorphic phantom of a 5-year-old child using the On-Board Imager with arbitrarily designated standard-dose (125 kVp, 80 mA, 25 milliseconds) and low-dose (125 kVp, 40 mA, 10 milliseconds) modes. The full-fan mode was used, and 20 MOSFET dosimeters were used to measure the absorbed doses in various organs. We calculated the ED, the lifetime attributable risk (LAR) for cancer incidence, and relative risk (RR) of cancer induction from a single scan for both standard- and low-dose modes in 5-year-old children. RESULTS: The highest absorbed doses were found in the skin, ascending colon, and stomach. The mean ED was 37.8+/-0.7 (SD) mSv for the standard-dose mode and 8.1+/-0.2 mSv for the low-dose mode. The LAR of cancer incidence ranged from 23 to 144 cases per 100,000 exposed persons for the standard-dose mode and from five to 31 cases per 100,000 exposed persons for the low-dose mode. The RR of cancer incidence ranged from 1.003 to 1.054 for the standard-dose mode and from 1.001 to 1.012 for the low-dose mode. CONCLUSION: The ED from pediatric CBCT using the standard-dose mode was considerably higher than that of MDCT, whereas the ED for CBCT using the low-dose mode was comparable to that of abdominal MDCT. For abdominal CBCT in the pediatric phantom, the highest LARs were for colon and bladder cancers and the highest RRs were for stomach and liver cancers.

Characteristics of elliptical sources in BEAMnrc Monte Carlo system: implementation and application.

Recently, several papers noticed that the electron focal spot of a linear accelerator (linac) could be elliptical which would cause dosimetric discrepancies between measurements and Monte Carlo simulations. To resolve the mismatch, two elliptical source models were developed in BEAMnrc code. The first was a parallel beam elliptical source with uniform distribution where the shape of the source was primarily considered. The other was a parallel beam elliptical source with Gaussian distribution whose source distribution follows the normal distribution. To validate the elliptical source models, uniform and Gaussian electron beams were impinged on a thin air target. Both models successfully reproduced the elliptical shapes and source distributions. Then, this study investigated the characteristics of the elliptical Gaussian source for a 6 MV photon beam in a Varian 21EX linac. The linac head model was implemented in the BEAMnrc/EGSnrc system and commissioned by comparing the lateral and depth dose profiles to the ion chamber measurements acquired from the annual quality assurance (QA). It was found that the circular Gaussian beam with 6 MeV/0.2 cm full width half maximum (FWHM) produces the best matches to the QA data. To explore the characteristics of the elliptical Gaussian source, this study employed an elliptical Gaussian electron source with 0.1 cm FWHM in the x axis and 0.2 cm FWHM in the y axis which was incident on the target of the linac head. Two circular Gaussian beams with 0.1 and 0.2 cm FWHM were employed to compare the differences between circular and elliptical sources. For all the sources, planar and energy fluences were acquired and analyzed. This study also compared the lateral and depth dose profiles in a water phantom by using a DOSXYZnrc user code. In results, a constricted shoulder effect was observed in both planar and energy fluence plots when the FWHM value was increased and the field size is larger than 30 x 30 cm2. The same effect was also noticed in the lateral dose profiles, while the depth dose profile did not vary much.

Kerma area product method for effective dose estimation during lumbar epidural steroid injection procedures: phantom study.

OBJECTIVE: The purpose of this study was to derive from the kerma area product the dose conversion coefficient for estimating the effective dose for lumbar epidural steroid injection procedures. MATERIALS AND METHODS: A mobile fluoroscopy system was used for fluoroscopic imaging guidance of lumbar epidural steroid injection procedures. For acquisition of organ dose measurements, 20 diagnostic metal oxide semiconductor field effect transistor detectors were placed at each organ in an anthropomorphic phantom of a man, and these detectors were attached to four mobile metal oxide semiconductor field effect transistor wireless bias supplies to obtain the organ dose readings. The kerma area product was recorded from the system console and independently validated with an ion chamber and therapeutic x-ray film. Fluoroscopy was performed on the phantom for 10 minutes for acquisition of the dose rate for each organ, and the average clinical procedure time was multiplied by each organ dose rate for acquisition of individual organ doses. The effective dose was computed by summing the product of each organ dose and the corresponding tissue weighting factor from International Commission on Radiologic Protection publication 60. RESULTS: The effective dose was computed as 0.93 mSv for an average lumbar epidural steroid injection procedure (fluoroscopic time, 40.7 seconds). The corresponding kerma area product was 2.80 Gy.cm(2). The dose conversion coefficient was derived as 0.33 mSv/(Gy.cm(2)). CONCLUSION: The effective dose for lumbar epidural steroid injection can be easily estimated by multiplying the derived dose conversion coefficient by the console-displayed kerma area product.